Here I will not mention the valencies of the first 20 elements in the periodic table.
But I will teach you some techniques by which you can find the valancy of any element situated anywhere in the periodic table.
I will mention the names of vertical groups in the periodic table and the common valencies of their elements. They all have common chemical character traits.
The ALKALI EARTH METALS They all have one electron in the outermost shell. Hydrogen is the only one in the group to have one less electron. With the exception of hydrogen, all alkali earth metals have one electron in the outermost shell as an extra one which make them very unstable and extremely reactive.
Hydrogen - 1
Lithium - (1+)
Sodium - (1+)
The ALKALI METALS They all have two electrons in the outermost shell. They are a bit less reactive than Alkali earth metals. It is much easier to give off one electron than two. So, that makes their valencies (2+).
Beryllium - (2+)
Magnesium - (2+)
Transition metals This is the big block in the middle. It has unique characteristics. The valency of transition metals is either 2 or 3.
Iron - 2, 3
Copper - 1, 2
After the Transition metals, you get another big stack of elements. There are non-metals. The first column in the non-metal block - the elements in this column all need 5 more electrons to fill the octet. Their valencies vary from situation to situation.
The elements in the next column need 4 more electrons to fill the octet. So, all the elements in this column have a valency of 4.
The elements of the third column in the block need 3 more electrons to fill the octet. This makes their valency 3.
The elements of the fourth column in the block need 2 more electrons to fill the octet. So, their valency is 2.
Then, comes the HALOGENS - they just need one more electron to fill their octet. Their valency is 1. They are extremely reactive.
Last of all, the NOBLE GASES. Their shells are filled. They are stable. They are non-reactive non-metals. Their valency is 0.