Carbon atom representation. Image credit: Pixabay
Carbon atom representation. Image credit: Pixabay

An atom is a fundamental piece of matter that makes up a substance – living or non-living. Tables, chairs, animals, humans, plants, etc. are made of atoms. However, an atom doesn’t make up objects on its own. It usually attaches to other similar atoms to form molecules and then the actual element. Atoms cannot, or do not combine with any random atom. They merge only if there is a chemical bond or physical attraction existing. A basic understanding of chemical bonding will throw light on how and why atoms get attracted to each other.

There are different atoms, and different substances have varying atomic compositions. For example, the atoms hydrogen, oxygen, nitrogen and carbon that make up living beings aren’t necessarily found in non-living objects. Since atoms are unbelievably small and cannot be viewed under a microscope, several thousand atoms are needed to make up a thing that’s remotely close to being visible to the naked human eye. Contrary to common perception though, atoms aren’t the smallest particles; a quark is the tiniest particle to exist.

Division and Structure

An atom can be chemically broken down further into subatomic particles or an atom’s building blocks: protons, neutrons, electrons. An atom has a nucleus – its center portion – which usually houses the protons and neutrons. At times, the nucleus may not have any neutron(s). The electrons – positioned outside the nucleus – are found encircling the nucleus. An atom may not have neutron(s), but it would certainly have a proton and an electron.

Generally, an atom has equal number of protons and electrons. At times, the neutron and proton count could be identical too, but there could be variations. The protons, in fact, determine the type of atom. For example, if an atom has one proton, it’s a hydrogen atom. Similarly, four protons indicate the atom is beryllium. The proton count of an atom is called atomic number.

Shells, Subshells & Orbitals

An atom’s nucleus is usually surrounded by shells that can be divided as subshells: sharp (s), principal (p), diffuse (d), fundamental (f), and g (since the alphabet comes after the letter f). And these subshells have a specific number of orbitals. As a rule, an orbital cannot hold more than 2 electrons.

The first shell or shell closest to the nucleus has one subshell, referred to as subshell s, which has one orbital. Therefore, shell 1 can hold only 2 electrons. Shell 2 has 2 subshells: s and p. Subshell p has three orbitals, which means it can hold a maximum of 6 electrons. Adding subshell s and p, shell 2 can hold 8 electrons. Shell 3 has 3 subshells: s, p and d.

Subshell d has 5 orbitals. This means shell 3 can house (2 + 6 + 8) 18 electrons. Shell 4 has 4 subshells: s, p, d, and f. Shell 5 has 5 subshells: s, p, d, f, and g. Subshells f and g have 7 and 9 orbitals, respectively. This means subshell f can hold a maximum of 14 electrons and subshell g can accommodate up to 18 electrons.

There is no limit to how many shells an atom can have. The more the number of electrons, the higher the number of shells. Scientists believe atoms could also have indefinite number of empty atomic shells.


The mass of an atom is measured as atomic mass. Typically, almost all of an atom’s mass can be attributed to its nucleus, since the neutron and proton are several thousand times heavier than electrons. In other words, electron(s) almost do not contribute to an atom’s size. Therefore, an atom having more neutrons and protons than another atom with fewer number of neutrons and protons would have a larger atomic mass.

Electric Charge

Every atom has an electric charge. The atom doesn’t carry any charge on its own; the charge is courtesy its constituents. A proton and electron have positive and negative charges, respectively. A neutron, on the other hand, doesn’t carry any charge. Therefore, if an atom has more protons than electrons, it’s positively charged, and vice-versa. If there are equal numbers of both, the atom would carry a positive charge. Neutrons do nothing in terms of providing the atom an electric charge.