Chapter 8: Journey Inside the Atom
Original notes — ready CBSE (NCERT, English Medium) • Class 9 • Science • Book: Exploration
Chapter 8: Journey Inside the Atom — EG's own notes, in simple words
Book: Exploration (CBSE, Class 9) | Chapter 8 | Medium: English
Diagram (labels are printed on the image): Diagram (labels are printed on the image): A Bohr model atom: a central nucleus with electrons moving in fixed orbits around it. A general diagram for study, not a figure copied from the book.
Scientists exploring the inside of the atom. A general picture about atomic discovery, not a book scene.
What the chapter is about
For a long time, people believed the atom could not be divided. Then experiments began to reveal smaller particles inside it. The discovery of the electron showed that atoms contain negatively charged particles, and later work revealed the positively charged proton and the neutral neutron.
Models of the atom changed as new evidence arrived. Thomson imagined electrons scattered in a positively charged 'plum pudding'. Rutherford's gold-foil experiment showed that most of an atom is empty space, with a tiny, dense, positively charged nucleus at the centre. Bohr then proposed that electrons move in fixed orbits, or shells, around the nucleus, each with a definite energy.
Today we know that protons and neutrons sit in the nucleus, while electrons move in shells around it. The number of protons gives the atomic number, which fixes the identity of an element, and protons plus neutrons give the mass number. Atoms of the same element with different numbers of neutrons are called isotopes.
What you should learn
Notice how each model of the atom replaced the last one because of new evidence — and how protons, neutrons and electrons are arranged in an atom.
Key Terms
- Electron — A negatively charged particle found in shells around the nucleus.
- Proton — A positively charged particle found in the nucleus.
- Neutron — A neutral particle, with no charge, found in the nucleus.
- Atomic number — The number of protons in the nucleus of an atom.
- Isotope — Atoms of the same element with different numbers of neutrons.
Remember
- Electrons are negative; protons are positive; neutrons have no charge.
- Protons and neutrons sit in the nucleus; electrons move in shells.
- Atomic number = number of protons.
- Mass number = protons + neutrons.
Exam Tip / Common Mistake
Common mistake: placing protons and electrons in the same place. Protons (and neutrons) are in the nucleus; electrons are outside it, in shells. In a neutral atom, the number of electrons equals the number of protons.
Chart / Table: The three subatomic particles
| Point | Detail |
|---|---|
| Electron | Negative charge; in shells around the nucleus |
| Proton | Positive charge; in the nucleus |
| Neutron | No charge; in the nucleus |
| Atomic number | Number of protons |
| Mass number | Protons + neutrons |
Practice set — try it yourself
- Name the three subatomic particles and their charges.
Hint: Think about the nucleus and the shells. - What did Rutherford's experiment show about the atom?
Hint: Think about empty space and a tiny nucleus. - An atom has 6 protons and 6 neutrons. What are its atomic and mass numbers?
Hint: Think about what each number counts. - Why are electrons arranged in shells?
Hint: Think about Bohr's fixed orbits. - What are isotopes? Give one example.
Hint: Think about same protons, different neutrons.
Questions with Answers
- What did Thomson's model of the atom propose?
Thomson proposed that the atom is a positively charged sphere with negatively charged electrons embedded in it, like plums in a pudding. - What did Rutherford's gold-foil experiment show?
It showed that most of an atom is empty space, because most alpha particles passed straight through, while a few were deflected by a tiny, dense, positively charged nucleus. - What did Bohr add to the model of the atom?
Bohr proposed that electrons move around the nucleus in fixed orbits, or shells, each with a definite energy, and that electrons jump between shells by gaining or losing energy. - How are atomic number and mass number different?
The atomic number is the number of protons in an atom. 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, and therefore different mass numbers.
How to use this page: Read the summary, say the Key Terms in your own words, then try the practice set without looking. Use the Questions with Answers to check yourself afterwards. Ask your teacher or parent to check your answers.