Atomic Number

An atom comprises electrons, protons, and neutrons. Electrons revolve around the nucleus of an atom just like the Earth around the Sun. Protons and neutrons reside in the centre of an atom called nucleus. Electrons are negatively charged while protons are positively charged. Neutrons are neutral i.e., no charge. In an unbounded stable atom, the number of protons equals the number of electrons. This makes the net charge of an atom zero. These number of protons are unique for a given element, and the number is called the atomic number or proton number.

Definition of Atomic Number

The Gold Book of the International Union of Pure and Applied Chemistry (IUPAC) defines an atomic number as “the number of protons in an atomic nucleus.”

Symbol

The symbol used for the atomic number is Z.

Representation

The atomic number is subscripted to the left of the symbol of an element. Consider an example of calcium, which has the atomic number of 20 (Z = 20). So, number 20 is subscripted to the symbol Ca i.e., ₂₀Ca.

Atomic Number vs Mass Number

As discussed, the atomic number is the number of protons presents in the nucleus while the mass number is the number of nucleons (protons and neutrons) presents in the nucleus of an atom. This both numbers are different by the definition itself. The below table highlights some of the key differences between the two numbers.

Relation with Mass Number and Neutron Number

The atomic number (Z) is related to the mass number (A) and the neutron number (N) by the following equation.

Atomic Number and Atomic Weight

The atomic weight or the relative atomic mass is the ratio of the average atomic mass of an atom to some reference standard. If the reference standard is considered 1 u, the atomic weight is the ratio of the average atomic mass to one unified mass unit (1 u). In other words, it is the numeric value of the average atomic mass. It is a dimensionless quantity. For lighter elements, the atomic weight almost twice the atomic number (see below figure). But for heavier elements, the atomic weight exceeds the twice of the atomic number. This is due to an increase in the neutron number compare to the proton number.

Atomic Number and Chemical Properties

The chemical properties of an atom are determined by the number of electrons particularly the number of electrons present in the outermost shell. For an uncharged, unbounded atom, the number of electrons always equals the number of protons. Thus, the atomic number decides the chemical nature of an atom.

Atomic Number and Isotopes

Isotopes are the atoms of the same elements, but with a different mass number. The atomic number can be used to distinguished the isotopes of an element. This is because isotopes of a given element have the same number of protons; they only differ in the number of neutrons. Instead, the mass number, which incorporates the neutron number, is used to identify the isotopes. Consider an example of oxygen. The common isotopes of oxygen are ¹⁶
₈O, ¹⁷
₈O, and ¹⁸
₈O. All these isotopes have the atomic number of 8; they only differ in the mass number.

List of Elements by Atomic Number

The below table lists all 118 elements by atomic number.

Associated Articles

• Atomic mass
• Mass number
• Atomic mass constant
• Dalton's atomic theory