An atom is composed of a dense core called the nucleus containing protons and neutrons and a series of outer shells occupied by orbiting electrons
An electron shell may be thought of as an orbit that the electrons follow. The closest shell to the nucleus is called the "1 shell" also called "K shell", followed by the "2 shells" "L shell"), then the "3 shells" (or "M shell"), and so on.

A valence electron is an outer shell electron that is associated with an atom, and the formation of a chemical bond
Each shell can contain only a fixed number of electrons: For the first 18 elements, the valence electrons follow these rules. The first shell can hold up to two electrons, the second shell can hold up to eight electrons, the third shell can hold up to 8 The transition metals vary from these guidelines.
You can use the periodic table for the groups 1,2,13,14,15,16,17,18 in order to find the number of valence electrons in the outer shell with the exception of helium. For example, all elements in group two have 2 electrons in the outer shell.
Elements in the same period have the same number of energy shells. Period 2 will have two energy shells.
So let’s go through the elements
Up first is period 1 Element 1 is Hydrogen in group 1 and contains one energy shell and 1 electron
Element 2 Helium period 1 group 18 and is the one exception because it has 2 valence electrons in the outer shell

Now period 2 which each have 2 energy shells.
Lithium found in group 1 has 2 electrons in k shell and 1 valence electron in L
Beryllium is found in group 2 and has 2 electrons in k shell and 2 valence electrons in L
Boron Is found in group 13 and has 2 electrons in k and 3 valence electrons in L
Carbon Is found in group 14 and has 2 electrons in k and 4 valence electrons in L
Nitrogen Is found in group 15 and has 2 electrons in k and 5 valence electrons in L
Oxygen Is found in group 16 and has 2 electrons in k and 6 valence electrons in L
Fluorine Is found in group 17 and has 2 electrons in k and 7 valence electrons in L
Neon Is found in group 18 and has 2 electrons in k and 8 valence electrons in L
Now let’s move on to period 3 with 3 electron shells
Sodium Is found in group 1 and has 2 electrons in k and 8 electrons in L and 1 valence electron in m
Magnesium Is found in group 2 and has 2 electrons in k and 8 electrons in L and 2 valence electrons in m shell.

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