Why is the valency of lead 2 ,even though the valence electrons are 4?
This can be explained only if you know about subshells which you will learn in higher classes
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p2. This is the subshell electronic configuration of lead. Let's look at the last subshells now. The highest energy can be found by seeing which one has largest n value (principal quantum number) which is 6.
In 6s2 6p2, these are clearly not the same as say 6p4, in which case all 4 would leave. The 2 electrons in 6p2 leave, as 6s2 is fully filled and is fairly stable.
The valency thus remains 2, rather than 4. This is the case in almost all heavier elements along the p block at least.
It doesn't require much energy to remove those p electrons.