The elements that compose the ordinary matter are formed by atoms, the smallest constituent unit with well-defined and specific characteristics of each chemical element, for example, the mass and atomic number. Every solid, liquid, gas, and plasma is composed of neutral or ionized atoms.
Although it is the smallest material structure (dimensions of the order of 10-10 m) exists from the chemical point of view, the atom does not represent the ultimate constituent of matter. Globally neutral, the atom consists mainly of three types of particles:
Protons and neutrons occupy the central part of the atom, the nucleus, whose radius is of the order of 10-15 m and in which almost all of the atomic mass is therefore concentrated.
The electron affinity of an element is a measurable physical quantity, namely, the energy released or absorbed when an isolated gas-phase atom acquires an electron, measured in kJ/mol. We must be careful not to confuse electron affinity with electronegativity.