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Ionisation Energies

p. 63-67

Ions and electron configuration

• Li: 1s22s1

• Li+: 1s2

• He: 1s2

• Cl: 1s22s22p63s23p5

• Cl-: 1s22s22p63s23p6

• Ar: 1s22s22p63s23p6

Ionisation Energy• the amount of energy required to remove

an electron from a gaseous atom to form a gaseous ion.

• Q(g) Q+(g) + e-

• Second ionisation energy: energy required to remove a second electron

• Q+(g) Q2+

(g) + e-

• Energy is required to remove an electron (endothermic)

Factors affecting ionisation energy

1. Charge on the nucleus

2. Repulsion (pushing away) or “shielding” of electron from inner orbitals

3. Repulsion on the electron from other electrons in the valence shell

Factors affecting ionisation energy

• Charge exerted on each electron by the positively charged nucleus

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Repel

AttractAttract

Repel

Effective Nuclear Charge

• Charge exerted on each electron by the positively charged nucleus

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-+15

+13+5

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-+12

+10+2

• Charge exerted on each electron by the positively charged nucleus

Successive (next) Ionization Energies

• The more electrons that have been removed, the greater the energy required to remove the next electron.– because of lower electron-electron repulsions– greater nucleus-electron attraction

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-+15

+13+5

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-+12

+10+2

--

Valence electrons repel

one another

No other electron in 3rd energy

level to repel the one remaining

electron

Remaining valence electron attracted more

strongly to nucleus

Successive Ionization Energy of Mg

Big increase in ionization energy when the next electron is removed from a filled inner energy level. This electron has a much higher effective nuclear charge on itself.

Stop.

Ionisation Energy - Groups• Going down a group on the periodic table,

ionisation energy decreases– successive electrons are lost from higher

energy levels• further from the nucleus• less attraction to nucleus

Ionization Energy - Periods

• Ionisation energy increases across a period– effective nuclear charge increases– not a smooth increase

Ionization Energy Increases

• Ionization energy decreases when going from the second to the third element in each period (Be to B, Mg to Al)– Because of which sub-level the electron

comes from (2p1 is higher energy than 2s2)– Lower ionization energy if removing one

electron from a doubly-filled orbital

Isoelectronic Species

• Iso = same

• Atoms and ions that have the same electronic structure

• S2-, Cl-, Ar, K+, Ca2+