Structure of the Atom
CBSE Class 9 · Science · Notes, formulas and practice questions
Structure of the Atom explains how electrons, protons and neutrons are arranged inside an atom. It covers the experiments behind Thomson's, Rutherford's and Bohr's models, shell distribution, valency, atomic number, mass number, isotopes and isobars.
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Inside every atom are three fundamental particles that decide an element's identity and chemical behaviour. The chapter begins with charged particles in matter and the discharge-tube experiments that revealed electrons and protons, and then traces how experiments by Thomson, Rutherford and Bohr refined the picture of atomic structure. Later ideas—shell distribution, valency, atomic number, mass number, isotopes and isobars—all grow out of these particle models.
Thomson proposed that electrons are embedded in a positive sphere, like plums in a pudding. Rutherford's alpha-particle scattering on gold foil overturned this: most particles passed straight through, showing atoms are mostly empty; a few bounced back, showing a tiny, dense, positively charged nucleus. Bohr then explained stability by suggesting electrons orbit the nucleus in fixed energy shells rather than radiating energy continuously. Each model added one important piece, and together they explain why atoms are neutral and why electrons stay near the nucleus.
Electron distribution follows a simple scheme: shells fill in order K, L, M, N, with maximum occupation 2n². The outermost shell governs valency—how many electrons an atom loses, gains, or shares to complete the shell. A sodium atom with one outer electron is ready to lose it, while a chlorine atom with seven needs just one more; compounds form in definite ratios because atoms tend to finish their outer shells. Understanding shell arrangement lets a student predict valency without memorising every element.
The atomic number is the number of protons, and it fixes the element's identity; the mass number is protons plus neutrons. Atoms of the same element with different numbers of neutrons are isotopes, while atoms of different elements that have the same mass number are isobars. These definitions matter practically: isotopes such as carbon-14 are used for dating, cobalt-60 for radiotherapy, and uranium-235 in nuclear reactors. This final part links particle numbers to real applications in science and medicine.
Key terms
- Electron
- A negatively charged particle found outside the nucleus. J.J. Thomson discovered it in cathode rays; it is very light and it is the particle lost, gained or shared when atoms combine.
- Proton
- A positively charged particle with relative mass about 1 u, present in the nucleus. The number of protons determines the atomic number of an element.
- Neutron
- A neutral particle with mass nearly equal to that of a proton, also present in the nucleus. Changing the number of neutrons does not change the element's identity, but it changes the mass number and may make the atom radioactive.
- Thomson's model of the atom
- Proposed by J.J. Thomson; called the plum-pudding model. It describes electrons as negatively charged 'plums' embedded in a uniform sphere of positive charge, explaining why the atom as a whole has no net charge.
- Rutherford's model of the atom
- Based on the alpha-particle scattering experiment, it states that almost all of the mass and all the positive charge lie in a tiny central nucleus, while the rest of the atom is empty space where electrons move around it.
- Bohr's model of the atom
- Electrons revolve around the nucleus only in fixed circular paths called energy levels or shells, labelled K, L, M, N. While in a particular shell, an electron does not radiate energy; shells are filled outwards in the order K, L, M, N, and a shell has maximum capacity 2n² with the outermost shell limited to 8.
- Valency
- The combining capacity of an atom, equal to the number of electrons it loses, gains, or shares to obtain a completely filled outer shell. Sodium loses 1 electron, so its valency is 1; chlorine gains 1 electron, so its valency is also 1; oxygen gains 2, so its valency is 2; noble gases have valency 0.
- Atomic number and mass number
- The atomic number (Z) is the number of protons in the nucleus and gives the element its identity. The mass number (A) is the total number of protons and neutrons; the number of neutrons is A − Z.
- Isotopes and isobars
- Isotopes are atoms of the same element with the same atomic number but different mass numbers, such as carbon-12 and carbon-14. Isobars are atoms of different elements with the same mass number, such as calcium-40 and argon-40.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Maximum number of electrons in a shell | 2n² | n is the shell number (K = 1, L = 2, M = 3, N = 4). This gives the maximum electron capacity of a shell, but an outermost shell can hold at most 8 electrons. |
| Mass number | A = Z + N | A = mass number, Z = atomic number (number of protons) and N = number of neutrons. It gives the total number of protons and neutrons in a nucleus. |
Practice questions with answers
1. What are cathode rays, and which fundamental particle did J.J. Thomson identify from them?
Cathode rays are streams of negatively charged particles produced in a discharge tube when a high voltage is passed through a gas at low pressure. Thomson showed that these particles have a fixed charge-to-mass ratio and are present in all atoms; they came to be called electrons.
2. An atom has atomic number 12 and mass number 24. How many protons, neutrons and electrons does a neutral atom of this element have? Write its electronic configuration and valency.
Protons equal Z = 12. Since the atom is neutral, electrons = 12. Neutrons = A − Z = 24 − 12 = 12. Electrons fill the shells as 2, 8, 2, with 2 electrons in the outermost M shell, so the atom can lose these 2 electrons and its valency is 2.
3. In Rutherford's gold-foil experiment, most alpha particles passed straight through, but a few bounced back. What do these two observations show about the atom?
Most alpha particles passing straight through shows that an atom is mostly empty space. A few particles bouncing back can only happen if they collide with a very small, dense and positively charged central region, which Rutherford called the nucleus.
4. An element has electronic configuration 2, 8, 7. What is its atomic number and why is its valency 1?
Adding the shell electrons gives 2 + 8 + 7 = 17, so the atomic number is 17. The atom has 7 electrons in its outermost shell and needs only 1 more electron to complete an octet, so its valency is 1.
5. What was the main problem in Rutherford's model that Bohr's model corrected?
According to Rutherford's model, an electron moving around the nucleus should continuously radiate energy and spiral into the nucleus, making the atom unstable. Bohr solved this by proposing that electrons can travel only in certain fixed orbits without radiating energy, and they radiate energy only when they jump from one level to another.
6. Define isotopes and isobars and give one example of each.
Isotopes are atoms of the same element having the same atomic number but different mass numbers, for example carbon-12 and carbon-14. Isobars are atoms of different elements having the same mass number, for example calcium-40 and argon-40.
7. An atom of calcium-40 has 20 protons, while an atom of argon-40 has 18 protons. Calculate the number of neutrons in each and state what these two atoms are called.
For calcium-40, neutrons = 40 − 20 = 20. For argon-40, neutrons = 40 − 18 = 22. Since both atoms have the same mass number 40 but different atomic numbers, they are isobars.
8. Give two important uses of isotopes.
Carbon-14 is used to estimate the age of ancient fossils and archaeological materials, while cobalt-60 is used in radiotherapy for treating cancer. Uranium-235 is used as fuel in nuclear reactors.
9. Using the formula for maximum electrons in a shell, find the capacity of the N shell (n = 4). Why can an outermost shell hold at most 8 electrons?
The maximum capacity is 2n², so for the N shell it is 2 × 4² = 32. However, according to the Bohr–Bury scheme, the outermost shell is allowed to hold at most 8 electrons because atoms are most stable when their outermost shell has 8 electrons.
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