Question

How to know the valency of different metals? For example Boron.

Answer 1

Boron has electronic configuration 2 and 3 .So it will lose 3 electrons. thus the valency of boron is 3

Answer 2

Leaving D&F-blocks, the valency of all other elements can be known with a logic.

The units digit of group number gives the number of valence electrons.

Valency is equal to valence electrons when valence electrons are 4 or less. 

See the periodic table,
The 1A and 2A groups have a valency of 1 and 2 respectively.

The 13 and 14 groups have a valency of 3 and 4 respectively.

Valency is equal to "8-valence electrons" when valence electrons are more than 4.

The 15, 16 and 17 groups have valencies 8-5=3, 8-6=2 and 8-7=1 respectively

The 18th group has 8-8=0 valency.

Boron is of 13th group and thus has a valency of 3.

:)