UNIT 4 ATOMIC THEORY 1. Atomic theory: Dalton s model Thomson s model Rutherford s model Bohr s model Electron cloud model 2. Particles inside the atom Atomic number Mass number 3. Ions Cations Anions 4. Isotopes 5. Relative atomic mass 86
1. ATOMIC THEORY Because we cannot see atoms, we use models to teach and learn about them. The atomic theory has changed over time as new technologies have become available. Scientific knowledge builds on past research and experimentation. Dalton s model. Scientist Information Model John Dalton Dalton said that: All matter was made of atoms. Atoms were too small to be seen, indivisible and indestructible. All atoms of a given element were identical. Compounds were formed by the joining of two or more elements 87
Thomson s model Scientist Information Model J.J Thompson Discovered the negative particles called electrons, and predicted that there also must be a positive charge to hold the electrons in place. He said that atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Rutherford s model Rutherford s experiment involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Most of the positively charged bullets passed right through the gold atoms in the sheet of gold foil without changing their course at all. Some of the positively charged bullets, however, did bounce off away from the gold sheet as if they had hit something solid. Few particles bounced back. 88
He knew that positive charges repel positive charges. This could only mean that the gold atoms in the sheet were mostly open space. Atoms were not a pudding filled with a positively charged material. Rutherford concluded that an atom had a small, dense, positively charged center that repelled his positively charged bullets. He called the center of the atom the nucleus. The nucleus is tiny compared to the atom as a whole. Rutherford reasoned that all the positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom s edge. After Rutherford had stated his model, Chadwick discovered that in the nucleus there were also other particles called neutrons that have no charge. Scientist Information Model Ernest Rutherford Discovered the nucleus of an atom. Named the positive particles in the nucleus protons. He said that electrons were scattered in empty space around the nucleus. 89
Bohr s model Scientist Information Model Neils Bohr Concluded that electrons are located in planet-like orbits around the nucleus in certain energy levels. Electron cloud model Scientist Information Model (Many Scientists!) The Modern Atomic Theory Nowadays we know that: Electrons do not orbit the nucleus in neat planet-like orbits but move at high speeds in an electron cloud around the nucleus. 90
2. PARTICLES INSIDE THE ATOM An atom is the smallest piece of matter that still retains the properties of the element. Three subatomic particles make up every atom: Subatomic Particle Charge Location Proton Positive (+) Nucleus Neutron No Charge (0) Nucleus Electron Negative (-) Shell-Electron Cloud ATOMIC NUMBER: It is the number of protons in an element. It defines the element. (it never changes) MASS NUMBER: It is the sum of the number of protons and neutrons. Knowing the atomic number and the mass number we can deduce the number of protons, electrons and neutrons and the other way round. 91
3.IONS As the atom is neutral it has the same number of protons as electrons so if we know the atomic number we know the number of protons and electrons of the atom. Sometimes an atom can gain or lose electrons, so that they became charged atoms, then we call them IONS When a neutral atom gains electrons from another atom it becomes a negative ion called ANION When a neutral atom loses electrons it becomes a positive ion called CATION. The charge of the ion depends on how many electrons it has gained or lost. To represent ions the charge is written in the top right corner of the symbol of the element: X +2 4.ISOTOPES ISOTOPES are atoms with the same number of protons, but different number of neutrons. They are atoms of the same element (same atomic number) with a different mass number. Despite these differences, isotopes are chemically the same because they have identical numbers of protons and electrons. Alike= similar 92
Examples: Isotopes of Neon Isotopes of Hydrogen 5. RELATIVE ATOMIC MASS The relative atomic mass is the mass of an atom when compared to a standard atom. It is represented by A r. It has no unit. Why do we use the relative atomic mass? The mass of an atom is too small to be measured in grams. We can only measure the mass of an atom by comparing its mass to the mass of another atom. 93
At the beginning all the atoms of the elements were compared with the hydrogen atom because it was the smallest. At present, carbon-12 is used as the standard of comparison because: It is solid (and can be easily handled) Its mass can be more easily measured (it is very stable) Carbon-12 is easily found as its compounds are a lot. Carbon-12 isotopes are assigned a mass of exactly 12 units. The RELATIVE ATOMIC MASS of an element is the mass of one atom of the element when compared to 1/12 of the mass of an atom of carbon -12. Examples: An atom of oxygen is 16 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative atomic mass is 16. An atom of sodium is 23 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative atomic mass is 23. To calculate the relative atomic mass of an element that has several isotopes, multiply the relative mass of each isotope by its natural abundance, expressed as a decimal, and then add all the products. Atomic mass is the average of all the isotopes of an element. 94
UNIT4 ACTIVITIES A1-PARTICLES IN THE ATOM Calculate the number of protons, neutrons and electrons of these elements: Element Z( atomic number) A(Mass number) Protons Electrons Neutrons Na, sodium 11 23 Li, lithium 3 7 K,potassium 19 39 Al, aluminium 13 27 Br, bromine 35 80 Ar,argon 18 40 A2- PARTICLES IN THE ATOM Complete the following table: Element Z( atomic number) A(Mass number) Protons Electrons Neutrons Ca 20 20 F 9 10 Sc 45 21 Fe 26 30 P 31 15 95
Be 9 4 A3- A4- IONS a) How many electrons does an atom of Mg have to lose to become the cation Mg2+? b) How many electrons does an atom of Cl have to gain to become the anion Cl-1? c) Does an atom of Ca have to gain or lose electrons to form the ion Ca2+? d) Does an atom of H have to gain protons or lose electrons to form the ion H+? A5-IONS Complete the table: Ion Z A Protons Electrons neutrons Li+ 3 7 O2-8 16 Fe3+ 26 56 Sr 2+ 38 88 I- 53 127 96
Zn2+ 30 65 A6-ISOTOPES Say which of the following atoms are isotopes of the same element: Atom Protons Electrons Neutrons A 6 6 6 B 17 17 18 C 5 5 5 D 6 6 7 E 4 4 5 F 17 17 20 A7-ISOTOPES Calculate the relative atomic mass of the element chlorine. You know that it has two isotopes and the abundance of each one is: Cl-35 75,5% Cl-37 24,5% A8-RELATIVE ATOMIC MASS Practice in your group the next sentence using different elements in order to learn it: An atom of oxygen is 16 times heavier than 1/12 of the mass of an atom of carbon-12. Its relative atomic mass is 16. You can practise with: Hydrogen 1, Helium 4, Lithium 7, Beryllium 9, Boron 10, Carbon 12, Nitrogen 14, Oxygen 16,Fluorine 19, Neon 20. Its relative time heavier than 1/12 of the An atom of is atomic mass mass of an atom of carbon-12. is 97
Try to say it following this chart times...of the An atom of is mass of... Its... is Then, try to say it following this chart An atom of is times its A9-Bingo activity for the review of the unit. Thomson atomic neutrons anion Rutherford number isotope nucleus Dalton mass number ion shell Bhor electrons cation neutral atom protons You have to draw a grid with 9 squares, then choose nine words from the above and write one in each square of the grid. The teacher reads one by one the following definitions randomly. You cross the word that corresponds with the definition until you have all of them crossed. That is full house! 98
1 UNIT 2 EXERCISES 2 3 99
4 5 6 100
7 8 9 101
10 11 12 13 Using the information in the unit. Calculate the relative atomic mass of Neon 14 Using the information in the unit. Calculate the relative atomic mass of Hydrogen 102
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