the groups (vertical columns) on the periodic table give the number of valence electrons of elements.
the periods (horizontal columns) on the periodic table give the number of electron shells.
by energy levels do you mean reactivity? it really depends.
for example, the alkali metals (group 1) become more reactive going down the group as they only contain 1 valence electron, therefore they want to lose 1 electron rather than gain 7 to have a full shell. (all elements react to have a full shell, so they become stable, like the noble gases in group 8.)
the halogens (group 7), however, become less reactive going down the group as they need to gain 1 rather than lose 7.
the reason for this:
the more electron shells, the weaker the attraction of the electrons to the nucleus. therefore, in the alkali metals case, when you go down the group (more valence shells), it will become easier to lose electrons (only 1 to lose for alkali metals).
in the case of halogens, it needs to gain 1. if the force between the nucleus and the electrons are week as you go down the group, so it becomes more difficult to gain 1, therefore less reactive.
does this mean that if, for example, lithium loses 1 electron, it will become helium? no, because the number of protons inside the lithium element remain, which determines its atomic number.
so the alkali metals are more reactive at the bottom of the group, and the halogens are more reactive at the top of the group.
hope this helped
