Chemical elements are divided into two primary categories, metals and nonmetals, each having distinct properties, with a small number being classed as a third group, the metalloids, which typically have some metallic and some nonmetallic properties.
Elements that readily lose electrons are all metals. Metals that normally lose one electron all form +1 ions with electron configurations we call “stable,” or “energetically favorable.”
Photoelectron spectroscopy (PES) enables us to measure the energy needed to remove the electrons from an element, one by one. When we look at PES data, we see that elements in Group 1 of the Periodic Table, the alkali metals, all have one electron that requires only a small amount of energy to remove, while removing any more electrons requires a great deal more energy. Generally, processes requiring a lot of energy occur rarely, so these elements normally lose one electron and no more.
Silver is another metal that forms +1 ions by losing a single electron. Copper and thallium can lose one or more, and hydrogen is a nonmetal that can lose an electron under certain circumstances.
The quantum mechanical model for the electrons in an atom tells us that electron configurations having ”filled valence shells,” or completely filled s and p orbitals at the highest principal quantum number, are stable. This is the case with the +1 ions of the alkali metals, silver, hydrogen, thallium, and copper.