Answer to Question #95664 in General Chemistry for Bruno

Question

Explain with examples, why;

i) atomic radius increases as you go down the group and decreases as you go across a period?
ii) the main trend for first ionization energy to increase across periods?
iii) the value of first electron affinity of oxygen (-142 kJ/mol) smaller than that of fluorine
(-328 kJ/mol)?

Answer 1

1. Elements of one group differ in numbers of electron shells, the more shells the bigger the atom and therefore its radius.

Moving across the period, radius decreases because nucleus charge increases and outer shell becomes closer to the nucleus.

2.Because the atomic radius decreases moving across a period, so the atom is smaller and outer electrons are closer to the nucleus, there is a greater effective attraction between the negatively charged electrons and positively-charged nucleus.

3. The electron brought in from the environment is being added to the 2pz orbital of fluorine which is closer to the nucleus (than the outer orbital of the oxygen atom) at a lower energy level where it becomes the outermost electron of the F- anion. This means that this electron has to release more energy in first electron affinity as its intrinsic energy is lower in fluorine than in oxygen, hence the first electron affinity of fluorine is more exothermic than that of the oxygen.