Concept:Elements in the same group of the periodic table have the same number of valence electrons, that is, the same number of electrons in their outermost shell.
Explanation:The periodic table is divided into vertical columns called groups.
The group number of an element depends on the number of electrons in its outermost shell.
For example, group 1 elements all have one valence electron, while group 17 elements all have seven valence electrons.
Since elements
X and
Y are placed in the same group, they must show an identical number of valence electrons.
The number of electronic shells determines the period, not the group.
Atomic size changes gradually down a group but is not the reason for grouping.
Atomic number is unique for each element and cannot be the same for two different elements.
Therefore, the common property linking two elements in the same group is the equal number of valence electrons.
Answer:B. number of valence electrons