Atoms are the fundamental unit of the matter we observe and interact with. The basis of chemical reactions is the interaction between atoms. More specifically, it involves the interactions between the positively charged particles of the nucleus, known as protons, and the negatively charged particles that surround the nucleus in a cloud, known as electrons. Understanding these subatomic particles can provide valuable insight into the mechanisms behind chemical reactions.
Electrons are subatomic particles with a negative charge. They are considered point particles, with no size and an incredibly small mass. Electrons are held a particular atomic nucleus by a force known as the Coulomb force. Their properties are described through a quantum state known as an orbital and only two electrons may occupy the same orbital and only if they have differing spins.
Protons, on the other hand, are positively charged particles that make up part of the nucleus, along with neutrons. The number of protons in the nucleus of a given atom determines the atom's element and is called the "atomic number". Atoms of a given element always have the same number of protons, but the number of neutrons may differ. These different types of the same element are known as isotopes.
The presence, absence and general properties of electrons give rise to the chemical reactivity of an atom. A neutral atom is one which has the same number of protons and electrons. Despite this, specific atoms may have varying degrees of reactivity despite neutrality. An atom with more or less electrons than protons is an ion and has a net charge, either positive or negative.
The basic interaction in chemistry is the chemical bond, which occurs as a result of electromagnetic forces. Sharing or transferring electrons between atoms creates this chemical bonds and holds two atoms together in units of matter known as molecules. Electrons can move freely within molecular orbitals in a molecules, stabilizing the entire structure.
A valence shell is the outermost region of a particular atom which electrons can occupy. These valence electrons are the primary particles of interaction. Most atoms interact in such a way that their valence shell receives the maximum possible number of electrons. A number of elements already satisfy this maximum outer shell, rendering them chemically inert. The periodic table is set up in such a way to line up the number of valence electrons. As a consequence, elements in the same column tend to have very similar chemical and physical properties, which give credibility to the understanding that most chemical reactivity is based on valence electron structure.