What primarily determines the chemical properties of an element?

The chemical properties of an element are primarily determined by the arrangement of electrons, particularly the electrons in the outermost shell, known as valence electrons. These electrons play a crucial role in how an element interacts with others, as they are involved in chemical bonding and reactions.

Elements tend to exhibit similar chemical properties when they have the same number of valence electrons, which is why elements in the same group or column of the periodic table often display similar reactivity and characteristics. For example, alkali metals all have one valence electron and thus have similar properties, such as being highly reactive with water.

Other factors, such as the number of protons or neutrons, influence an element's identity and mass but do not directly determine its chemical reactivity or bonding behavior in the same way that electron arrangement does.