An atom is the smallest unit of a particular element that still retains the properties of that element. The various characteristics of an atom give rise to its chemical properties, or the way that it interacts with other atoms. These characteristics include atomic number, mass, charge, and valence structure. These properties determine the atom’s interactions and chemical reactivity.
The basic particles within an atom are electrons, neutrons and protons. Electrons are subatomic particles with a negative charge, with essentially no size and an incredibly small mass. They are held to 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. Neutrons are particles with no net charge that reside in the nucleus alongside protons. Their presence or absence determines certain chemical properties of a given atom, creating versions of the element known as isotopes.
The atom, in general, is made up of a nucleus and an electron cloud. The nucleus contains the protons and neutrons. The sum of the mass of the protons and neutrons is called the mass number and is accepted as the mass of that given atom. This can be done because electrons have such incredibly small masses that they can be effectively ignored in this regard. The electron cloud is the space which the electrons occupy. The space closest to the nucleus has the greatest force, so electrons in that region are fairly resistant to removal. However, electrons furthest from the nucleus are much more capable of being removed, so they form the basis of many chemical reactions.
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.
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.