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Atomic Number and Mass Number of an Atom

Atomic Number and Mass Number of an Atom
M.Sc (Industrial analytical chemistry)(Reading), B.Sc (USJ) (Chemistry, Environmental management, Bio-Technology), HND (Software Development)

In chemistry, understanding fundamental concepts such as the atomic number, and the mass number are essential. Before we move in to learn about those concepts, it is important to get a basic understanding of the fundamental particles (subatomic particles) of an atom.

Fundamental particles of an atom

Elements are made out of atoms. An atom can be defined as the smallest unit that an element can be divided. In other words, an atom is the smallest unit that resembles the chemical and physical properties of an element.

Atoms are made out of fundamental subatomic particles, electrons, protons, and neutrons. Those subatomic particles further can be divided into much smaller units. But we are not going to discuss them in this basic introduction.

When considering subatomic particles, neutrons, and protons are contained in the nucleus of an atom. Electrons occupy the region of space around the nucleus.

The mass of protons and neutrons are approximately similar, and the mass of an electron is negligible when compared to neutron and proton. That means nearly all the mass of an atom is concentrated in its nucleus.

The radius of a nucleus is about 10-15 m. When comparing this with the radius of an atom this is a very small fraction. So, the volume of the nucleus of an atom is a small fraction of the volume of an atom. That means the density of the nucleus is very high.

Eg: The atomic radius of a Hydrogen atom is about 3.7×10-11. This is nearly 104 times the radius.

Protons are positively charged, and neutrons are neutral. Electrons are negatively charged, and the magnitude of the charge is equal to the charge of protons.

Subatomic particleCharge – in Coulombs (C)Relative charge (Charge number)Rest mass (kg)Relative mass

What is nuclide?

A Nuclide can be defined as a specific type of atom (or nucleus) that possesses a characteristic atomic number and a characteristic mass number. This is represented by the symbol ‘E’ in the shorthand method.

What is the atomic number?

The atomic number of a nuclide (atom) is equal to the number of protons present in the nucleus of the atom. Because, in the ground state atom is electrically neutral, its atomic number is also equal to the number of electrons in the atom. The atomic number is represented by the symbol ‘Z’ in the shorthand method.

What is mass number?

The mass number of the nuclide (atom) is equal to the total number of neutrons and protons in the nucleus. In the shorthand method, the mass number is represented by the symbol ‘A’.

A Shorthand method of showing the atomic number and mass number of a nuclide.
Figure 01: A Shorthand method of showing the atomic number and mass number of a nuclide.


Atomic Number and Mass Number of an Atom eq 01

To learn about concepts of mass calculations of atoms (Atomic mass unit, relative atomic mass), you can visit the linked article.

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References and Attributes

Housecroft, C.E. and Sharpe, A.G. (2018) Inorganic Chemistry. Harlow: Pearson Education.


The cover image was created using an image by Dede from Pixabay

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