What is the periodic trend in valency as one moves from left to right across a period?

1. Valency depends on the number of valence electrons across the period. 2. In period 2, valency rises Li(1), Be(2), B(3), C(4), then falls N(3), O(2), F(1). 3. So valency increases up to the middle and then decreases. 4. Rule out B and C: a single monotonic trend ignores the mid-period peak. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 2 "Periodic Classification of Elements", p.33_