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1.1 ATOM – Significance of Sub-atomic Particles

What is an atom? What is the significance of sub-atomic particles- Proton, neutron and electron?

Quiz – Build an Atom / Element / Ion


As we know, an atom is the smallest building block of everything that exists. It is composed of three sub-atomic particles: the proton, neutron, and electron. Protons and neutrons make up the nucleus of the atom, a dense and positively charged core, while the negatively charged electrons can be found around the nucleus in an electron cloud.
Atoms can be different by having different numbers of protons, neutrons and electrons. Atoms are the units of chemical change.

Significance of Sub-atomic particles

(A) Proton

A proton (p+) is a positively charged sub-atomic particle.

Please check in detail – Characteristics of Protons

  • The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element. Its value determines the identity of the atom.
    For example, any atom that contains eight protons is the element oxygen and has the atomic number 8, regardless of how many neutrons or electrons it may have. Similarly, Hydrogen has one proton and no neutron and its atomic number is 1.
  • All atoms with the same atomic number behave in the same way chemically and are classified as the same chemical element. Elements are the substances consisting of one type of atoms.
  • Atomic number(Z) = number of protons

(B) Electron

An electron (e) is an extremely light, negatively charged sub-atomic particle.

Please check in detail – Characteristics of Electrons

  •  A neutral atom must contain the same number of positive and negative charges, so the number of protons equals the number of electrons. Therefore, the atomic number also indicates the number of electrons in an atom. 

(C) Neutron

A neutron is a neutral subatomic particle. It is a constituent of every atomic nucleus except ordinary hydrogen.

Please check in detail – Characteristics of neutrons

  • The total number of protons and neutrons in an atom is called its mass number (A).
  • Mass number (A)=number of protons + number of neutrons
  • The number of neutrons is therefore the difference between the mass number and the atomic number:
  • Number of neutrons = A – Z.

When describing the properties of tiny objects such as atoms, we use appropriately small units of measure, such as the atomic mass unit (amu) and the fundamental unit of charge (e).



Quiz – Build an Atom / Element / Ion

ATOM – Its Symbol

ATOM – Its Size (Atomic radius)

IONS – Cation and Anion