Chapter 5The Periodic Table

Organizing the Elements

Mendeleev’s original periodic table was arranged by atomic mass.

A periodic table is an arrangement of elements in columns, based on a set of properties that repeat from row to row.

Mendeleev’s Periodic Table

Mendeleev named missing elements after elements in the same group. He gave the name eka-aluminum to the missing element one space below aluminum in the table. Mendeleev predicted that eka-aluminum would

• be a soft metal,• have a low melting point, and• have a density of 5.9 g/cm3.

Mendeleev’s Periodic Table

In 1875, a French chemist discovered a new element. He named the element gallium (Ga) in honor of France. (The Latin name for France is Gallia.) Gallium

• is a soft metal, • has a melting point of 29.7°C, and • has a density of 5.91 g/cm3.

Mendeleev’s Periodic Table

Assessment Questions

1. In Mendeleev’s periodic table, elements with similar properties were grouped a. in the same row.b. in the same column.c. in diagonal lines that run from top left to the

bottom right.d. in pairs of two.

Assessment Questions

1. In Mendeleev’s periodic table, elements with similar properties were grouped a. in the same row.b. in the same column.c. in diagonal lines that run from top left to the

bottom right.d. in pairs of two.

ANS: B

Assessment Questions

2. For which element did Mendeleev correctly predict the properties even before it had been discovered?a. galliumb. hydrogenc. bromined. aluminum

Assessment Questions

2. For which element did Mendeleev correctly predict the properties even before it had been discovered?a. galliumb. hydrogenc. bromined. aluminum

ANS: A

The Periodic Law

The modern periodic table is arranged by atomic number.

A period is a row in the periodic table.

A group is a column in the periodic table.

The Periodic Law

Classes of ElementsElements are classified as metals, nonmetals, and metalloids.

Metals are elements that are good conductors of electric current and heat.

Nonmetals are elements that are poor conductors of heat and electric current.

Metalloids are elements with properties that fall between those of metals and nonmetals.

Atomic Mass UnitsThe mass of an atom in grams is extremely small. In order to have a convenient way to compare the masses of atoms, scientists chose one isotope to serve as a standard.

• Scientists assigned 12 atomic mass units to the carbon-12 atom, which has 6 protons and 6 neutrons.

• An atomic mass unit (amu) is defined as one twelfth the mass of a carbon-12 atom.

Atomic Mass

There are four pieces of information for each element.

Atomic Mass

Atomic number

Element symbol

Element name

Atomic mass

Isotopes of ChlorineIn nature, most elements exist as a mixture of two or more isotopes. The element chlorine has an atomic mass of 35.453 amu. Where does the number 35.453 come from?

• There are two natural isotopes of chlorine, chlorine-35 and chlorine-37.

• An atom of chlorine-35 has 17 protons and 18 neutrons.

• An atom of chlorine-37 has 17 protons and 20 neutrons.

Atomic Mass

Weighted AveragesThis table shows the atomic masses for the two naturally occurring chlorine isotopes. The value of the atomic mass for chlorine is a weighted average. If you add the atomic masses of the isotopes and divide by 2, you get 35.967, not 35.453.

Atomic Mass

MetalsThe majority of the elements on the periodic table are classified as metals. Metals are elements that are good conductors of electric current and heat.

• Except for mercury, metals are solids at room temperature.

• Most metals are malleable. • Many metals are ductile; that is, they can

be drawn into thin wires.

Classes of Elements

The metals in groups 3 through 12 are called transition metals. Transition metals are elements that form a bridge between the elements on the left and right sides of the table.

• Transition elements, such as copper and silver, were among the first elements discovered.

• One property of many transition metals is their ability to form compounds with distinctive colors.

Classes of Elements

NonmetalsNonmetals generally have properties opposite to those of metals.

• Nonmetals are elements that are poor conductors of heat and electric current.

• Nonmetals have low boiling points–many nonmetals are gases at room temperature.

• Nonmetals that are solids at room temperature tend to be brittle. If they are hit with a hammer, they shatter or crumble.

Classes of Elements

MetalloidsMetalloid elements are located on the periodic table between metals and nonmetals.

• Metalloids are elements with properties that fall between those of metals and nonmetals.

• For example, a metalloid’s ability to conduct electric current varies with temperature. Silicon (Si) and germanium (Ge) are good insulators at low temperatures and good conductors at high temperatures.

Classes of Elements

Across a period from left to right, the elements become less metallic and more nonmetallic in their properties.

Variations Across a Period

From left to right across Period 3, there are three metals (Na, Mg, and Al), one metalloid (Si), and four nonmetals (P, S, Cl, and Ar).

Variations Across a Period

Assessment Questions

1. What determines the atomic mass of an element? a. the natural distribution of isotopes and

the atomic numbers of those isotopesb. the natural distribution of isotopes and

the masses of those isotopesc. the mass of the isotope of the element

that has the most neutronsd. the average number of protons in the

element’s nucleus

Assessment Questions

1. What determines the atomic mass of an element? a. the natural distribution of isotopes and

the atomic numbers of those isotopesb. the natural distribution of isotopes and

the masses of those isotopesc. the mass of the isotope of the element

that has the most neutronsd. the average number of protons in the

element’s nucleus

ANS: B

Assessment Questions

2. Which of the following is not characteristic of metals? a. ductileb. good electrical conductorc. typically solid at room temperatured. brittle

Assessment Questions

2. Which of the following is not characteristic of metals? a. ductileb. good electrical conductorc. typically solid at room temperatured. brittle

ANS: D

Assessment Questions

3. Within a period of the periodic table, how do the properties of the elements vary? a. Metallic characteristics increase from

left to right.b. Metallic characteristics decrease from

left to right.c. Reactivity increases from left to right.d. Reactivity decreases from left to right.

Assessment Questions

3. Within a period of the periodic table, how do the properties of the elements vary? a. Metallic characteristics increase from

left to right.b. Metallic characteristics decrease from

left to right.c. Reactivity increases from left to right.d. Reactivity decreases from left to right.

ANS: B

Assessment Questions

1. In the modern periodic table, elements are arranged in order of increasing atomic mass.

TrueFalse

Assessment Questions

1. In the modern periodic table, elements are arranged in order of increasing atomic mass.

TrueFalse

ANS: F, atomic number

Valence ElectronsWhy do elements in a group have similar properties?

Elements in the same group have similar properties because they have the same number of valence electrons.

A valence electron is an electron that is in the highest occupied energy level of an atom.

Valence ElectronsThe number of valence electrons is

equal to the group number for groups 1A – 8A in the periodic table.

Elements in the same group have similar properties because they have the same number of valence electrons.

The properties are not identical because the valence electrons are in different energy levels.

The GroupsThe elements in Group 1A are called alkali metals. All have 1 valence electronExtremely reactiveFound only in compoundsStored in oil or argon gas to keep

from reacting with water in air.

The reactivity for Group 1A elements increases from top to bottom.

The GroupsThe elements of Group 2A are called alkaline earth metalsAll have 2 valence electronsHarder than Group 1A metalsLess reactive than Group 1A

metalsHigher melting points than Group

1A metals

The reactivity for Group 2A elements also increases from top to bottom.

The Groups

Group 3A elements are called the boron family.All elements have 3

valence electronsBoron is a metalloids,

aluminum, gallium, indium, and thallium are metals.

The GroupsGroup 4A elements are called the carbon family.All elements have 4 valence

electronsCarbon is a nonmetal,

silicon and germanium are metalloids, and tin and lead are metals.

Carbon is found in most compounds in your body, except water

The Groups

Group 5A elements are called the nitrogen family.All elements have 5

valence electronsNitrogen and phosphorus

are nonmetals, arsenic and antimony are metalloids, and bismuth is a metal

The Groups

Group 6A elements are called the oxygen family.All elements have 6

valence electronsOxygen, sulfur, and

selenium are nonmetals, tellurium and polonium are metalloids

The GroupsGroup 7A elements are called the halogens.Each element has 7 valence electronsFluorine and Chlorine are gases,

Bromine is a liquid, and Iodine is a solid

They are all nonmetals, except astatine is a metalloid

React most easily with metals (Group 1A especially)

The reactivity for Group 7A decreases from top to bottom. Fluorine is the most reactive halogen.

The GroupsGroup 8A elements are called the noble gases. Helium has 2 valence electronsEach of the other noble gases

has 8 valence electronsWhen electric current passes

through noble gases, they emit different colors.

The noble gases are colorless, odorless, and extremely unreactive.

Assessment Questions

1. Why do elements in a group have similar chemical properties?

a) They have the same number of valence electrons

b) They have the same ratio of protons to neutrons.

c) Their atomic masses are multiples of the least massive element of the group.

d) They have the same number of total electrons.

Answer: A

Assessment Questions

2. True or False: In general, the metallic characteristics of A group elements within a period decreases from left to right.

Answer: True