1

Electron Shells

Move down P. table: Principal quantum number (n) increases.

Distribution of electrons in an atom is represented with a radial electron density graph. Radial electron density is probability of

finding an electron at a particular distance from the nucleus.

Electron shells are diffuse and overlap a great deal.

2

Examples of Electron shells

He: 1s2

Radial plot shows 1 maximum Ne: 1s2 2s2 2p6 Radial plot shows 2 maxima ( 1 each for

the 1st and 2nd energy levels ) Ar: 1s2 2s2 2p6 3s2 3p6 Radial plot shows 3 maxima ( 1 each for

the 1st,2nd and 3rd energy levels )

Effective Nuclear Charge

3

Charge actually “experienced” by the valence electrons

Two factors: 1. distance from the nucleus 2. shielding by the core electrons

Attraction nucleus declines as a function of 1/distance2.

If an electron is shielded, it is less attracted to the nucleus and is more easily removed.

Effective Nuclear Charge (Zeff)

Zeff = Z - S Z = the atomic number of the element S is equal to the number of electrons that lie in shells with n

values smaller than the n for the electron you're interested in. Ex: Calculate the shielding for a 3p electron: All electrons with n values of 1 or 2 contribute to the

shielding: 2 e- (n=1) + 8 e- (n=2) = 10 total electrons so S = 10

4

Zeff = Z - S

Consider only the electrons in lower shells to contribute to shielding. The small shielding contribution by other electrons in the same shell is ignored.

s and p and d electrons each contribute differently to the shielding of an outer electron, but this simple method does not take that into account.

5

Effective Nuclear Charge (Zeff)

6

Protons Species ElectronConfiguration

Protons - Core e-

Zeff

12 Mg 1s22s22p63s2 12 - 10 +2

12 Mg2+ 1s22s22p6 12 - 2 +10

7 N 1s22s22p3 7 - 2 +5

7 N3- 1s22s22p6 7-2 +5

13 Al 1s22s22p63s23p1 13 - 10 +3

13 Al3+ 1s22s22p6 13 - 2 +11

7

Protons Species ElectronConfiguration

Protons - Core e-

Zeff

4 Be 1s22s2 4-2 +2

5 B 1s22s23s1 5-2 +3

7 N 1s22s22p3 7 - 2 +5

7 N 2s↑↓ 2p ↑ ↑ ↑ .

8 O 1s22s22p4 8-2 +6

8 O 2s↑↓ 2p ↑↓ ↑ ↑ .

Zeff Example: Iron (Fe)

Each of the 3p electrons: Zeff = 26 - 10 = 16

Each of the 3d electrons: Zeff = 26 - 10 = 16 (so even though these electrons go in later, they are still considered the n=3 shell, so they have the same shielding as the 3s and 3p e-.)

Each of the 4s electrons: Zeff = 26 - 24 = 2 (Add the 3d electrons into the shielding S, since they are in the n=3 shell and will therefore shield the 4s electrons.)

8

9

Atomic Sizes

As we move down a group, atoms become larger. Larger n = more shells = larger radius

As we move across a period, atoms become smaller. More protons = more effective nuclear

charge, Zeff More positive charge increases the

attraction of nucleus to the electrons in the outermost shell, so the electrons are pulled in more “tightly,” resulting in smaller radius

Atomic Radius

10

Fig. 8.6

11

Ion size Isoelectronic = Having exactly the same number and configuration of electrons)

The oxide ion is isoelectronic with neon, and yet O2– is bigger than Ne.

O2–: 8 protons (+) Ne: 10 protons (+)10 electrons(-) 10 electrons(-)

This is Coulomb's law at work. In any isoelectronic series the species with the highest nuclear charge will have the smallest radius.

12

Ionization energy

Ionization energy of an ion or atom is the minimum energy required to remove an electron from the ground state of the isolated gaseous atom or ion.

The first ionization energy, I1 is the energy required to remove one electron from an atom.

Na(g) Na+(g) + e-

The 2nd ionization energy, I2, is the energy required to remove an electron from an ion.

Na+(g) Na2+(g) + e-

Larger ionization energy, harder to remove electron.

13

Fig. 8.11

14

Periodic Trends in Ionization Energy

Highest = Fluorine Ionization energy decreases down a

group. Easier to remove electrons that are

farther from the nucleus. Ionization energy increases across a

period. Zeff increases, so it’s harder to remove

an electron. Exceptions: Removing the 1st and 4th p

electrons

15

Electron Affinity

Electron affinity is the energy change when a gaseous atom gains an electron to form a gaseous ion.

Electron affinity: Cl(g) + e- Cl-(g) Ionization energy: Cl(g) Cl+(g) + e-

Affinity for reaction above is exothermic: ∆E = -349 kJ/mol

If adding the electron makes the species more stable, it will be exothermic.

Gain

Lose

16

Metals Metallic character increases down a group and

from left to right across a period. Metal properties:

Lustrous (shiny) Malleable (can be shaped) Ductile (can be pulled into wire) Conduct electricity

Metal oxides form basic ionic solids:Metal oxide + water metal hydroxide Metal oxides react with acids to form salt and

water

Metallic Character

A metal tends to lose its outer electron(s)

The more likely it is to lose the electrons, the more metallic it is

Low ionization energy is associated with high metallic character

17

18

Metals Metal oxides form basic ionic solids:Metal oxide + water metal hydroxide MgO(s) + H2O(l) Mg(OH)2(s) Metal oxides react with acids to form

salt and waterMgO(s) + 2HCl(aq) MgCl2(aq) + H2O(l) Most neutral metals are oxidized rather

than reduced. Metals have low ionization energies.

19

Metal reactivity

Which of the alkali metals would you expect to react most violently with water? Li, Na, K, Rb

Of these four, rubidium has the lowest ionization energy, making it the most reactive. Rubidium reacts explosively with water.

20

Nonmetals

Lower melting points than metals Diatomic molecules are nonmetals. Most nonmetal oxides are acidic:Nonmetal oxide + water acidP4O10(s) + 6H2O(l) 4H3PO4(aq) Nonmetal oxides react with bases to form

salt and water:CO2(g) + 2NaOH(aq) Na2CO3(aq) + H2O(l)

21

Nonmetallic oxides

Which nonmetallic oxide would you expect to be the strongest acid? NO2, N2O, N2O4, N2O5

N2O5: Nitrogen has an oxidation state of +5 in this compound. In general, the higher the oxidation state of the nonmetal, the more acidic the nonmetal oxide.

22

General Trend SummaryElectronegativity, Ionization Energy, Electron Affinity

Ele

ctro

negati

vit

y,

Ion

izati

on E

nerg

y,

Ele

ctro

n A

ffinit

y

F

Atomic Radius, Metallic Character

Ato

mic R

adiu

s, M

eta

llic Chara

cter

Fr

Mullis 23

24

A. 1s22s22p63s2 C. 1s22s22p63s23p4 B. 1s22s22p63s23p2 D. 1s22s22p63s23p6

E. 1s22s22p63s23p64s1

Use the choices above to answer each item below.

1.Represents an element in the oxygen family.2.Represents an electron configuration for a chloride ion.3.Represents an electron configuration of a common ion of an alkali metal.4.Represents an atom with four valence electrons.

25