Question

how can we Say that carbon is non metal and boron is metalloid since carbon has valency of four and boron has velency of 3?

Answer 1

Metals have generally a valency of 1 to 3 while non-metals have valencies ranging from 4 to 8. Metals are placed on the left side of the periodic table whereas non-metals are placed at the upper right and extreme right side of the periodic table. There are some elements which are placed between metals and non-metals in each period. These are called metalloids which have properties in between those of metals and non-metals. Boron is one of the metalloids.
The properties of carbon show that it is a non-metal.