Ionisation energies & Periodicity

Define 1st ionisation energy

amount of energy required to remove one electron from each atom in a mole of gaseous atoms

Give Mg 1-4 ionisation energies

• Mg₍𝓰₎ → Mg⁺₍𝓰₎  + e-

• Mg⁺₍𝓰₎ → Mg²⁺₍𝓰₎ + e-

• Mg²⁺₍𝓰₎ → Mg³⁺₍𝓰₎ + e

• Mg³⁺₍𝓰₎ → Mg⁴⁺₍𝓰₎ + e-

What factors affect ionisation energy?

• atomic radius

• nuclear charge (no. of protons)

• electron shielding

How does atomic radius affect IE?

the greater the atomic radius, the weaker the electrostatic attraction between nucleus and outermost electrons

How does nuclear charge affect IE?

the greater the nuclear charge, the greater the forces of attraction between the nucleus and electrons

How does shielding affect IE?

• inner shells electrons repel outer shell electrons → repulsion = shielding

• as the number of inner shells increase, shielding effect increases

• weaker nuclear attraction between nucleus and outer electrons

What is the trend of 1st IE down groups?

• IE decreases down the group

• outer electron is in another shell so atomic radius increases

• electron shielding increases

• distance and shielding outweigh the increased nuclear charge

• so less nuclear attraction between nucleus and outer shell electrons

What is the trend of 1st IE across periods?

• IE increases across period

• increased nuclear charge so atomic radius decreases

• so greater nuclear attraction between nucleus and outer electron

• shielding remains same / outer electrons are in the same shell

Why does it require more energy to remove each successive electron?

• atomic radius decreases as same amount of protons attract fewer electrons

• nuclear attraction between remaining electrons and nucleus increases

Successive ionisation energy (potassium example)

• large increase indicates change of shell

• amount of e- lost in last shell indicate group number

• every element left with 2e- in inner shell

Why is there a decrease in energy from group 2-3?

• Be outer electron is in s subshell whereas B outer electron is in a p subshell

• p subshell is in a higher energy level than s subshell so less energy required to remove e-

Why is there a decrease in energy from group 5-6?

• N outer electron has 3e- in 3 p orbitals whereas O outer electron has 4e- in 3 p orbitals

• O has electron pair in p orbital which repel

• O has increased shielding

Whys there a sharp decrease in IE from the end of the period to the start of the next period

• there is a new subshell so outer electron is removed from a higher energy level

• increased shielding → increased atomic radius → less nuclear attraction

Whats periodicity

trend in physical and chemical properties that is repeated across each period

Why does the B/MP increase from Na-Al

• the charges of each ion increases and the number of delocalised electrons increase

• ionic size decreases

• so greater ef attraction between +ve ions and -ve delocalised electrons

• more energy required to overcome

Why does Si have a higher MP than Al

• Si is a giant covalent structure and atoms held tightly by strong covalent bonds which require more energy to break

Why does Si have a higher MP than P

• Si is a giant covalent structure with atoms held tightly by strong covalent bonds

• whereas P is a simple molecular molecular structure which has weak London forces between molecules which requires less energy to break than Si

Why is there a general decrease in M/BP from P-Ar

• all simple molecular structures

• molecules held together by weak London forces

• requires a small amount of energy to break

Why does S have higher M/BP than P4 and Cl2

• S is bigger molecule than both

• S has more e- so has stronger london forces

• requires more energy to break

Why is second ionisation energies more endothermic than first ionisation energies

• more energy required to remove an electron from a positive ions

• smaller ion size, same number of protons attracting less electrons, greater attraction