Because of these vacancies in the outermost shells, we see the formation of chemical bonds, or interactions between two or more of the same or different elements that result in the formation of molecules. Not all elements have enough electrons to fill their outermost shells, but an atom is at its most stable when all of the electron positions in the outermost shell are filled. Lithium has only one electron in its outermost shell, beryllium has two electrons, boron has three, and so on, until the entire shell is filled with eight electrons, as is the case with neon. These all have electrons that occupy only the first and second shells. If you look at the second row of the periodic table, you will find lithium (Li), beryllium (Be), boron (B), carbon (C), nitrogen (N), oxygen (O), fluorine (F), and neon (Ne). Although each element has the same number of shells, not all of the shells are completely filled with electrons.

The elements within a particular row have increasing numbers of electrons as the columns proceed from left to right. These rows correspond to the number of shells that the elements within that row have. Looking at the periodic table again (Figure 1), you will notice that there are seven rows. The eight electrons are arranged in four pairs and one position in each pair is filled with an electron before any pairs are completed. The second and third energy levels can hold up to eight electrons. Hydrogen and helium are the only two elements that have the lowest shell and no other shells. This is because they only have electrons in their first shell. If you look at the periodic table, you will see that hydrogen and helium are the only two elements in the first row. Helium has two electrons therefore, it can completely fill the lowest shell with its two electrons. Hydrogen has one electron therefore, it has only one spot occupied within the lowest shell. The closest shell to the nucleus is always filled first, before any other shell can be filled. The closest shell can hold up to two electrons.

Electrons exist at energy levels that form shells around the nucleus. How elements interact with one another depends on how their electrons are arranged and how many openings for electrons exist at the outermost region where electrons are present in an atom.

Ion bonding action