Element

Definition¹

A kind of atoms that have the same number of protons in the atomic nucleus is called an element.

Explanation

The number of protons in the atomic nucleus, which serves as the criterion for distinguishing elements, is called the atomic number and is denoted by $Z$. Atomic numbers and elements correspond one-to-one, and each element is given an element symbol of one or two letters, such as $\mathrm{H}$ (hydrogen, $Z=1$), $\mathrm{C}$ (carbon, $Z=6$), and $\mathrm{O}$ (oxygen, $Z=8$). To date, $118$ elements, with atomic numbers from $1$ to $118$, have been confirmed. A table that arranges the elements in order of atomic number, laid out so that elements with similar chemical properties fall in the same column, is called the periodic table.

Distinction between Element and Atom

The terms element and atom are often used interchangeably, but they are distinct concepts.

An element is a concept referring to a kind of atom. The classified names such as hydrogen, oxygen, and carbon are themselves elements.
An atom refers to each individual particle corresponding to that element.

For example, "hydrogen" is an element, whereas each of the "two hydrogen atoms in a water molecule" is an atom. By analogy with the classification of biological species, an element corresponds to the species "Homo sapiens", and an atom corresponds to each individual person.

Why the Number of Protons?

An atom consists of protons, neutrons, and electrons, but among these only the number of protons determines the element.

The number of electrons changes when the atom loses or gains electrons and becomes an ion. However, a sodium ion $\mathrm{Na}^{+}$, a sodium atom that has lost one electron, is still sodium.
The number of neutrons can differ from atom to atom even within the same element (see isotopes below).
If the number of protons changes, the charge of the atomic nucleus changes, and so does the number of electrons it holds when electrically neutral. Since the chemical properties of an atom are determined by the number and arrangement of its electrons, a different number of protons yields a substance with entirely different chemical properties.

In other words, the number of protons is a quantity that determines the chemical properties of an atom yet does not change through chemical reactions.

Isotopes

Atoms that have the same atomic number but different numbers of neutrons, and hence different mass numbers, are called isotopes of one another. The mass number $A$ is the sum of the number of protons and the number of neutrons. For example, hydrogen has the following three isotopes.

Name	Notation	Protons	Neutrons	Mass Number
Hydrogen (protium)	$^{1}\mathrm{H}$	$1$	$0$	$1$
Deuterium	$^{2}\mathrm{H}$	$1$	$1$	$2$
Tritium	$^{3}\mathrm{H}$	$1$	$2$	$3$

Since isotopes have the same number and arrangement of electrons, their chemical properties are nearly identical, while their masses and the stability of their nuclei differ. For example, tritium has an unstable nucleus and undergoes radioactive decay over time.