Question

Chlorine is an element of atomic number 17. It is a mixture of two isotopes having mass numbers of 35 and 37.

(i) What is meant by “atomic number of an element”?

(ii) What do you understand by an 'atom'?

Answer 1

(i) Atomic number(Z)- The total number of protons in the nucleus of an atom gives us the atomic number of that atom.

• For example Chlorine(Cl) has atomic number 17, it has 17 protons in its nucleus.
• Proton- Proton is a positively charged subatomic particle in the nucleus of an atom.

(ii) Atom- An atom is defined as the smallest unit that retains the properties of an element.

• For example, the Hydrogen atom, is symbolized by the letter H in chemistry.
• An atom is composed of sub-atomic particles and these cannot be made or destroyed.
• An atom has protons and neutrons in its nucleus.
• Electrons which are negatively charged subatomic particles, revolve around the nucleus in a fixed path called an orbit.
• All atoms of the same element are identical and different elements have different types of atoms.
• Chemical reactions occur when the rearrangement of atoms takes place.