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Chemical elements and nuclear structure

An element can be represented by a unique chemical symbol $$\ce{X}$$ (e.g. $$\ce{H}$$ for hydrogen, $$\ce{He}$$ for helium).

The number of electrons and protons must be equal for an atom's charge to be zero. The number of electrons an atom contains in its neutral state determines the chemical characteristics of the atom.

Sometimes the chemical symbol is written with a superscript and subscript to denote the composition of the nucleus of a particular atom: $$$\ce{^{A}_{Z}X}$$$

This is used in the periodic table, or when discussing different isotopes. In general, elements are represented just by the symbol $$\ce{X}$$.

$$\ce{Z}$$ is the proton number (also called the atomic number), which gives the number of protons in an atom.

$$\ce{A}$$ is the nucleon number (also called the mass number), which gives the total number of protons and neutrons (i.e. nucleons) in the atom.

The number of neutrons in an atom can be determined by taking the difference between the nucleon number and proton number (i.e. $$\ce{A}-\ce{Z}$$).

The number of electrons is equal to the proton number, $$\ce{Z}$$, when an atom is in its uncharged state.

Isotopes are atoms of the same chemical element that have different numbers of neutrons.

Most elements have several isotopes, but usually only one isotope tends to be stable. Unstable isotopes emit nuclear radiation and are thus said to be radioactive.

Hydrogen has three main isotopes. The most common one is $$\ce{^{1}_{1}H}$$ and is usually just referred to as hydrogen. The isotope $$\ce{^{2}_{1}H}$$ is called deuterium and the isotope $$\ce{^{3}_{1}H}$$ is called tritium.

Deuterium and tritium are produced by nuclear fusion in stars, but are uncommon on earth.