Question

describe how valency changes across a period

Answer 1

The valency of an element is determined by its valence electrons. The electrons in outer shell are called as valence electrons. Valence electrons  increase in number as you go left to right in a periodic table, but  transition metals will usually have 1 or 2 electrons in outer shell.

Elements in Alkali Metals Group I are  having one valence electron where as, Alkali Earth Metals Group II are having 2 valence electrons. Group 13 elements Boron and Aluminum are having 3 valence electrons.  Halogens Group 17 Florine and Chlorine hare having 7 valence electrons . Where as  Noble Gases Group 18 Neon and Argon are having 8 valence electrons.

Answer 2

The number of electrons lost or gained by one atom of an element  to achieve the nearest inert gas configuration is called Valency of the element
on moving from left to right in each short period, the valency of elements increases from 1 to 4 and then decreases to zero.
Example :
Third Period elements Na     Mg     Al     Si       P       S           Cl       Ar
       Valency                 1       2        3      4        3      2              1        0
so elements in a period has different valency.