Circle the element in each of the following pairs with the greater atomic size:
a) sodium or lithium
b) silicon or magnesium
c) carbon or boron
d) selenium or oxygen

a)  sodium of lithium:  Na, since it has a new outer shells of electrons ( 2p and 3s over Li's 2s).

b) silicon or magnesium:  Si, since it has added the 3p shell.

c) carbon or boron:  B, the atomic size generally decreases as one moves from the left to the right on the same row.  The nuclear charge is more powerful as protons are added to the nucleus.

d) selenium or oxygen:  Se, because it lies two complete rows down from O, so has several additional electron shells.  The other electrons are pushed outward due to the proximity of other electron in the same shell, and since they are also further from the nucleus, so the electrical attraction to the protons is dimished.

or

a. sodium b. strontium

c. germanium d. selenium

Explanation:   here is the answer

a. sodium b. strontium

c. germanium d. selenium

a. energy is absorbed in the reaction.

if in a given reaction the energy of products is greater than the energy of reactants it means that reaction is endothermic and during the reaction, the energy is absorbed by reactants. in an endothermic reaction, the products have more stored chemical energy than the reactants. and in this reaction the reactants are more stable than products before the compounds with less energy are always more stable.