Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Atomic Structure is part of Term 1 in Class 8 Science. This chapter introduces the atom as the basic unit of matter, explains the discovery and properties of subatomic particles, the Bohr model of the atom, and how electrons are arranged in shells.
Key Definitions
- Atom: The smallest particle of an element that retains the chemical properties of that element.
- Proton: Positively charged particle in the nucleus; mass = 1 amu.
- Neutron: Neutral particle in the nucleus; mass = 1 amu.
- Electron: Negatively charged particle orbiting the nucleus; mass ≈ 0 amu (negligible).
- Nucleus: The dense central core of an atom containing protons and neutrons.
- Atomic number (Z): Number of protons in an atom.
- Mass number (A): Total number of protons and neutrons.
- Isotopes: Atoms of the same element with the same atomic number but different mass numbers.
Key Facts
| Particle | Charge | Location | Mass (amu) |
|---|---|---|---|
| Proton | +1 | Nucleus | 1 |
| Neutron | 0 | Nucleus | 1 |
| Electron | −1 | Shells (orbitals) | ≈ 0 |
Important Formulae
- Mass number (A) = Protons + Neutrons
- Number of neutrons = A − Z
- Maximum electrons in shell n = 2n²
- Shell 1 (K): max 2 electrons
- Shell 2 (L): max 8 electrons
- Shell 3 (M): max 18 electrons
Bohr's Model of the Atom
- Electrons revolve around the nucleus in fixed circular orbits called shells or energy levels.
- Each shell has a fixed energy.
- Electrons do not lose energy as long as they stay in the same shell.
- Electrons jump to higher shells when they absorb energy; fall back when they emit energy.
Electronic Configuration — Examples
| Element | Symbol | Atomic No. | Electronic Config. |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Carbon | C | 6 | 2, 4 |
| Nitrogen | N | 7 | 2, 5 |
| Oxygen | O | 8 | 2, 6 |
| Sodium | Na | 11 | 2, 8, 1 |
| Chlorine | Cl | 17 | 2, 8, 7 |
| Calcium | Ca | 20 | 2, 8, 8, 2 |
Valency
- Valency is the combining capacity of an atom.
- Determined by the number of electrons in the outermost shell.
- If outermost shell has ≤ 4 electrons, valency = number of outermost electrons.
- If outermost shell has > 4 electrons, valency = 8 − number of outermost electrons.
Important Questions
One Mark
- Define atomic number.
- What is the charge of an electron?
- Write the formula to find the number of neutrons in an atom.
- What is valency?
- How many electrons can the second shell (L shell) hold?
Short Answer (3 marks)
- Write the electronic configuration of sodium (atomic number 11) and chlorine (atomic number 17). What is the valency of each?
- Distinguish between atomic number and mass number with examples.
- Explain Bohr's model of the atom with a diagram of the hydrogen atom.
Long Answer (5 marks)
- Describe the structure of an atom with a neat diagram. Name the three subatomic particles, their charges, masses, and locations in the atom.
- Explain isotopes with examples. Write the electronic configuration and valency of oxygen (Z=8), sodium (Z=11), and calcium (Z=20).
Common Mistakes
- Confusing atomic number (protons) with mass number (protons + neutrons).
- Forgetting to fill inner shells before outer shells when writing electronic configuration.
- Saying valency is always the number of outermost electrons — it can be 8 minus that number.
Revision Checklist
- [ ] Textbook reading completed
- [ ] Three subatomic particles with properties memorised
- [ ] Electronic configuration of first 20 elements practised
- [ ] Bohr model diagram drawn
- [ ] Important questions answered in notebook
Frequently Asked Questions
The three subatomic particles are: 1) Proton — positively charged, in the nucleus. 2) Neutron — no charge, in the nucleus. 3) Electron — negatively charged, orbiting the nucleus.