b. strontium, magnesium
c. carbon, germanium
d. selenium, oxygen
How do you know?
The radius of an atom is a function of the number of electron shells, especially the OUTERMOST or valence shell, where all the interesting chemistry occurs. The ionization also affects atomic radius, but is not relevant to your problem.
Atomic radius increases with each period (or row) of the table. So as one goes down in the table, the radius increases.
a) Na and Li are in the same group, but Na is one period down, so its radius is greater
b) Sr and Mg are in same group (alkaline earth), but Sr is two periods down…so Sr
c) C and Ge are also in same group, and Ge is two periods down…so Ge
d) O and Se are in same group, and Se is two periods down…so Se
Germanium Atomic Radius
You can answer this question by looking at the trend of atomic radius in the periodic table. Atomic radius generally decreases on going left to right (electrons add on in the same shell) in a series and increases down the group. Example: Sodium has larger atomic radius than lithium, as it’s situated just below Li. Hope it helps
As you move down and towards the left side of the periodic table, the atomic radius of elements increases, being largest at the lower left corner of the periodic table (francium). The reason for this has to do with an increase in the number of protons in the nucleus (i.e. more positive charge at the center of the atom), but moving across a row in the periodic table, electrons are consecutively added into a single orbital, which doesn’t produce electrons in additional, higher-energy orbitals which would be further away from the nucleus.
Hope that helps!Source(s): M.S. Organic Chemist
The atomic radius increases as you go down a group: the size of the atom increases for each layer of electrons that is added.