Matter and Atomic Structure
Matter and Atomic Structure
Matter exists as solid, liquid and gas, and changes state by melting, boiling, evaporation, condensation and sublimation. Inside every atom are protons, neutrons and electrons. NTPC asks the names of state changes, who discovered each particle, atomic and mass numbers, isotopes and electronic configuration.
1. States of Matter and Changes of State
| Change | From and to | Example |
|---|---|---|
| Melting (fusion) | Solid to liquid | Ice to water at 0 °C |
| Freezing | Liquid to solid | Water to ice |
| Boiling | Liquid to gas throughout the liquid, at a fixed temperature | Water at 100 °C |
| Evaporation | Liquid to gas at the surface, at any temperature | Drying clothes; causes cooling |
| Condensation | Gas to liquid | Dew, droplets on a cold glass |
| Sublimation | Solid directly to gas | Camphor, naphthalene, dry ice, iodine |
| Deposition | Gas directly to solid | Frost formation |
| Mixture separation | Method |
|---|---|
| Salt from sea water | Evaporation |
| Cream from milk | Centrifugation |
| Dyes in ink | Chromatography |
| Alcohol from water | Fractional distillation |
| Camphor from salt | Sublimation |
2. Atomic Structure
| Particle | Discovered by | Charge |
|---|---|---|
| Electron | J.J. Thomson (1897), cathode rays | Negative |
| Proton | Ernest Rutherford (credited, 1917-19); Goldstein saw canal rays earlier | Positive |
| Neutron | James Chadwick (1932) | None |
| Nucleus | Rutherford's gold foil experiment (1911) | Positive |
| Model | Key idea |
|---|---|
| Thomson | Plum pudding: electrons set in a positive sphere |
| Rutherford | Tiny dense positive nucleus; most of the atom is empty |
| Bohr | Electrons move in fixed energy shells (K, L, M, N) |
Atomic number Z = number of protons; mass number A = protons + neutrons
Maximum electrons in a shell = 2n²: K 2, L 8, M 18
Isotopes: same Z, different A (protium, deuterium, tritium; C-12 and C-14)
Isobars: same A, different Z (argon-40 and calcium-40)
✗ Evaporation happens only at the boiling point | ✓ Evaporation happens at all temperatures at the surface; boiling happens at a fixed temperature
✗ Chadwick discovered the electron | ✓ Chadwick discovered the neutron; J.J. Thomson discovered the electron
हिंदी नोट: ठोस से सीधे गैस बनना ऊर्ध्वपातन है, जैसे कपूर और नैफ़्थलीन। इलेक्ट्रॉन की खोज जे.जे. थॉमसन, न्यूट्रॉन की चैडविक और नाभिक की रदरफ़ोर्ड ने की। समस्थानिकों में परमाणु संख्या समान और द्रव्यमान संख्या भिन्न होती है।
Exam Pointer: Verified NTPC patterns: naming the change from liquid to gas (April 2016 CBT-1) and the discoverer of the electron (March 2026 Graduate CBT-1). Isotope and configuration questions are tagged syllabus-based.
Pariksha Pattern: Every Way NTPC Asks This Topic
Pattern 1: Changes of state
[PYQ: NTPC CBT-1 5-Apr-2016 Shift-1]
EXAM LEVEL
Q. What is the direct change of a solid into vapour, without becoming liquid, called?
Sublimation, as seen with camphor and naphthalene balls.
Answer: Sublimation
EXAMATLAS LEVEL
Q. Give two differences between evaporation and boiling, and explain why water in an earthen pot stays cool.
Evaporation happens at any temperature and only at the surface; boiling happens at a fixed temperature throughout the liquid. Water seeps out through the pores of the pot and evaporates, taking heat from the remaining water.
Answer: Surface vs whole liquid, any vs fixed temperature; evaporation causes cooling
Pattern 2: Subatomic particles and their discoverers
[PYQ: NTPC Graduate CBT-1 18-Mar-2026 Shift-1]
EXAM LEVEL
Q. Which particle did James Chadwick discover, and in which year?
The neutron, in 1932.
Answer: Neutron, 1932
EXAMATLAS LEVEL
Q. Match the scientist with the discovery or model: J.J. Thomson, Rutherford, Niels Bohr.
Thomson discovered the electron and proposed the plum pudding model. Rutherford discovered the nucleus with the gold foil experiment. Bohr proposed fixed energy shells for electrons.
Answer: Electron; nucleus; energy shells
Pattern 3: Atomic number, mass number and isotopes
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. An atom has 11 protons and 12 neutrons. Find its atomic number and mass number.
Atomic number = 11 (sodium); mass number = 11 + 12 = 23.
Answer: 11; 23
EXAMATLAS LEVEL
Q. Which pair are isotopes and which are isobars: carbon-12 and carbon-14; argon-40 and calcium-40? How many neutrons does tritium have?
Carbon-12 and carbon-14 have the same atomic number, so they are isotopes. Argon-40 and calcium-40 share mass number 40, so they are isobars. Tritium has 1 proton and mass 3, so 2 neutrons.
Answer: Isotopes: C-12, C-14; isobars: Ar-40, Ca-40; 2
Pattern 4: Electronic configuration and separation of mixtures
[Pattern: syllabus-based, PYQ-style]
EXAM LEVEL
Q. Write the electronic configuration of chlorine (Z = 17).
K 2, L 8, M 7, written 2, 8, 7.
Answer: 2, 8, 7
EXAMATLAS LEVEL
Q. Which method separates the dyes in black ink, and which separates cream from milk? What is the maximum number of electrons in the M shell?
Chromatography separates dyes; centrifugation separates cream. The M shell (n = 3) can hold 2 × 3² = 18 electrons.
Answer: Chromatography, centrifugation; 18
60-Second Revision
- Sublimation: camphor, naphthalene, dry ice, iodine.
- Electron Thomson; nucleus Rutherford; neutron Chadwick 1932; shells Bohr.
- Z = protons; A = protons + neutrons; shell capacity 2n².
- Isotopes same Z; isobars same A.