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ATOMIC THEORY It sounds so mysterious doesn t it? Atomic theory conjures up images of secret science in the 1940 s, the atom bomb, and explosions! But the truth is, atomic theory is simply the internal structure of the atom. It s not sinister, secret, or scary it s nothing more than a blueprint! How we came up with the blueprint for something as small as the atom is quite another story and yes this does involve chalkboards covered with equations, funky experimental contraptions, and science nerds. Lots and lots of science nerds! You ve probably learned a bit about the atom before so let s just skim over the basics of the what, why, and how of atoms before digging deeper. The first thing to discuss is the definition of an element. An element is a basic building block of matter. All the known elements are contained in a special chart called the Periodic Table of the Elements and everything on the table cannot be broken down into any more parts. For instance gold is gold. It does not contain any other element other than gold. Same with silver, copper, tin, iron, cobalt, uranium and so on. The best way to describe an element is a fundamental (basic) building block of other compounds. A compound is a combination of two or more elements. Table salt is a compound because it contains sodium and chlorine which when combined makes sodium chloride or table salt. 2
In order to classify elements we must understand their basic structure and internal components. An atom is the basic unit of matter but it also has fundamental components inside of it called protons, electrons, neutrons. These things inside the atom are called subatomic particles. The specific arrangement of the subatomic particles is what makes each element unique. A molecule is a collection of atoms. For example, water is a collection of two hydrogen atoms and one oxygen atom. That s H 2 O and H 2 O is a molecule because it has two atoms hydrogen and oxygen. H is an atom, O is an atom and both H and O are also elements because they cannot be broken down into smaller atoms. They can be broken down into smaller particles such as protons, electrons, and neutrons. But those are called subatomic particles and are not atoms. To understand atomic theory you must first understand the subatomic particles within the atom and where each of these particles can be found. The nucleus is very small and dense and resides towards the center of the atom. The nucleus is not a particle like a proton, electron, or neutron; rather it is a place. A proton is a subatomic particle that resides in the atom; more specifically, it resides inside of the nucleus. The proton is a positively charged particle. We show a positive charge by using the plus symbol (+) next to an element. A neutron has no charge we call that an uncharged particle. The neutron also resides inside the nucleus. Don t get the nucleus and the neutron mixed up. The nucleus is a place inside the atom, while the neutron is a subatomic particle. Electrons are probably the most interesting thing about atoms because they don t reside inside the nucleus and are free to move about inside the atom. In some cases, the electrons can even leave the atom and join another atom. 3
This is how chemistry is done. Electrons, as we understand them today, exist in a cloud state and do not occupy a specific place inside the atom. Electrons are negatively charged (when we write this we use a minus sign to show a charge, so an electron would have a charge of -1). The negative charge on an electron cancels out the positively charged proton inside the nucleus. Now that you have a basic understanding of the atom and the subatomic particles that reside inside the atom, you should next understand matter. Matter is anything that has both mass and volume so these two properties, mass and volume, are used to describe the substances which are matter. Scientists also say that matter is something that occupies space. That is a pretty broad definition. In fact, it includes everything you come into contact with in your daily life. Cars are made of matter, hair, eyes, books, water, and air are all made up of matter. Since the beginning of time humans have wondered about what stuff is made of, and over the centuries they came up with some pretty good, and pretty rotten, ideas about the concept. Today we use the word atom to describe the basic and fundamental stuff. This word comes from the ancient Greek word atomos, which means unable to divide or cut. The ancient Greek philosopher Democritus (who lived between 460 and 370 BC) and his mentor Leucippus are generally credited with the naming of these tiny invisible particles. They also came up with a theory on their basic characteristics. Unfortunately, the view held by Democritus and other atomists didn t hold for long and was generally forgotten until the seventeenth century when scientists 4
began to think about the properties of gases. When these seventeenth century scientists began to think critically about air, something which is colorless (invisible) and had no odor, they began to wonder a little more about how one could accurately describe such a thing. What was air? What was it made of? How could they measure it? One thing scientists had figured out during this period is that things are made up of elements and before there were scientists there were alchemists. An alchemist was a person who practiced alchemy, and alchemy was the science of understanding, deconstructing, and reconstructing matter. It is also used interchangeable as the pursuit of turning common metals such as lead or copper into gold. In 1789 a French scientist named Antoine Lavoisier discovered that even though matter may change its form or shape, its mass always remains the same, thus he was the first to formulate the Law of Conservation of Mass. During Lavoisier s time air and water were considered elements, but he rejected this notion and eventually described the individual components of air (nitrogen, oxygen, argon) and water (hydrogen and oxygen). These discoveries, like all previous discoveries, propelled the thinking and reasoning of other scientists who were looking to explain what atoms were. 5
The next major step in the process of discovering what atoms were was the theory proposed by an English schoolteacher named John Dalton, around the year 1803. His theory states the following: 1. Each element is composed of extremely small particles called atoms. 2. All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of another element. 3. Atoms of an element are not changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions. 4. Compounds are formed when atoms of more than one element combine, a given compound always has the same relative number and kind of atoms. As you have already learned, Dalton s theory explains that atoms are the smallest particles of any given element and are the substance which contains and retains the chemical identity of the element even during a chemical reaction. John Dalton based his theory on other chemical laws known to exist at the time including the Law of Conservation of Mass and the Law of Constant Composition which states that the molecular make-up of a substance is always the same, regardless of how the substance was made or where the substance is found. Using water as an example we know with certainty that all water molecules contain 2 hydrogen atoms for each oxygen atom, regardless of whether we got it from the sink or collected it from a mountain stream. Dalton also deduced from his observations the Law of Multiple Proportions, which states when two elements can combine to form more than one compound and the same amount of the first element is used in each, then the ratio of the amounts of the other element will be a whole number. 6
The significance of this law is not important to understand right now but it is the basis of stoichiometry or how the products and reactants of a chemical equation relate to one another. Stoichiometry is a fundamental concept of high school and college chemistry. It is important to understand that John Dalton had no direct evidence to support his theory instead he relied on chemical observations. He could not see the oxygen and hydrogen atoms thus there was really no evidence that atoms actually existed until more modern times when powerful microscopes were developed. It is for this reason that atomic theory has changed quite a bit over the past 2 centuries and so we must discuss several simpler models before arriving at the current theory of atomic structure. We now know that the atom is not the smallest and most basic fundamental particle. We know this because inside the atom are electrons, protons, and neutrons. While this might seem elementary to you, being a student of the 21st century, it was not at all obvious to scientists in the mid 1800 s. The first subatomic particle to be discovered was the electron. Scientists all over Europe had been busy looking at electrical energy they knew that some elements could carry an electrical charge and that charges could be either positive or negative. One way in which scientists studied electrical discharge, which is the giving off of electrical energy, was to partially empty a glass tube of air and push a voltage through it. This created a negative charge on one end and a positive charge on the other and was called a cathode ray. Watch this animation about cathode rays and JJ Thompson s electron discovery. During this time more scientist were experimenting with subatomic particles and the new mystery became How do all these particles fit together inside the atom? JJ Thompson, who calculated the mass of the electron, also came up with the first definitive model for the inside of the atom. He called it the Plum Pudding Model 7
and thought that electrons were embedded inside the atom like raisins in a cup of plum pudding. This theory didn t last long, mostly because it was wrong, but also because a scientist from New Zealand named Ernest Rutherford proved with his famous backscattering experiment that most of the mass of an atom was concentrated into a compact nucleus, with electrons occupying the bulk of the atom's space and orbiting the nucleus at a distance. This eventually led to the discovery of protons (the positive charge in the center of an atom), by Rutherford, and neutrons, by an English scientist named James Chadwick. Watch this animation about Rutherford s backscattering experiments. Since the discovery of the three major subatomic particles scientists have learned a lot more about how atoms are structured. There are in fact, many more sub atomic particles inside the atom, but only these three are terribly important for the study of basic chemistry. With the Plum-Pudding model out, and the backscatter experiment by Rutherford proving that most of the atom is empty space, atomic structure evolved into the Niels Bohr planetary model. The planetary model is still used for elementary students today because it is simple; but it is not correct. Today we know that the electrons don t orbit the nucleus like planets orbiting the sun, instead they exist in the cloud outside the nucleus. 8
Look at the chart below. In the chart you are presented with a compare and contrast view of each of the subatomic particles. Study the chart before moving on. Now let s talk about each subatomic particle individually and how they are significant in chemistry. The proton is a positively charged subatomic particle which resides inside the nucleus of the atom. The nucleus is the center mass. Since the mass of subatomic particles is so small, we use a special unit of measurement called the atomic mass unit or AMU to measure it. A proton has a mass of approximately 1 amu (atomic mass unit). Protons also carry a charge, in fact they have a charge of +1, which means they are positive. The proton is the most important particle because it is what gives the element its identity. If an element has one proton it is hydrogen, if it has two it is helium. If it has 14 it is nitrogen. There is no way a helium atom can have 1 proton because if it does it is not helium, it is hydrogen. 9
Since the protons in the atom dictate which element it is, this number is used as an identifying marker and is called the atomic number. So the number of protons is the same as the atomic number. The neutron is an uncharged particle which resides inside the nucleus with the proton. It too has the approximate mass of 1 amu. Some students might wonder what the purpose of the neutron is because it sort of just sits there and doesn t seem to do anything. But it does do something it helps to hold the nucleus together! The neutrons are necessary to prevent the positively charged protons from repelling each other right outside the nucleus. The electron is the rock star of subatomic particles in chemistry. It is the electron which moves around in a chemical reaction because remember you can t move protons around without changing the element completely. The electron has a mass but it is so small that we pretty much just say it is zero! It also has a charge of -1. Electrons are located outside of the nucleus in what we call the electron cloud. The electron cloud is really more of an approximate place where an electron might be found at any given time, not an actual point in space. You now know that it is the arrangement of subatomic particles which give each atom its individual properties. Now we want to explore how little differences in the number of neutrons and electrons can change the element slightly not enough to be another element entirely but enough to have a significant effect on the way the element behaves in a chemical reaction. 10
Recall that the number of protons in the nucleus is the same as the atomic number and that the atom is naturally occurring in an uncharged state so this tells us that the number of protons equals the number of electrons. Right off the bat we are going to shake things up a bit with the neutrons because the number of neutrons can change. Earlier you were told that the number of protons must always be the same, but that is not true for neutrons or electrons. When we change the number of neutrons we have a slightly different version of the same element. This new version is called an isotope. If the atom has a different number of neutrons than number of protons this means that the atomic mass of the atom has changed. Recall that both protons and neutrons have a mass of about 1 amu each. Naturally if the number of neutrons changes, then the atomic mass of the atom also changes. If you add more neutrons the atom gets heavier imagine it is like you picking up a large stone and stepping on the scale. You d weight more, right? If you subtract some neutrons then the atom weighs less makes sense if you drop that stone while you re on the scale, you lose weight immediately! The atomic number and the atomic mass number are both included when using elemental notation. Elemental notion is a visual shorthand way of describing the number of subatomic particles inside an atom. Elemental notation consists of the element symbol, the atomic number, the mass number, and charge (if there is one). It looks like this: 11
Students and scientists use elemental notation to determine how many proton, electrons, and neutrons are inside an atom, as well as which atom it is, and whether or not it is an isotope, or contains a charge. Atoms with the same atomic numbers but different mass numbers (have a different number of neutrons) are called isotopes. Hydrogen has three well know isotopes called protium, deuterium, and tritium. Protium is by far the most common it has one proton, one electron, and no neutron. Deuterium, also called heavy hydrogen, is twice as heavy as protium because it has one proton, one neutron, and one electron. Tritium is even heavier and is also radioactive. It is rare on Earth. Let s look at an example of elemental notation for each of the three hydrogen isotopes: Each atom of hydrogen has one proton in its nucleus, so the atomic number of each is "1" and that number can be found in the lower left corner. The mass number of each of these atoms varies because they each have a different number of neutrons. The first hydrogen has 2 neutrons in addition to the 1 proton in the nucleus, for a total mass number of 3. The second atom has 1 neutron in addition to the 1 proton in the nucleus, for a total mass number of 2. The third atom has no neutrons, only 1 proton in its nucleus, for a mass number of 1. 12
Hydrogen is a good example to explain isotopes because it is easy to calculate, so it is also a good example to describe how we come up with the atomic mass number. Atomic mass is actually an average of all the isotopes of that element so it is not precise. If you were working on a critical experiment in subatomic chemistry it might be worth your while to use the actual mass number for the correct isotope of hydrogen you are using, but right now that level of accuracy is overkill. Carbon is another good example not because it is simpler to work with but because an isotope of carbon is how scientists came up with the unit for measuring atomic mass. Remember that atomic mass is measured in amu s or atomic mass units? It turns out that the u or unit used for this measurement is based on an isotope of carbon called carbon-12. Carbon-12 has a mass of exactly 12 units and all other elemental isotopes are measured according to this isotope. It is the most common of the stable carbon isotopes, but certainly not the most interesting. Carbon-14 is a radioactive isotope of carbon, consisting of the following elemental notation: This notation states that the atomic number of Carbon is 6, the number of protons is 6, and the number of neutrons is 8 which gives us an atomic mass number of 14. Carbon 14 is the basis of radiocarbon dating which is a technique scientists use to approximate the age of organic matter, such as archeological remnants. It works like this: radiocarbon (carbon-14) is radioactive so it takes a very long time to decay into nothingness. The half life for carbon-14 is about 5,700 years. That means it takes about 5,700 years for one half of the isotope to decay. Scientists use this half-life estimate of carbon -14 (and other radioactive elements) to calculate how long an old organic object (which contains carbon) has ceased to take in any new carbon which is a scientific way to say since it died. 13
If an element has a charge on it then it either has extra electrons (- charge) or is missing electrons (+ charge), and the charge can also be depicted using elemental notation. The notation to the right shows us calcium, which has an atomic number of 20, and an atomic mass of 40. That means it is not an isotope because the number of protons equals the number of neutrons. In addition, this notation has a 2+ in the upper right corner; this means it has a net positive charge of 2. Since an electron is negative in order to get a 2+ charge you must be missing 2 electrons or have 2 extra protons. The only way to change the charge of an element is to move electrons around because moving protons around is not allowed! So we cannot actually have 2 extra protons! The way we find out how many electrons there are in an element with a charge is to subtract a positive charge from the number of electrons there should be, or to add a negative charge to the number of electrons there should be. For example Ca has a positive charge of 2 (+2) and we know from the bottom left number that the atomic number is 20 that means a normal atom of calcium has 20 protons and 20 electrons. We subtract 2 from 20 to get 18 total electrons. An element which has more electrons than protons or less electrons than protons is called an ion. Remember any element with a charge must have extra or missing electrons since you cannot move protons around. 14
Now let s practice determining the number of protons, electrons, and neutrons in an atom. Fill in the chart before going on. Finding the number of protons is the easiest part it is simply the atomic number which is located on the bottom left of the notation. The number of neutrons and protons is contained in the atomic mass number, which is on the upper left, so all you have to do is subtract the protons from the atomic mass number and you get the number of neutrons in the element. Finding the number of electrons is not as easy as the other two, but it is still fairly straight forward. Notice that bromine has a little negative sign in the upper right corner that means it has a negative charge. Now the ONLY way we can get an overall negative charge on an element is to add electrons. So the first thing we must do is determine how many electrons this element would have if there was no charge. 15
That s easy because the number of electrons = the number of protons. So we begin with 35 electrons, but we aren t done. We must add an additional electron because of the negative charge; this gives us an overall number of 36 electrons. Remember that a negative charge means you have EXTRA electrons and a positive charge means you are MISSING some electrons. To find the number of electrons for Na (sodium) we must subtract from the number of electrons we start with, so 11-1 = 10 electrons. To find the number of electrons for Sr (strontium) we must subtract 2 from our original number of electrons because we had a +2 charge on the atom. This gives us 36. And that s pretty much it as far as basic atomic theory goes. You now know that atoms are elements that contain subatomic particles. The subatomic particles give the elements their properties and identity. You now also understand what an isotope and an ion are, and you can figure out the basic characteristics of elements by decoding their elemental notation. You re really on your way to understanding how chemistry works! 16
Glossary Alchemist A practitioner of alchemy. Alchemy A medieval philosophy and early form of chemistry whose aims were the transmutation of base metals into gold, the discovery of a cure for all diseases, and the preparation of a potion that gives eternal youth. The imagined substance capable of turning other metals into gold was called the philosophers' stone. Antoine Lavoisier French chemist known as the father of modern chemistry and who discovered oxygen. Atom A unit of matter, the smallest unit of an element, having all the characteristics of that element and consisting of a dense, central, positively charged nucleus surrounded by a system of electrons. Atomic Mass The total mass of protons, neutrons and electrons in a single atom. Atomic Mass Unit A unit that is used for indicating mass on an atomic or molecular scale. Atomic Number The number of protons found in the nucleus of an atom. Atomic Theory The physical theory of the structure, properties, and behavior of the atom. Atomos The Greek word for divide or cut. Carbon 12 The more abundant of the two stable isotopes of the element carbon, accounting for 98.89% of carbon; it contains 6 protons, 6 neutrons, and 6 electrons. Carbon 14 A radioactive isotope of carbon with a nucleus containing 6 protons and 8 neutrons. Cathode Ray A beam of electrons streaming from the negatively charged end of a vacuum tube (the cathode) toward a positively charged plate (the anode). Charge A fundamental property of the elementary particles of which matter is made that gives rise to attractive and repulsive forces. Compound A pure substance consisting of atoms or ions of two or more different elements in definite proportions that cannot be separated by physical means. Democritus Greek philosopher who developed one of the first atomist theories of the universe. Deuterium The hydrogen isotope which contains one proton and one neutron. Electrical Charge A form of charge, designated positive, negative, or zero, found on the elementary particles that make up all known matter. Electron A negatively charged particle inside an atom and which resides outside the nucleus. Electron Cloud An area inside the atom where electrons are likely to be found. 17
Element A substance composed of atoms having an identical number of protons in each nucleus. Elements cannot be reduced to simpler substances by normal chemical means. Elemental Notation A shorthand way of writing information about a particular type of element, isotope or atom. Ernest Rutherford Proved that most of the mass of an atom is due to the nucleus and not the electron. Ion - An atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. Isotope Atoms that contain the same number of protons but a different number of neutrons. JJ Thompson The scientist who discovered the electron using a cathode tube. John Dalton English chemist and physicist who formulated atomic theory. Matter Something that has mass and exists as a solid, liquid, gas, or plasma. Molecule- A group of two or more atoms linked together by sharing electrons in a chemical bond. Neutron An uncharged particle inside an atom and which resides inside the nucleus. Niels Bohr Came up with the planetary model of the atom. Nucleus The positively charged central region of an atom, composed of protons and neutrons and containing almost all of the mass of the atom. Periodic Table A tabular arrangement of the elements according to their atomic numbers so that elements with similar properties are in the same column. Planetary Model A model of the atom where electrons orbit the nucleus like planets orbit the sun. Plum Pudding Model Model of how electrons were positioned inside the atom. Protium The most common isotope of hydrogen, with one proton and no neutrons. Proton A positively charged particle inside an atom and which resides inside the nucleus. Radioactive Process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles. Radiocarbon Dating a dating method that uses the naturally occurring radioisotope carbon-14 to estimate the age of carbonaceous materials up to about 58,000 to 62,000 years. Stoichiometry Calculation of the quantities of reactants and products in a chemical reaction. Subatomic Particle Any of various particles of matter that are smaller than a hydrogen atom including protons, neutrons, and electrons. 18
Tritium A radioactive isotope of hydrogen which has one proton and two neutrons in the nucleus. 19
Student Activities Exercise One Write the letter of the correct match next to each problem. 1. Atomic Theory a. A negatively charged particle inside an atom. b. A table of elements which are arranged according to 2. Element atomic number. 3. Periodic Table c. The physical theory of the structure of the atom. d. A group of two or more atoms linked together by 4. Compound chemical bonds. 5. Atom e. A unit of matter, the smallest unit of an element. f. A pure substance consisting of atoms or ions of two or 6. Proton more different elements. g. Any of various particles of matter that are smaller than 7. Electron an atom. 8. Neutron h. A positively charged particle inside an atom. Subatomic i. An uncharged particle inside an atom. 9. Particle 10. Molecule j. A substance composed of atoms that are identical. 20
Exercise Two Write the letter of the correct match next to each problem. a. Calculation of the quantities of reactants and products in 1. Nucleus a chemical reaction. b. English chemist and physicist who formulated atomic 2. Charge theory. c. Greek philosopher who developed one of the first 3. Matter atomist theories. 4. Atomos d. A pre-science chemistry. 5. Democritus e. Attractive and repulsive forces attached to matter. 6. Alchemist f. French chemist who discovered oxygen. 7. Alchemy g. A practitioner of alchemy. 8. Antoine Lavoisier h. Something that has mass and exists as a solid, liquid, gas, or plasma. 9. John Dalton i. The Greek word for divide or cut. 10. Stoichiometry j. The positively charged central mass of an atom. 21
Exercise Three Write the letter of the correct match next to each problem. Electrical a. Came up with the planetary model of the atom. 1. Charge b. The scientist who discovered the electron using a 2. Cathode Ray cathode tube. 3. JJ Thompson c. The number of protons found in the nucleus of an atom. Plum Pudding d. Model of how electrons were positioned inside the atom. 4. Model 5. Ernest Rutherford e. An area inside the atom where electrons are likely to be found. f. Proved that most of the mass of an atom is due to the 6. Niels Bohr nucleus and not the electron. g. A form of charge, designated positive, negative, or zero, 7. Planetary Model Atomic Mass found on the elementary particles. h. A unit that is used for indicating mass on an atomic or 8. Unit molecular scale. i. A beam of electrons streaming from the negatively 9. Atomic Number charged end of a vacuum tube. j. A model of the atom where electrons orbit the nucleus 10. Electron Cloud like planets orbit the sun. 22
Exercise Four Write the letter of the correct match next to each problem. a. A radioactive isotope of carbon with a nucleus containing 1. Isotope 6 protons and 8 neutrons. b. A radioactive isotope of hydrogen which has one proton 2. Atomic Mass Elemental and two neutrons in the nucleus. c. Process by which an atomic nucleus of an unstable atom 3. Notation loses energy by emitting ionizing particles. d. The hydrogen isotope which contains one proton and one 4. Protium neutron. e. An atom or molecule in which the total number of 5. Deuterium electrons is not equal to the total number of protons. f. The total mass of protons, neutrons and electrons in a 6. Tritium single atom. g. Atoms that contain the same number of protons but a 7. Radioactive different number of neutrons. h. The more abundant of the two stable isotopes of the 8. Carbon 12 element carbon. i. The most common isotope of hydrogen, with one proton 9. Carbon 14 and no neutrons. j. A shorthand way of writing information about a particular 10. Ion type of element, isotope or atom. 23
Label the diagram of the atom using the word bank below. Proton Electron Neutron Nucleus Electron Cloud 24
Study the elemental notation of this hypothetical potassium ion, and then fill in the correct number for each subatomic particle. 25
Parent Solutions Exercise One Write the letter of the correct match next to each problem. 1. c Atomic Theory a. A negatively charged particle inside an atom. 2. j Element b. A table of elements which are arranged according to atomic number. 3. b Periodic Table c. The physical theory of the structure of the atom. 4. f Compound d. A group of two or more atoms linked together by chemical bonds. 5. e Atom e. A unit of matter, the smallest unit of an element. 6. h Proton f. A pure substance consisting of atoms or ions of two or more different elements. 7. a Electron g. Any of various particles of matter that are smaller than an atom. 8. i Neutron h. A positively charged particle inside an atom. 9. g Subatomic Particle i. An uncharged particle inside an atom. 10. d Molecule j. A substance composed of atoms that are identical. 26
Exercise Two Write the letter of the correct match next to each problem. a. Calculation of the quantities of reactants and products in j 1. Nucleus a chemical reaction. 2. e Charge b. English chemist and physicist who formulated atomic theory. 3. h Matter c. Greek philosopher who developed one of the first atomist theories. 4. i Atomos d. A pre-science chemistry. 5. c Democritus e. Attractive and repulsive forces attached to matter. 6. g Alchemist f. French chemist who discovered oxygen. 7. d Alchemy g. A practitioner of alchemy. 8. f Antoine Lavoisier h. Something that has mass and exists as a solid, liquid, gas, or plasma. 9. b John Dalton i. The Greek word for divide or cut. 10. a Stoichiometry j. The positively charged central mass of an atom. 27
Exercise Three Write the letter of the correct match next to each problem. Electrical a. Came up with the planetary model of the atom. g 1. Charge 2. i Cathode Ray b. The scientist who discovered the electron using a cathode tube. 3. b JJ Thompson c. The number of protons found in the nucleus of an atom. 4. d Plum Pudding Model d. Model of how electrons were positioned inside the atom. 5. f Ernest Rutherford e. An area inside the atom where electrons are likely to be found. 6. a Niels Bohr f. Proved that most of the mass of an atom is due to the nucleus and not the electron. 7. j Planetary Model g. A form of charge, designated positive, negative, or zero, found on the elementary particles. 8. h Atomic Mass Unit h. A unit that is used for indicating mass on an atomic or molecular scale. 9. c Atomic Number i. A beam of electrons streaming from the negatively charged end of a vacuum tube. 10. e Electron Cloud j. A model of the atom where electrons orbit the nucleus like planets orbit the sun. Exercise Four 28
Write the letter of the correct match next to each problem. a. A radioactive isotope of carbon with a nucleus containing g 1. Isotope 6 protons and 8 neutrons. 2. f Atomic Mass b. A radioactive isotope of hydrogen which has one proton and two neutrons in the nucleus. 3. j Elemental Notation c. Process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles. 4. i Protium d. The hydrogen isotope which contains one proton and one neutron. 5. d Deuterium e. An atom or molecule in which the total number of electrons is not equal to the total number of protons. 6. b Tritium f. The total mass of protons, neutrons and electrons in a single atom. 7. c Radioactive g. Atoms that contain the same number of protons but a different number of neutrons. 8. h Carbon 12 h. The more abundant of the two stable isotopes of the element carbon. 9. a Carbon 14 i. The most common isotope of hydrogen, with one proton and no neutrons. 10. e Ion j. A shorthand way of writing information about a particular type of element, isotope or atom. 29
Label the diagram of the atom using the word bank below. Proton Electron Neutron Nucleus Electron Cloud 30
Study the elemental notation of this hypothetical potassium ion, and then fill in the correct number for each subatomic particle. 31