Discovery of the electron: cathode rays
Take a glass tube, pump out almost all the air, and put a very high voltage across two metal ends. The negative end is the cathode, the positive end is the anode. A stream called cathode rays travels from the cathode to the anode.
- They move in straight lines (they cast a sharp shadow).
- They bend towards a positive plate in an electric field, so they carry negative charge.
- They are the same whatever gas or metal you use, so these particles are in every atom.
These particles are electrons. J. J. Thomson measured their charge-to-mass ratio: e/m = 1.758820 × 1011 C kg−1. Millikan's oil-drop experiment then gave the charge: e = −1.602 × 10−19 C. So the mass of an electron is m = e ÷ (e/m) = 9.1094 × 10−31 kg.
Discovery of the proton: canal rays
Goldstein used a cathode with holes (channels) in it. Behind it he saw rays going the opposite way. These canal rays (anode rays) bend towards the negative plate, so they are positive.
- Their e/m depends on the gas in the tube. They are positive ions made when electrons are knocked off gas atoms.
- With hydrogen gas we get the smallest and lightest positive particle. It is called the proton: charge +1.602 × 10−19 C, mass 1.6726 × 10−27 kg, about 1836 times an electron.
Discovery of the neutron
Atoms were heavier than their protons alone could explain. In 1932 James Chadwick hit a thin sheet of beryllium with alpha particles and got a very penetrating radiation made of neutral particles. Their mass (1.6749 × 10−27 kg) is a little more than a proton. They were named neutrons.
| Particle | Charge (C) | Relative charge | Mass (kg) | Mass (u) |
|---|---|---|---|---|
| Electron | −1.602 × 10−19 | −1 | 9.109 × 10−31 | 0.00054 |
| Proton | +1.602 × 10−19 | +1 | 1.6726 × 10−27 | 1.00727 |
| Neutron | 0 | 0 | 1.6749 × 10−27 | 1.00867 |
Thomson model of the atom
Thomson (1898) said: the atom is a sphere of radius about 10−10 m. Positive charge is spread evenly through it, and electrons sit inside so that the total charge is zero. People call it the plum pudding or watermelon model.
Good point: it explains why the atom is neutral. Problem: it could not explain Rutherford's alpha scattering result, so it was given up.
Rutherford's alpha-particle scattering experiment
Rutherford fired a narrow beam of fast alpha particles (He2+) at a very thin gold foil (about 100 nm). A zinc sulphide screen round the foil flashed wherever an alpha particle landed.
- Most went straight through → most of the atom is empty space.
- A few bent by small angles → there is positive charge that repels them.
- Very few (about 1 in 20,000) bounced almost straight back → the positive charge and nearly all the mass sit in a very tiny region.
In Thomson's spread-out ball, no alpha particle could bounce back. So Thomson's model failed.
Rutherford's nuclear model and its drawbacks
Nuclear model: all the positive charge and almost all the mass are in a tiny nucleus (radius about 10−15 m, 100,000 times smaller than the atom). Electrons move round the nucleus in circles, like planets round the Sun, held by electrostatic attraction.
Drawbacks:
- Stability: a charge moving in a circle is accelerating, and an accelerating charge should give out energy as light. The electron would spiral into the nucleus in about 10−8 s. Real atoms do not collapse.
- Line spectra: it says nothing about the energies of electrons, so it cannot explain why atoms give sharp lines.
Atomic number, mass number, isotopes and isobars
Atomic number Z = number of protons = number of electrons in a neutral atom. Mass number A = protons + neutrons (together called nucleons). So neutrons N = A − Z. We write the symbol as AZX, for example 3517Cl.
- Isotopes: same Z, different A (1H protium, 2H deuterium, 3H tritium). Same chemistry, because chemistry depends on electrons.
- Isobars: same A, different Z (14C and 14N).
- Isotones: same number of neutrons (13C and 14N, both N = 7).
For an ion: electrons = Z − charge. Na+ has 11 − 1 = 10 electrons; Cl− has 17 + 1 = 18.
Try it: build atoms in 3D
Go to the last story step. Set Z = 6 and A = 12 (carbon-12). Now change only A to 13 and 14. The number of protons stays 6, only neutrons change: these are isotopes of carbon. Then set Z = 7, A = 14 and compare with carbon-14: same A, different element, so they are isobars.
Board exam corner
This unit (Structure of Atom) carries about 9 marks in CBSE Class 11. From this part expect: 1–2 mark questions on properties of cathode/canal rays, counting p, n, e in atoms and ions, isotopes/isobars; 3 mark questions on Rutherford's experiment (observations → conclusions) and drawbacks of his model.
Key formulas and definitions
- Neutrons N = A − Z
- Electrons in an ion = Z − (charge on ion)
- Mass of electron m = e ÷ (e/m) = 9.109 × 10⁻³¹ kg
- e = 1.602 × 10⁻¹⁹ C
- Proton mass ≈ 1836 × electron mass
- Nucleus radius ≈ 10⁻¹⁵ m, atom radius ≈ 10⁻¹⁰ m
Worked examples
1. How many protons, neutrons and electrons are in ³⁵₁₇Cl?
Protons = Z = 17. Electrons = 17 (neutral). Neutrons = A − Z = 35 − 17 = 18.
2. Find p, n and e in ⁵⁶₂₆Fe³⁺.
p = 26. n = 56 − 26 = 30. The ion has lost 3 electrons: e = 26 − 3 = 23.
3. An ion has 18 electrons, 16 protons and 16 neutrons. Write its symbol.
Z = 16, so it is sulphur. A = 16 + 16 = 32. Charge = 16 − 18 = −2. Symbol: ³²₁₆S²⁻.
4. Using e/m = 1.76 × 10¹¹ C/kg and e = 1.6 × 10⁻¹⁹ C, find the mass of an electron.
m = e ÷ (e/m) = 1.6 × 10⁻¹⁹ ÷ 1.76 × 10¹¹ = 0.909 × 10⁻³⁰ kg = 9.09 × 10⁻³¹ kg.
5. The nucleus radius is about 10⁻¹⁵ m and the atom radius about 10⁻¹⁰ m. What fraction of the atom's volume is the nucleus?
Volume goes as r³. Ratio = (10⁻¹⁵ ÷ 10⁻¹⁰)³ = (10⁻⁵)³ = 10⁻¹⁵. The nucleus fills only one part in a million billion: the atom is almost all empty.
6. An element has mass number 81 and 31.7 % more neutrons than electrons. Find its symbol.
Let electrons = protons = x. Neutrons = 1.317x. So x + 1.317x = 81, x = 81 ÷ 2.317 ≈ 35. Z = 35 is bromine: ⁸¹₃₅Br (n = 46).
7. Which of these are isotopes and which are isobars: ⁴⁰₁₈Ar, ⁴⁰₂₀Ca, ³⁹₁₉K, ⁴⁰₁₉K?
³⁹K and ⁴⁰K: same Z = 19 → isotopes. ⁴⁰Ar, ⁴⁰K, ⁴⁰Ca: same A = 40 → isobars.
Common mistakes
- Saying A = number of neutrons. A = protons + neutrons; N = A − Z.
- For a positive ion adding electrons. Na⁺ LOST one electron: 11 − 1 = 10.
- Thinking canal rays are always protons. They are positive ions of whatever gas is in the tube; only hydrogen gives protons.
- Thinking Rutherford found the neutron. Rutherford proposed the nucleus; Chadwick discovered the neutron in 1932.