TOPIC 11: THE PERIODIC TABLE

Question 1

Trend of atomic radius across period 3

Answer 1

Protons increase, nuclear charge increase
Same no. of inner shell electrons, shielding effect approximately constant

Effective nuclear charge increases, EFA between nucleus and valence electrons increases

Atomic radii decreases

Question 2

Trend of ionic radii across period 3
(same for both cation - Na+ to Si4+ & anion P3- to Cl-)

Answer 2

Protons increase, nuclear charge increase
Same no. of inner shell electrons, same INTER-ELECTRONIC REPULSION (not shielding effect for ionic radii)

Effective nuclear charge increases, EFA between nucleus and valence electrons increases

Ionic radii decreases

Question 3

Difference between cationic and anionic radius across period 3

Answer 3

Anions have one more fully filled principal quantum shell

Valence electrons in anion further away from nucleus than in cation

Weaker EFA between nucleus and valence electrons

Anionic radii > Cationic radii

Question 4

General trend of 1st IE across period 3

Answer 4

Protons increase, nuclear charge increase
Same no. of inner shell electrons, shielding effect approximately constant

Effective nuclear charge increases, EFA between nucleus and valence electrons increases

More energy needed to remove valence electrons, increase in 1st IE

Question 5

Exceptions to 1st IE trend across period 3

Answer 5

[DROP FROM Mg TO Al)

The 3p electron to be removed from Al is of a higher energy level than the 3s electron to be removed from Mg.

Less energy needed to remove, 1st IE of Al lower than Mg

[DROP FROM P to S]

There is inter-electronic repulsion between the paired electrons in the same orbital for S.

Less energy is needed to remove the 3p electron due to inter-electron repulsion between the paired electrons in the same orbital. So 1st IE of S lower than P

Question 6

Electronegativity across period 3

Answer 6

Protons increase, nuclear charge increase
Same no. of inner shell electrons, shielding effect approximately constant

Effective nuclear charge increases, EFA between nucleus and valence electrons increases

Greater tendency to attract electrons, electronegativity increases

Question 7

Trend of melting point across period 3 (metals - Na to Al)

Answer 7

Increase in number of delocalised valence electrons contributed per atom +
Smaller cationic Radius

Higher charge density

More energy needed to overcome the stronger EFA between the metal cations and the sea of delocalised valence electrons

Question 8

Trend of melting point across period 3 (non-metals - P4 to Cl2)

Answer 8

Increase in number of electrons in the molecule, increase in polarisability of electron cloud

More energy needed to overcome the stronger intermolecular id-id interactions