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Chemistry · Class 9 · Chapter 2: Atomic Structure

8 min

Key points

Atom is the smallest particle of an element that can take part in chemical reactions
Atom consists of three fundamental particles: electrons, protons and neutrons
Nucleus contains protons and neutrons while electrons revolve around the nucleus in orbits
Atomic number (Z) equals the number of protons in the nucleus of an atom
Mass number (A) equals the sum of protons and neutrons in the nucleus
Isotopes are atoms of the same element having same atomic number but different mass numbers
Electronic configuration shows the arrangement of electrons in different shells around the nucleus
Maximum number of electrons in any shell is given by 2n² formula where n is shell number
Valence shell is the outermost shell of an atom and valence electrons determine chemical properties

Definitions

Atom: Atom is the smallest particle of an element that can take part in chemical reactions.
Atomic Number: The number of protons present in the nucleus of an atom is called atomic number.
Mass Number: The sum of the number of protons and neutrons present in the nucleus of an atom is called mass number.
Isotopes: Atoms of the same element having same atomic number but different mass numbers are called isotopes.
Electronic Configuration: The arrangement of electrons in different shells around the nucleus is called electronic configuration.

Worked example

Define atomic number and mass number. Calculate the number of neutrons in chlorine atom having mass number 35 and atomic number 17. (5 marks)

Atomic number is the number of protons present in the nucleus of an atom. Mass number is the sum of protons and neutrons present in the nucleus of an atom. Number of neutrons = Mass number - Atomic number = 35 - 17 = 18 neutrons.

Common mistakes

Students confuse mass number with atomic mass unit
Writing wrong electronic configuration by not following 2n² rule
Mixing up protons and neutrons in nucleus composition
Forgetting that isotopes have same chemical properties but different physical properties
Not writing complete symbol notation showing mass number and atomic number

Quick recap

Atom is the smallest particle of an element that retains its chemical properties. It consists of nucleus (protons + neutrons) and electrons in shells. Atomic number equals number of protons, mass number equals protons plus neutrons. Isotopes are atoms of same element with different mass numbers but same atomic number.

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