Atomic Structure – Class 9 Notes
1. Atom
An atom is the smallest unit of an element that retains its chemical properties.
2. Subatomic Particles
Atoms are made up of three main particles:
• Electrons (e■): Negatively charged particles, present outside the nucleus.
• Protons (p■): Positively charged particles, present in the nucleus.
• Neutrons (n■): Neutral particles, present in the nucleus.
3. Structure of an Atom
The nucleus is located at the center of the atom and contains protons and neutrons. Electrons
revolve around the nucleus in fixed energy levels or shells.
4. Atomic Number (Z)
Atomic number is the number of protons present in the nucleus of an atom. In a neutral atom, the
number of protons equals the number of electrons.
5. Mass Number (A)
Mass number is the sum of the number of protons and neutrons in the nucleus.
6. Isotopes
Isotopes are atoms of the same element having the same atomic number but different mass
numbers.
7. Important Points
• Electrons are much lighter than protons and neutrons.
• Most of the mass of an atom is concentrated in the nucleus.
• Atoms are electrically neutral.
1. Atom
An atom is the smallest unit of an element that retains its chemical properties.
2. Subatomic Particles
Atoms are made up of three main particles:
• Electrons (e■): Negatively charged particles, present outside the nucleus.
• Protons (p■): Positively charged particles, present in the nucleus.
• Neutrons (n■): Neutral particles, present in the nucleus.
3. Structure of an Atom
The nucleus is located at the center of the atom and contains protons and neutrons. Electrons
revolve around the nucleus in fixed energy levels or shells.
4. Atomic Number (Z)
Atomic number is the number of protons present in the nucleus of an atom. In a neutral atom, the
number of protons equals the number of electrons.
5. Mass Number (A)
Mass number is the sum of the number of protons and neutrons in the nucleus.
6. Isotopes
Isotopes are atoms of the same element having the same atomic number but different mass
numbers.
7. Important Points
• Electrons are much lighter than protons and neutrons.
• Most of the mass of an atom is concentrated in the nucleus.
• Atoms are electrically neutral.
