When counting valence electrons, we typically focus on the outermost (highest energy level) electrons. This is because the outermost electrons are the ones involved in the majority of chemical reactions and bonding with other atoms.
The innermost shell, often referred to as the core electrons, is closer to the nucleus and tightly bound to it. These electrons are shielded by the outer shells, making them less accessible for chemical interactions. They are generally not involved in bonding or determining the chemical properties of an atom.
Valence electrons, on the other hand, are found in the outermost energy level (also known as the valence shell) and have the highest energy. These electrons are crucial for determining an element's reactivity and its ability to form chemical bonds with other elements.
In the case of lithium, the 1s² electrons are considered core electrons and are not typically involved in chemical reactions. Therefore, when counting valence electrons, we focus on the 2s¹ electron, which is in the outermost shell and determines lithium's chemical behavior.