CHEMISTRY · Multi-part · A Level · Chemistry · Periodicity and periodic trends · Periodicity and periodic trends
Problem
Four period-3 elements—sodium (Na), magnesium (Mg), aluminum (Al), and silicon (Si)—are listed in order of increasing atomic number.
Relevant information for the elements is given below.
• Atomic numbers: Na = 11, Mg = 12, Al = 13, Si = 14
• Ground-state outer-electron configurations:
– Na: 3s¹
– Mg: 3s²
– Al: 3s²3p¹
– Si: 3s²3p²
• Reported atomic radii (pm): Na = 186, Mg = 160, Al = 143, Si = 118
(a) Rank Na, Mg, and Al in order of increasing effective nuclear attraction experienced specifically by a 3s electron. Justify your ranking.
(b) Using your result from part (a), account for the observed trend in atomic radius among Na, Mg, and Al.
(c) A valence electron in Si occupies a 3p orbital rather than a 3s orbital. Compare the effective nuclear attraction on a 3p electron in Si with that on a 3s electron in Al, and explain any difference you predict.