By David Chandler
Updated Aug 30, 2022
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Atoms are composed of three fundamental sub‑particles: the positively charged proton, the negatively charged electron, and the electrically neutral neutron. The magnitudes of the proton and electron charges are equal but opposite, and they define the electrical behavior of an atom. Protons and neutrons bind together in the nucleus via the strong nuclear force, while electrons orbit the nucleus in a cloud held by the comparatively weaker electromagnetic force.
Electrons carry a negative charge, protons a positive charge, and neutrons are neutral.
Each element’s identity is determined by the number of protons in its nucleus, known as the atomic number. For instance, a carbon atom contains six protons, while a nitrogen atom contains seven. This proton count remains unchanged during chemical reactions, meaning reactants and products are the same elements.
Atoms of a single element can contain different numbers of neutrons, giving rise to isotopes. Hydrogen, for example, has three isotopes: protium (no neutrons), deuterium (one neutron), and tritium (two neutrons). While isotopes differ in mass, they exhibit virtually identical chemical behavior.
Electrons are not tightly bound to the nucleus, allowing them to be lost, gained, or shared between atoms. An atom that loses an electron becomes a +1 ion; one that gains an electron becomes a –1 ion. Chemical bonds arise from these electron rearrangements.
The mass of an atom is essentially the sum of its protons and neutrons; electrons contribute negligibly and are usually omitted from mass calculations. The resulting figure, the atomic mass, varies with isotope. Protium (1 p + 0 n) has an atomic mass of 1; deuterium (1 p + 1 n) has an atomic mass of 2.
In natural samples, atoms exist as mixtures of isotopes. Atomic weight is the weighted average of these isotopic masses, reflecting their natural abundance. For hydrogen, the predominance of protium (mass 1) with minor deuterium (mass 2) and tritium (mass 3) yields an average atomic weight of 1.008. While isotope ratios can slightly vary between samples, they remain consistently similar.
