Journey Inside the Atom
NCERT Class 9 Science • Chapter 8
Quick Answer
This Class 9 chemistry chapter explores atomic structure: the discovery of electrons, protons, and neutrons, and models proposed by Thomson, Rutherford, and Bohr. It covers atomic number, mass number, isotopes, isobars, the distribution of electrons in shells, and valency, explaining how the atom's structure determines its behaviour.
↓ Free PDF, NCERT solutions, summary, key terms, FAQs below.
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Key Terms
- Atom
- The smallest particle of an element that retains its chemical properties, made of protons, neutrons, and electrons.
- Electron
- A negatively charged subatomic particle that revolves around the nucleus in shells and has negligible mass.
- Proton
- A positively charged subatomic particle found in the nucleus; the number of protons defines the element's atomic number.
- Neutron
- An electrically neutral subatomic particle present in the nucleus that contributes to an atom's mass.
- Atomic Number
- The number of protons in the nucleus of an atom, which determines the identity of the element.
- Isotopes
- Atoms of the same element having the same atomic number but different mass numbers due to differing numbers of neutrons.
- Valency
- The combining capacity of an atom, determined by the number of electrons it gains, loses, or shares to achieve a stable outer shell.
Frequently Asked Questions
What are the three main subatomic particles in an atom?▾
An atom contains three main subatomic particles: protons, which carry a positive charge, neutrons, which are neutral, and electrons, which carry a negative charge. Protons and neutrons sit in the nucleus while electrons move around it.
What is the difference between atomic number and mass number?▾
The atomic number is the number of protons in an atom and identifies the element. The mass number is the total number of protons and neutrons in the nucleus.
What are isotopes?▾
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This gives them the same atomic number but different mass numbers, such as the isotopes of carbon.
How did Rutherford's model change our understanding of the atom?▾
Rutherford's experiment showed that an atom has a small, dense, positively charged nucleus at its centre, with electrons moving around it. Most of the atom is empty space, which replaced earlier uniform models.
What is valency?▾
Valency is the combining capacity of an atom, decided by the electrons in its outermost shell. Atoms gain, lose, or share electrons to complete their outer shell, and valency tells us how many bonds they can form.
How are electrons arranged in an atom?▾
Electrons are arranged in shells around the nucleus, with each shell holding a maximum number of electrons. They fill the innermost shells first, and the outermost electrons determine the atom's chemical behaviour.
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