Atomic Structure and The Periodic Table. Unit 3

Transcription

1 Atomic Structure and The Periodic Table Unit 3

2 Lesson 1: Atoms Unit 5: Atomic Structure & The Periodic Table

3 Atoms How small can things get? If you break a stone wall into smaller and smaller pieces, you would have a pile of smaller stones. If you could break the smaller stones into the smallest pieces possible, you would have a pile of atoms. An atom is the smallest unit of matter. Greek philosophers proposed the first atomic theory of matter in the fifth century BC. This idea that atoms are the basic building blocks has been confirmed by modern-era scientists. Atoms are so small a single teaspoon of water contains approx. 500,000,000,000,000,000,000,000 atoms!

4 History About 2400 years ago, Greek philosophers proposed that everything on Earth was made of only 4 things: air, water, fire, and earth. Everything else was a mixture of these four basic substances. As time went on, chemists came to realize that there had to be more than four basic substances. Today chemists know that about 100 basic substances or elements account for everything we see or touch. Sometimes these elements appear by themselves. However, most often, they appear in combination with other elements and compounds.

5 Elements An element is a substance that contains only a single type of an atom. The number of atoms is not important, as long as all the atoms are of the same type. You cannot separate an element into other substances. Examples: silver, oxygen, hydrogen, helium, aluminum, gold There are more than 100 elements.

6 Chemical Symbols Each element has its own unique symbol, referred to as its chemical symbol. For some, it s the first letter of its name. Examples: hydrogen (H), carbon (C) For others, it s the first letter plus one other letter of the element s name. Examples: aluminum (Al), platinum (Pt)

7 Composition of Elements Each element is made of tiny particles called atoms. All atoms of a particular element are identical, but different from atoms of all other elements. For example, every atom of silver is similar to every other atom of silver. But it is different from an atom of iron.

8 Electric Charges Atoms contain subatomic particles that have electric charges. A particle s charge can be either positive or negative. Subatomic particles with the same charge repel each other. Subatomic particles with different charges attract each other.

9 The Structure of an Atom Atoms are composed of three types of subatomic particles. A proton is a positively charged particle A neutron is a neutral particle. Neutral means that is neither negatively or positively charged; there is no charge. An electron is a negatively charged particle. The protons and neutrons make up the nucleus of the atom. The nucleus is the central region of an atom, where most of the atom s mass is found in protons and neutrons. The electrons move around the outside of the atom.

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11 The Size of an Atom It would take you about 500 years to count the number of atoms in one grain of salt. Electrons are much less massive than protons or neutrons. Therefore, most of the atom s mass is in the nucleus. The negative electrons stick around the nucleus because they are attracted to the positively charged protons.

12 Atomic Numbers The identity of an atom is determined by the number of protons in its nucleus, called the atomic number. Example: Every hydrogen atom has exactly 1 proton in its nucleus. Therefore, its atomic number is 1. Example: Every gold atom has exactly 79 protons in its nucleus. Therefore, its atomic number is 79.

13 Atomic Mass Numbers Remember, most of the atom s mass is in the nucleus. The total number of protons and neutrons in an atom s nucleus is called its atomic mass number. Atomic Mass Number Atomic Mass Number = Protons + Neutrons

14 Finding the Number of Neutrons In order to find the number of neutrons: 1. Round the atomic mass number to the nearest whole number. 2. Subtract the atomic number from the atomic mass number.

15 Checkpoint: Atoms 1. Draw a model of an atom. Label the parts of the atom.

16 Lesson 2: Elements, Compounds, and Mixtures Unit 5: Atomic Structure & The Periodic Table

17 Atomic Mass Numbers The atoms of a certain element always have the same number of protons. However, they do not always have the same number of neutrons. Therefore, NOT all atoms of the same element have the same atomic mass number.

18 Isotopes For example, all chlorine atoms have 17 protons. However, some chlorine atoms have 18 neutrons, while other chlorine atoms have 20 neutrons. The chlorine atoms that have 18 or 20 neutrons are called chlorine isotopes. An isotope is an atom of the same element that has a different number of neutrons.

19 Isotopes An isotope is identified by the name of the element and its atomic mass number. You can find the number of neutrons in an isotope by subtracting the atomic number from the atomic mass number. Example: Chlorine-35 indicates the isotope of chlorine that has 18 neutrons (35 17).

20 Valence Electrons A valence electron is an electron in an atom that can form a chemical bond with another atom. A valence electron is typically found in the outermost electron shell. Valence electrons can only be found in atoms that do not have a full outermost electron shell.

21 Sharing Electrons Na = 11 protons Cl = 17 protons

22 Ions A neutral atom has an equal number of electrons and protons. An ion is an atom that has lost or gained one or more electrons A positive ion has lost an electron. It is represented by the symbol for the element, with a raised plus sign (+) to indicate a positive charge. Examples: Na +, Ca 2+ A negative ion has gained an electron. It is represented by the symbol for the element, with a raised negative sign (-) to indicate a negative charge. Examples: Cl -, O 2-

23 Pure Substances Matter can be pure, or it can be two or more substances mixed together. Often, the difference between pure and mixed substances is apparent only on the atomic or molecular level. A pure substance has only one type of component. Example: Pure water contains only water molecules; pure silver contains only silver atoms. A substance is considered pure if it contains only a single type of atom, such as gold, or a single combination of atoms that are bonded together, such as a water molecule. An element is a type of a pure substance.

24 Molecules When two or more atoms bond together, or combine, they make a particle called a molecule. A molecule can be made of atoms that are different or atoms that are alike. Example: A molecule of water is a combination of different atoms two hydrogen atoms and one oxygen atom. H 2 O Example: Hydrogen gas molecules are made of the same atoms two hydrogen atoms bonded together. H 2 A molecule is the smallest amount of a substance made of combined atoms that is considered to be a substance.

25 Compounds A compound is a substance that consists of two or more different types of atoms bonded together. Some types of compounds are made of molecules, such as water (H 2 O) and carbon dioxide (CO 2 ). Other compounds are made of atoms that are bonded together in a different way, like table salt (NaCl). A compound can have very different properties from the individual elements that make up that compound. For example, table salt is a combination of sodium and chlorine. Although table salt is safe to eat, the individual elements can be poisonous.

26 Combining Atoms A subscript is a number written slightly below and to the right of a chemical symbol that shows how many atoms of an element are in a compound. A chemical formula is an expression that shows the number and types of atoms joined in a compound.

27 Mixtures A mixture is a combination of different substances that remain the same individual substances and can be separated by physical means. Examples: Fruit salad, trail mix

28 Compounds vs. Mixtures The substances in mixtures remain the same substances (physical change). Compounds are new substances formed by atoms that bond together (chemical change). Mixtures can be separated by physical means. Compounds can be separated only by breaking the bonds between atoms (chemical change). The proportions of different substances in a mixture can vary throughout the mixture or from mixture to mixture. The proportions of different substances in a compound are fixed because the type and number of atoms that make up a basic unit of the compound are always the same.

29 Checkpoint: Atoms 1. What is the difference between an element, a compound, and a mixture?

30 Lesson 3: The Periodic Table Unit 5: Atomic Structure & The Periodic Table

31 Organizing Elements What characteristics would you use to organize a pile of all sorts of buttons? One way of organizing elements is by the masses of their atoms. Finding the mass of a single atom is incredibly difficult. Instead, chemists had to find the mass of a very large number of atoms of a certain element. Then they could infer the mass of a single one of them. Remember that not all the atoms of an element have the same atomic mass number. Elements have isotopes. When chemists try to measure the mass of an atom, therefore, they are actually finding the average mass of all its isotopes. The atomic mass is the average mass of all the element s isotopes.

32 Atomic Mass Numbers Remember, most of the atom s mass is in the nucleus. The average mass of the atoms is called its atomic mass. The total number of protons and neutrons in an atom s nucleus is called its atomic mass number. Atomic Mass Atomic Mass Number

33 Mendeleev s Periodic Table In the early 1800s several scientists proposed systems to organize the elements based on their properties. None worked very well. Finally, a Russian chemist named Dmitri Mendeleev came up with a system in the 1860s. Mendeleev made a set of elements cards; each card contained the atomic mass of an atom of the element and information about the element s properties. He then searched for a relationship between the properties and the atomic mass. Mendeleev produced the first periodic table of the elements in Periodic = repeating pattern of properties of elements

34 Rows: Similar chemical properties Columns: Increasing atomic masses

35 Predicting New Elements Mendeleev left some empty spaces where no known elements fit the pattern. He predicted that new elements would be discovered. He even described some of the properties of these unknown elements. At first, many chemists found it hard to accept Mendeleev s prediction of unknown elements. However, only 6 years after he published the table, the first of these elements was discovered. In the next 20 years, two other elements Mendeleev predicted were discovered.

36 Modern Periodic Table The modern periodic table differs from Mendeleev s in several ways: Elements with similar properties are found in columns (not rows). The elements are arranged by their atomic number, not by atomic masses. We can now define the periodic table as a table of the elements, arranged by atomic number, that shows the patterns in their properties.

37 Reading the Periodic Table Each square of the periodic table gives particular information about the atoms of an element.

38 Periodic Table

39 Groups Elements in a vertical column of the periodic table are similar in chemical and physical properties, known as a group. Each group is identified by a number at the top of the column. Sometimes a group is called a family of elements, because these elements are related.

40 Group 17 is commonly called the halogen group / family. They tend to combine easily with many other elements and compounds, especially with those in Group 1 and Group 2. They have similar chemical properties, so an element that combines with one halogen will likely combine with another. Halogen Family

41 Noble Gases Group 18 is commonly referred to as the Noble Gases. They are called inert because they are inactive. All of their energy shells are full, so they are stable.

42 Periods Each horizontal row in the periodic table is called a period. Properties of elements change in predictable ways from one end of a period to the other.

43 Electron Arrangement The chemical properties of an element depend on the way the electrons are arranged about the nucleus. Remember, we don t know exactly where the electrons are. We only know the region (the energy shell, or electron shell) is where the electrons move.

44 Electron Arrangement The periods on the periodic table end when an energy shell gets full of electrons. Elements located on the left (lithium, sodium) have one electron added to a new energy shell. Elements located on the right (chlorine, iodine) are near the end of a period. Therefore, they have energy shells that are almost full. Elements in the same group have similar electron arrangement.

45 Checkpoint: Periodic Table 1. What is the difference between the groups and periods on a periodic table of elements? Group: Elements in a vertical column of the periodic table are similar in chemical and physical properties. Period: Properties of elements change in predictable ways as you move through the horizontal row.

46 Lesson 4: Classification Unit 5: Atomic Structure & The Periodic Table

47 Distinct Regions of the Periodic Table The periodic table is a kind of map of the elements. It has 3 distinct regions: Metals = left Nonmetals (except hydrogen) = right Metalloids = in between

48 Metals A metal is an element that conducts electric current and heat well and has a shiny appearance. Most of the elements are metals. Properties of metals: Are solids at room temperature (except Mercury) Can be shaped easily Shiny Good conductor

49 Reactive Metals An element s position on the table also indicates how reactive it is. Reactive means how likely an element is to undergo a chemical change. Elements in Groups 1 and 17 are the most reactive. Elements in Group 18 are almost completely unreactive.

50 Reactive Metals The metals in Group 1 are the alkali metals. These metals are very reactive. When exposed to air, these elements react rapidly with oxygen and water vapor. The metals in Group 2 are the alkaline earth metals. These metals are less reactive than the alkali metals, but more reactive than most other metals.

51 Transition Metals The metals in Groups 3-12 are called the transition metals. These metals are some of the earliest known elements, such as copper, gold, silver, and iron. They are generally less reactive than most other metals. For thousands of years these metals have been used to make jewelry and coins. Some, like steel, iron, and copper, are also important for industry. They remain relatively unchanged since the time they were made.

52 Rare Earth Metals Two rows of elements are set below the periodic table to save space. The top row is the rare earth metals. Scientists once thought they were available only in tiny amounts in Earth s crust. Through mining we have learned that are not so rare, but hard to isolate in pure form.

53 Nonmetals A nonmetal is an element that is not a metal and has properties generally opposite to those of a metal. Properties of nonmetals: Many are a gas at room temperature (Bromine is a liquid) Dull surfaces Not easily shaped Generally poor conductors

54 Metalloids A metalloid has properties of both metals and nonmetals. They lie on either side of the zigzag line separating metals from nonmetals. The most common metalloid is silicon, the second most common element in Earth s crust. Metalloids are used in semiconductors found in electronics. They conduct an electric current under some conditions, but not under others.

55 Expansion of the Periodic Table Before 1940, no one had found an element whose atoms had more than 92 protons in its nucleus. That year, scientists Edwin McMillan and Philip Abelson thought they may be able to make one using a cyclotron. A cyclotron can hurl protons and neutrons towards the nuclei of other atoms. If some of these particles stick to a nucleus, there is a chance a different element can form. They were successful in assembling an element with 93 protons. The next year scientist Glenn Seaborg assembled an elements with 94 protons. He also went on to predict properties of elements with higher numbers of protons.

56 Trends of the Periodic Table Remember, a group is one column of the periodic table. Atoms of the elements in a group have the same number of electrons in their outer energy shells. This gives the elements similar chemical properties. It also gives trends to the periodic table.

57 Trends of the Periodic Table: Atomic Size We measure the size of an atom by the size of its electron cloud. The size of the cloud depends on how many electrons it contains and how strongly the protons in the nucleus pull on the electrons. If the nucleus exerts a strong pull on the electrons, the atom is smaller. If the nucleus pulls only weakly on the electrons, the atom is larger. The electron clouds around the nucleus grows as more energy shells are filled. Even though there are more electrons, the nucleus pulls more strongly on these electrons, so the atoms are smaller.

58 Trends of the Periodic Table: Ions Metals usually lose electrons to form positive ions. The metals in Group 1 lose one electron easily. As you move from left to right across the periodic table, more energy is required to remove an electron. However, the further down a column you go, the less energy is required to remove an electron.

59 Trends of the Periodic Table: Ions Nonmetals usually gain electrons to form negative ions. The nonmetals in Group 17 gain one electron easily. Atoms farther to the right of the periodic table gain electrons more easily. Remember, Noble Gases normally do not form ions because their electron shells are full.

60 Trends of the Periodic Table: Physical Properties Density: With each period, the elements are at the right and left edges of the row are least dense. The elements in the middle of the row are the most dense. Hardness: Dense elements tend to be hard. Hardness means a material can t be scratched or dented easily. Conductivity: Elements on the right side of the table (nonmetals) tend to be poor conductors. Elements in the middle and left side of the table (metals) tend to be good conductors. Melting / Boiling Points: Elements on the right side of the table (nonmetals) tend to have lower melting and boiling points. Elements in the middle and left side of the table (metals) tend to have higher melting and boiling points.

61 Checkpoint: Classification 1. Identify and explain a trend found on the periodic table.