The number of electrons in the outermost shell of an atom determines the chemical properties of the atom, including its reactivity and the types of chemical bonds it can form.
The outermost shell of an atom is also known as the valence shell, and the electrons in this shell are called valence electrons. These valence electrons are involved in chemical bonding and determine the number and type of chemical bonds that an atom can form with other atoms.
Atoms with a full valence shell of electrons are generally stable and unreactive, while atoms with incomplete valence shells tend to be more reactive and will seek to either gain or lose electrons in order to achieve an entire valence shell.
The number of valence electrons can also determine the position of an element in the periodic table. Elements in the same group have the same number of valence electrons and similar chemical properties. For example, all elements in group 1 of the periodic table have one valence electron and are highly reactive metals that readily form positive ions by losing that electron.