What this chapter is about
This chapter takes you inside the atom to understand its internal structure. In earlier classes, you learned that all matter is made of atoms and molecules. Now you will discover that atoms themselves are not the smallest particles — they contain even tinier particles called subatomic particles. The chapter traces how scientists discovered electrons, protons and neutrons through careful experiments.
You will learn about different atomic models proposed by scientists like Thomson, Rutherford and Bohr, and understand why each model was improved upon. The chapter explains how electrons are arranged in shells around the nucleus, which helps explain why elements behave the way they do in chemical reactions.
By the end of this chapter, you should be able to describe the structure of an atom, calculate the number of protons, neutrons and electrons in any atom, write electronic configurations, and explain what isotopes and isobars are.
Key ideas
- An atom consists of three subatomic particles: electrons (negative charge), protons (positive charge) and neutrons (no charge). Electrons are found outside the nucleus, while protons and neutrons are packed inside the nucleus.
- Thomson proposed the plum pudding model where electrons are embedded in a sphere of positive charge, like seeds in a watermelon.
- Rutherford's gold foil experiment showed that most of the atom is empty space, with a tiny, dense, positively charged nucleus at the centre.
- Bohr improved Rutherford's model by proposing that electrons move in fixed circular orbits (shells) around the nucleus without losing energy.
- The atomic number (Z) equals the number of protons in an atom. The mass number (A) equals the sum of protons and neutrons.
- Electrons fill shells in a specific order. The maximum electrons in a shell is given by 2n², where n is the shell number (1, 2, 3...).
- Isotopes are atoms of the same element with the same atomic number but different mass numbers (different number of neutrons). Isobars are atoms of different elements with the same mass number but different atomic numbers.
Formulas and facts to remember
- Atomic number (Z) = Number of protons = Number of electrons (in a neutral atom)
- Mass number (A) = Number of protons + Number of neutrons
- Number of neutrons = A − Z
- Maximum electrons in shell n = 2n² (K-shell: 2, L-shell: 8, M-shell: 18, N-shell: 32)
- Charge of electron = −1.6 × 10⁻¹⁹ coulomb
- Mass of electron is approximately 1/1836 times the mass of a proton
- Protons and neutrons have nearly equal mass, about 1 atomic mass unit (1 u) each
- The nucleus is about 10⁻¹⁵ m in size, while the atom is about 10⁻¹⁰ m — the nucleus is 100,000 times smaller than the atom
Worked examples
Example 1: Finding subatomic particles
A sodium atom has atomic number 11 and mass number 23. Find the number of protons, electrons and neutrons.
Step 1: Number of protons = Atomic number = 11
Step 2: Number of electrons = Number of protons (for a neutral atom) = 11
Step 3: Number of neutrons = Mass number − Atomic number = 23 − 11 = 12
Answer: Sodium has 11 protons, 11 electrons and 12 neutrons.
Example 2: Writing electronic configuration
Write the electronic configuration of chlorine (atomic number 17).
Step 1: Total electrons to arrange = 17
Step 2: K-shell (n = 1) can hold 2 × 1² = 2 electrons. Fill it first: 2 electrons
Step 3: L-shell (n = 2) can hold 2 × 2² = 8 electrons. Fill it next: 8 electrons
Step 4: Remaining electrons = 17 − 2 − 8 = 7 electrons go into M-shell
Answer: Electronic configuration of chlorine is 2, 8, 7.
Example 3: Identifying isotopes
Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons. Are these isotopes?
Step 1: Check atomic numbers. Both have 6 protons, so both have atomic number 6. They are the same element (carbon).
Step 2: Check mass numbers. Carbon-12: A = 6 + 6 = 12. Carbon-14: A = 6 + 8 = 14. Mass numbers differ.
Answer: Yes, they are isotopes because they have the same atomic number but different mass numbers.
Common mistakes
Confusing atomic number with mass number → Atomic number is only protons; mass number includes both protons and neutrons.
Thinking the nucleus contains electrons → Electrons are always outside the nucleus, moving in shells around it.
Adding more than 8 electrons in the outermost shell → The outermost shell of atoms with low atomic numbers holds maximum 8 electrons, regardless of which shell it is.
Believing isotopes are different elements → Isotopes are the same element with identical chemical properties; only the number of neutrons differs.
Assuming most of the atom is solid matter → Rutherford showed that atoms are mostly empty space; the nucleus occupies a tiny fraction.
Quick revision
- Atom = nucleus (protons + neutrons) + electrons in shells around it.
- Atomic number Z = protons; Mass number A = protons + neutrons.
- Fill electrons using 2n² rule: K holds 2, L holds 8, M holds 18.
- Outermost shell should not have more than 8 electrons in simple atoms.
- Isotopes: same Z, different A. Isobars: same A, different Z.
- The nucleus is tiny and dense; most of the atom is empty space.