Periodic Trends Chapter 7. Development of Periodic Table Elements in the same group generally have...

Periodic Trends

Chapter 7

Development of Periodic Table

• Elements in the same group generally have similar chemical properties.

• Properties are not identical, however.

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.

What are the electronic Configurations of the “always”

ions?What does Isoelectronic mean?

Electron Configurations of Cations and Anions

Na [Ne]3s1 Na+ [Ne]

Ca [Ar]4s2 Ca2+ [Ar]

Al [Ne]3s23p1 Al3+ [Ne]

Atoms lose electrons so that cation has a noble-gas outer electron configuration.

H 1s1 H- 1s2 or [He]

F 1s22s22p5 F- 1s22s22p6 or [Ne]

O 1s22s22p4 O2- 1s22s22p6 or [Ne]

N 1s22s22p3 N3- 1s22s22p6 or [Ne]

Atoms gain electrons so that anion has a noble-gas outer electron configuration.

Of Representative Elements

8.2

8.2

ns1

ns2

ns2 n

p1

ns2 n

p2

ns2 n

p3

ns2 n

p4

ns2 n

p5

ns2 n

p6

d1

d5 d10

4f

5f

Ground State Electron Configurations of the Elements

+1

+2

+3 -1-2-3

Cations and Anions Of Representative Elements

8.2

Na+: [Ne] Al3+: [Ne] F-: 1s22s22p6 or [Ne]

O2-: 1s22s22p6 or [Ne] N3-: 1s22s22p6 or [Ne]

Na+, Al3+, F-, O2-, and N3- are all isoelectronic with Ne

What neutral atom is isoelectronic with H- ?

H-: 1s2 same electron configuration as He

8.2

Atomic Radii Trends

Who is the biggest?

Who is the smallest?

Why?

Atomic Radii

8.3

8.3

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 Charge

The 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.

Effective nuclear charge (Zeff) is the “positive charge” felt by an electron.

Na

Mg

Al

Si

11

12

13

14

10

10

10

10

1

2

3

4

186

160

143

132

ZeffCoreZ Radius

Zeff = Z - 0 < < Z ( = shielding constant)

Zeff Z – number of inner or core electrons

Within a Periodas Zeff increasesradius decreases

8.3

Sizes of Ions• Ionic size depends

upon:– Nuclear charge.

– Number of electrons.

– Orbitals in which electrons reside.

Sizes of Ions• Cations are smaller

than their parent atoms.– The outermost

electron is removed and repulsions are reduced.

Sizes of Ions• Anions are larger

than their parent atoms.– Electrons are added

and repulsions are increased.

Sizes of Ions

• Ions increase in size as you go down a column.– 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.

Cation is always smaller than atom from which it is formed.Anion is always larger than atom from which it is formed.

8.3

8.3

Do Metals lose or gain electrons?

What do we call the energy required to do this process?

Ionization energy is the minimum energy (kJ/mol) required to remove an electron from a gaseous atom in its ground state.

I1 + X (g) X+

(g) + e-

I2 + X+(g) X2+

(g) + e-

I3 + X2+(g) X3+

(g) + e-

I1 first ionization energy

I2 second ionization energy

I3 third ionization energy

8.4

I1 < I2 < I3

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.

The next slide has patterns.

Find them.

Filled n=1 shell

Filled n=2 shell

Filled n=3 shellFilled n=4 shell

Filled n=5 shell

8.4

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.

General Trend in First Ionization Energies

8.4

Increasing First Ionization Energy

Incr

ea

sing

Firs

t Io

niz

atio

n E

ner

gy

What implications can we make from these trends?

What is a stable electronic configuration?

Which electron is removed from a transition element first?

Other questions answered?

Trends in First Ionization Energies

• The first occurs between Groups IIA and IIIA.

• Electron removed from p-orbital rather than s-orbital– Electron farther from

nucleus

– Some repulsion by s electrons.

Trends in First Ionization Energies

• The second occurs between Groups VA and VIA.– Electron removed comes

from doubly occupied orbital.

– Repulsion from other electron in orbital helps in its removal.

Electron Configurations of Cations of Transition Metals

8.2

When a cation is formed from an atom, electrons are removed from the highest n orbital! Do not just remove electrons from the “end” of the electronic configuration!

Fe: [Ar]4s23d6

Fe2+: [Ar]4s03d6 so … [Ar]3d6

Fe3+: [Ar]4s03d5 so … [Ar]3d5

Mn: [Ar]4s23d5

Mn2+: [Ar]4s03d5 so … [Ar]3d5

Why is this so?

What is the energy called for adding an electron to an atom?

What are the trends?

Why?

Electron affinity is the negative of the energy change that occurs when an electron is accepted by an atom in the gaseous state to form an anion.

X (g) + e- X-(g)

8.5

F (g) + e- F-(g)

O (g) + e- O-(g)

H = -328 kJ/mol EA = +328 kJ/mol

H = -141 kJ/mol EA = +141 kJ/mol

8.5

Trends in Electron Affinity

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

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.– Added electron must

go in p-orbital, not s-orbital.

– Electron is farther from nucleus and feels repulsion from s-electrons.

Trends in Electron Affinity

• The second occurs between Groups IVA and VA.– Group VA has no empty

orbitals.

– Extra electron must go into occupied orbital, creating repulsion.

Metals versus Nonmetals

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

Metals

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

• Dull, brittle substances that are poor conductors of heat and electricity.

• Tend to gain electrons in reactions with metals to acquire noble gas configuration.

Nonmetals

• Substances containing only nonmetals are molecular compounds.

• Most nonmetal oxides are acidic.

Metalloids

• Have some characteristics of metals, some of nonmetals.

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

Group Trends

Alkali Metals

• Soft, metallic solids.• Name comes from

Arabic word for ashes.

Alkali Metals

• Found only as compounds in nature.• Have low densities and melting points.• Also have low ionization energies.

Alkali Metals

Their reactions with water are famously exothermic.

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

form peroxides.• K, Rb, and Cs also form super oxides:

K + O2 KO2

• Produce bright colors when placed in flame.

Group 1A Elements (ns1, n 2)

M M+1 + 1e-

2M(s) + 2H2O(l) 2MOH(aq) + H2(g)

4M(s) + O2(g) 2M2O(s)

Incr

easi

ng r

eact

ivity

8.6

M(s) + O2(g) MO2 (s) K, Rb, Cs make super oxides

Li and Na make oxides

Alkaline Earth Metals

• Have higher densities and melting points than alkali metals.

• Have low ionization energies, but not as low as alkali metals.

Alkaline Earth Metals

• Be does not react with water, Mg reacts only with steam, but others react readily with water.

• Reactivity tends to increase as go down group.

Group 2A Elements (ns2, n 2)

M M+2 + 2e-

Be(s) + 2H2O(l) No Reaction

Incr

easi

ng r

eact

ivity

8.6

Mg(s) + 2H2O(g) Mg(OH)2(aq) + H2(g)

M(s) + 2H2O(l) M(OH)2(aq) + H2(g) M = Ca, Sr, or Ba

Group 3A Elements (ns2np1, n 2)

8.6

4Al(s) + 3O2(g) 2Al2O3(s)

2Al(s) + 6H+(aq) 2Al3+

(aq) + 3H2(g)

Group 6A

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

Oxygen• Two allotropes:

– O2

– O3, ozone

• Three anions:– O2−, oxide– O2

2−, peroxide– O2

1−, superoxide

• Tends to take electrons from other elements (oxidation)

Sulfur

• Weaker oxidizing agent than oxygen.

• Most stable allotrope is S8, a ringed molecule.

Group VIIA: Halogens

• Prototypical nonmetals• Name comes from the Greek halos and gennao:

“salt formers”

Group VIIA: Halogens

• Large, negative electron affinities– Therefore, tend to oxidize other

elements easily

• React directly with metals to form metal halides

• Chlorine added to water supplies to serve as disinfectant

Group 7A Elements (ns1np5, n 2)

X + 1e- X-1

X2(g) + H2(g) 2HX(g)

Incr

easi

ng r

eact

ivity

8.6

Group VIIIA: Noble Gases

• Astronomical ionization energies• Positive electron affinities

– Therefore, relatively unreactive

• 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.