Concept:Elements in the same group of the periodic table share the same number of valence electrons, which controls their chemical behaviour.
Explanation:The periodic table is arranged in columns called groups.
The group number of an element is determined by the number of electrons present in its outermost shell.
These outermost electrons are called valence electrons.
For example, all alkali metals in Group 1 have one valence electron.
Elements in Group 17 have seven valence electrons.
Thus, it is the count of valence electrons, not total electrons or core electrons, that fixes an element's group.
Answer:The group is determined by the number of
valence electrons.
Therefore, the correct option is
D. valence electrons.