Chapter 7Periodic Properties

of the Elements

Development of Periodic Table• Elements in the same

group generally have similar chemical properties.

• Physical properties are not necessarily similar, however.

Development of Periodic TableDmitri Mendeleev and Lothar Meyer independently came to the same conclusion about how elements should be grouped.

Development of Periodic Table

Mendeleev, for instance, predicted the discovery of germanium (which he called eka-silicon) as an element with an atomic weight between that of zinc and arsenic, but with chemical properties similar to those of silicon.

Periodic Trends• In this chapter, we will rationalize observed

trends in– Sizes of atoms and ions.– Ionization energy.– Electron affinity.

6© 2009, Prentice-Hall, Inc.

Effective Nuclear Charge• In a many-electron atom,

electrons are both attracted to the nucleus and repelled by other electrons.

• The nuclear charge that an electron experiences depends on both factors.

Effective Nuclear ChargeThe effective nuclear

charge, Zeff, is found this

Way: Zeff = Z − S

where Z is the atomic number

and S is a screening constant,

Usually close to the number of

Inner electrons.

What Is the Size of an Atom?

The bonding atomic radius is defined as one-half of the distance between covalently bonded nuclei.

Sizes of AtomsBonding atomic radius tends to…

… decrease from left to right

across a row(due to

increasing Zeff).

…increase from top to bottom of a column (due to increasing value of n).

11© 2009, Prentice-Hall, Inc.

Sample Exercise 7.1 Bond Lengths in a Molecule

Natural gas used in home heating and cooking is odorless. Because natural gas leaks pose the danger of explosion or suffocation, various smelly substances are added to the gas to allow detection of a leak. One such substance is methyl mercaptan, CH3SH, whose structure is shown in the margin. Use Figure 7.7 to predict the lengths of the C—S, C—H, S—H and bonds in this molecule.

Using Figure 7.7, predict which will be greater, the P—Br bond length in PBr3 or the As—Cl bond length in AsCl3.

P—Br

Referring to a periodic table, arrange (as much as possible) the following atoms in order of increasing size: 15P, 16S, 33As, 34Se. (Atomic numbers are given for the elements to help you locate them quickly in the periodic table.)

Arrange the following atoms in order of increasing atomic radius:

11Na, 4Be, 12Mg.

14

Sizes of Ions

• Ionic size depends upon:– The nuclear charge.– The number of

electrons.– The orbitals in which

electrons reside.

Sizes of Ions

• Cations are smaller than their parent atoms.– The outermost

electron is removed and repulsions between electrons are reduced.

Sizes of Ions

• Anions are larger than their parent atoms.– Electrons are added

and repulsions between electrons are increased.

Sizes of Ions• Ions increase in size

as you go down a column.– This is due to

increasing value of n.

Sizes of Ions

• In an isoelectronic series, ions have the same number of electrons.

• Ionic size decreases with an increasing nuclear charge.

Arrange these atoms and ions in order of decreasing size: Mg2+, Ca2+, and Ca.

Which of the following atoms and ions is largest: S2–, S, O2–?

Arrange the Isoelectronic Series of ions K+, Cl–, Ca2+, and S2– in order of decreasing size.

Which of the following ions is largest, Rb+, Sr2+, or Y3+?

21© 2009, Prentice-Hall, Inc.

Ionization Energy• The ionization energy is the amount of

energy required to remove an electron from the ground state of a gaseous atom or ion.– The first ionization energy is that energy required

to remove first electron.– The second ionization energy is that energy

required to remove second electron, etc.

Ionization Energy• It requires more energy to remove each

successive electron.• When all valence electrons have been removed,

the ionization energy takes a quantum leap.

Trends in First Ionization Energies• As one goes down a

column, less energy is required to remove the first electron.– For atoms in the same

group, Zeff is essentially the same, but the valence electrons are farther from the nucleus.

25© 2009, Prentice-Hall, Inc.

Trends in First Ionization Energies• Generally, as one

goes across a row, it gets harder to remove an electron.– As you go from left

to right, Zeff increases.

Trends in First Ionization Energies

However, there are two apparent discontinuities in this trend.

Trends in First Ionization Energies• The first occurs between

Groups IIA and IIIA.• In this case the electron is

removed from a p-orbital rather than an s-orbital.– The electron removed is

farther from nucleus.– There is also a small

amount of repulsion by the s electrons.

Trends in First Ionization Energies• The second occurs

between Groups VA and VIA.– The electron removed

comes from doubly occupied orbital.

– Repulsion from the other electron in the orbital aids in its removal.

Which will have the greater third ionization energy, Ca or S?

Referring to a periodic table, arrange the following atoms in order of increasing first ionization energy: Ne, Na, P, Ar, K.

Which has the lowest first ionization energy, B, Al, C, or Si? Which has the highest first ionization energy?

Write the electron configuration for (a) Ca2+ (b) Co3+, and (c) S2–.

Write the electron configuration for (a) Ga3+, (b) Cr3+, and (c) Br–.

Electron Affinity

Electron affinity is the energy change accompanying the addition of an electron to a gaseous atom:

Cl + e− ⎯⎯→ Cl−

33© 2009, Prentice-Hall, Inc.

Trends in Electron Affinity

In general, electron affinity becomes more exothermic as you go from left to right across a row.

35© 2009, Prentice-Hall, Inc.

Trends in Electron Affinity

There are again, however, two discontinuities in this trend.

Trends in Electron Affinity• The first occurs

between Groups IA and IIA.– The added electron

must go in a p-orbital, not an s-orbital.

– The electron is farther from nucleus and feels repulsion from the s-electrons.

Trends in Electron Affinity• The second occurs

between Groups IVA and VA.– Group VA has no

empty orbitals.– The extra electron

must go into an already occupied orbital, creating repulsion.

39© 2009, Prentice-Hall, Inc.

Properties of Metal, Nonmetals,and Metalloids

Metals versus Nonmetals

Differences between metals and nonmetals tend to revolve around these properties.

Metals versus Nonmetals• Metals tend to form cations.• Nonmetals tend to form anions.

Metals

They tend to be lustrous, malleable, ductile, and good conductors of heat and electricity.

Metals• Compounds formed

between metals and nonmetals tend to be ionic.

• Metal oxides tend to be basic.

Nonmetals• These are dull, brittle

substances that are poor conductors of heat and electricity.

• They tend to gain electrons in reactions with metals to acquire a noble gas configuration.

Nonmetals• Substances

containing only nonmetals are molecular compounds.

• Most nonmetal oxides are acidic.

Metalloids• These have some

characteristics of metals and some of nonmetals.

• For instance, silicon looks shiny, but is brittle and fairly poor conductor.

Group Trends

Alkali Metals• Alkali metals are

soft, metallic solids.• The name comes

from the Arabic word for ashes.

Alkali Metals• They are found only in compounds in nature,

not in their elemental forms.• They have low densities and melting points.• They also have low ionization energies.

Alkali Metals

Their reactions with water are famously exothermic.

53

Alkali Metals• Alkali metals (except Li) react with oxygen to form

peroxides.• K, Rb, and Cs also form superoxides:

K + O2 ⎯⎯→ KO2

• They produce bright colors when placed in a flame.

56© 2009, Prentice-Hall, Inc.

Alkaline Earth Metals

• Alkaline earth metals have higher densities and melting points than alkali metals.

• Their ionization energies are low, but not as low as those of alkali metals.

Alkaline Earth Metals• Beryllium does not

react with water and magnesium reacts only with steam, but the others react readily with water.

• Reactivity tends to increase as you go down the group.

Group 6A

• Oxygen, sulfur, and selenium are nonmetals.• Tellurium is a metalloid.• The radioactive polonium is a metal.

Oxygen• There are two allotropes of oxygen:

– O2

– O3, ozone

• There can be three anions:– O2−, oxide– O2

2−, peroxide– O2

1−, superoxide

• It tends to take electrons from other elements (oxidation).

Sulfur• Sulfur is a weaker

oxidizer than oxygen.• The most stable

allotrope is S8, a ringed molecule.

Group VIIA: Halogens

• The halogens are prototypical nonmetals.• The name comes from the Greek words halos

and gennao: “salt formers”.

Group VIIA: Halogens• They have large, negative electron

affinities.– Therefore, they tend to oxidize other

elements easily.

• They react directly with metals to form metal halides.

• Chlorine is added to water supplies to serve as a disinfectant

64© 2009, Prentice-Hall, Inc.

Group VIIIA: Noble Gases

• The noble gases have astronomical ionization energies.• Their electron affinities are positive.

– Therefore, they are relatively unreactive.

• They are found as monatomic gases.

Group VIIIA: Noble Gases• Xe forms three compounds:

– XeF2

– XeF4 (at right)

– XeF6

• Kr forms only one stable compound:– KrF2

• The unstable HArF was synthesized in 2000.

(a) Would you expect scandium oxide to be a solid, liquid, or gas at room temperature?

(b) Write the balanced chemical equation for the reaction of scandium oxide with nitric acid.

(c) Write the balanced chemical equation for the reaction between copper(II) oxide and sulfuric acid.

Write the balanced chemical equation for the reaction of solid tetraphosphorus hex-oxide with water.

Write the balanced chemical equations for the reactions of solid selenium dioxide with (a) water, (b) aqueous sodium hydroxide.

Write a balanced equation that predicts the reaction of cesium metal with (a) Cl2(g), (b) H2O(l), (c) H2(g).

Write a balanced equation for the reaction between potassium metal and elemental sulfur.

Sample Integrative Exercise Putting Concepts Together

The element bismuth (Bi, atomic number 83) is the heaviest member of group 5A. A salt of the element, bismuth subsalicylate, is the active ingredient in Pepto-Bismol®, an over-the-counter medication for gastric distress.(a) The covalent atomic radii of thallium (Tl) and lead (Pb) are 1.48 Å and 1.47 Å, respectively. Using these values and those in Figure 7.7, predict the covalent atomic radius of the element bismuth (Bi). Explain your answer.(b) What accounts for the general increase in atomic radius going down the group 5A elements?(c) Another major use of bismuth has been as an ingredient in low-melting metal alloys, such as those used in fire sprinkler systems and in typesetting. The element itself is a brittle white crystalline solid. How do these characteristics fit with the fact that bismuth is in the same periodic group with such nonmetallic elements as nitrogen and phosphorus?(d) Bi2O3 is a basic oxide. Write a balanced chemical equation for its reaction with dilute nitric acid. If 6.77 g of Bi2O3 is dissolved in dilute acidic solution to make 0.500 L of solution, what is the molarity of the solution of Bi3+ ion?(e) 209Bi is the heaviest stable isotope of any element. How many protons and neutrons are present in this nucleus?(f) The density of Bi at 25 °C is 9.808 g/cm3. How many Bi atoms are present in a cube of the element that is 5.00 cm on each edge? How many moles of the element are present?