Question

how to know atom numebrs of metals to eg

Answer 1

You actually have to memorise the atomic numbers. But after knowing that, you can easily find the valency and shell distribution, etc

Well...byheart the atomic numbers. Then take the shells. The first one can hold only 2, the second one 8, the third one 18,etc. An example, Mg. Its atomic number is 12. So, one the first shell, only 2 electrons can be placed which means that 10 electrons are left. On the second shell, 8 can be placed so only 2 are left. On the last shell, the remaining 2 are placed. For an atom to be stable, the last shell has to be full, which means it has to have 8 electrons. So, for Mg, it is easier to remove 2 electrons than to gain 6. So, its valency is 2.