WHICH ONE IS TRUE WHICH ONE IS FALSE? The number of valence electrons decreases from left to right across each period.
The number of valence electrons increases from top to bottom within each group.
All of the elements in group 9 have nine valence electrons.
Elements with the most valence electrons are in group 18.

The number of valence electrons increases from top to bottom within each group.

Explanation:

The valence electrons are the outermost shell electrons.

Therefore, the number of valence electrons does not increase from top to bottom within each of the group.

Down a group, the number of valence electrons remains the same.

For group I elements, the number of electrons is 1.

 The group is the vertical arrangement of elements.

From left to right across a period, the number of valence electrons increases because you move from group to group.

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the accuracy of the data.