Atomic models

What do the atomic models say?

In plain words: once electrons were known, the question was how the positive charge and the mass are arranged around them. Thomson spread them evenly through the atom. Rutherford’s experiment showed instead that they are packed into a nucleus about 100 000 times smaller than the atom.

Thomson’s model

In 1898 J.J. Thomson proposed that an atom is a sphere of uniform positive charge in which the electrons are embedded so as to give the most stable arrangement (NCERT Section 2.2.1, page 33). It has been called the plum pudding, raisin pudding or watermelon model: a pudding of positive charge with electrons as the plums, or a watermelon with electrons as the seeds.

An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom.

NCERT Chemistry Part I (Class 11), Section 2.2.1, page 33

The model explained why atoms are electrically neutral, but it failed the test of later experiments. Thomson received the 1906 Nobel Prize in Physics for his work on the conduction of electricity through gases (page 33).

Two discoveries of the 1890s gave the tools for that test (pages 33–34):

• X-rays. Wilhelm Röntgen (1845–1923) found in 1895 that electrons striking a material in a cathode ray tube produce rays that make fluorescent materials glow outside the tube. These X-rays are not bent by electric or magnetic fields, penetrate matter to a degree depending on photon energy and material thickness and have very short wavelengths, about 0.1 nm. NCERT spells his name “Wilhalm Röentgen”.

• Radioactivity. Henri Becquerel (1852–1908) found that some elements give off radiation on their own; Marie Curie, Pierre Curie, Rutherford and Frederick Soddy developed the field. (NCERT misspells three of these names: “Becqueral”, “Piere” and “Fredrick”.) The radiation is of three kinds:

  • α-rays: particles with two units of positive charge and mass 4 u, helium nuclei;
  • β^⁻-rays: fast-moving electrons (the negative beta radiation described here by NCERT); some radioactive decays instead emit β^⁺ particles, positrons;
  • γ-rays: high-energy electromagnetic radiation like X-rays, neutral, not material particles.

  For typical radioactive sources, alpha particles have the shortest range, beta particles penetrate farther, and gamma rays are more penetrating. NCERT quotes rough factors of 100 and 1000, but these are not universal constants: penetration depends on particle/photon energy, material and thickness.

Rutherford’s scattering experiment

Rutherford and his students Hans Geiger and Ernest Marsden fired a beam of high-energy α-particles from a radioactive source at a very thin gold foil, about 100 nm thick. A circular zinc sulphide screen around the foil gave a flash of light wherever an α-particle struck it (NCERT Section 2.2.2, page 34).

On Thomson’s model, the mass of each gold atom is spread evenly, so the α-particles should have passed through, slowing slightly and changing direction only by small angles. Instead (page 34):

1. most α-particles passed through the foil undeflected;
2. a small fraction were deflected by small angles;
3. a very few, about 1 in 20 000, bounced back, deflected by nearly 180°.

Rutherford concluded (pages 34–35):

Most of the space in the atom is empty as most of the α–particles passed through the foil undeflected.

NCERT Chemistry Part I (Class 11), Section 2.2.2, page 34

• The few large deflections need an enormous repulsive force, so the positive charge is not spread through the atom but concentrated in a very small volume.
• That volume, the nucleus, is tiny: the atom’s radius is about 10^⁻¹⁰ m and the nucleus’s about 10^⁻¹⁵ m. If a cricket ball were the nucleus, the atom’s radius would be a few kilometres (NCERT says about 5 km).

His nuclear model (page 35):

• the positive charge and most of the mass are concentrated in an extremely small nucleus;
• electrons move round the nucleus at very high speed in circular paths, called orbits, like planets round the sun;
• electrons and nucleus are held together by electrostatic attraction.

Atomic number and mass number

The positive charge of the nucleus is carried by its protons, each with a charge equal and opposite to an electron’s (NCERT Section 2.2.3, page 35). The neutrons add mass but no charge.

In a neutral atom the number of electrons equals Z: hydrogen has 1 proton and 1 electron, sodium 11 of each (page 35). An atom is written with A as a left superscript and Z as a left subscript, as in ²³₁₁Na.

For an ion, first decide whether it is neutral, a cation or an anion (NCERT note, page 36):

• number of protons = Z, always;
• number of neutrons = A − Z, always;
• number of electrons = Z for a neutral atom, Z − (charge) for a cation, Z + (size of the charge) for an anion.

Isobars and isotopes

• Isotopes are atoms of the same element, with the same Z, but different mass numbers, because they have different numbers of neutrons (NCERT Section 2.2.4, page 35).
• Isobars are atoms of different elements with the same mass number, such as ¹⁴₆C and ¹⁴₇N.

NCERT’s examples of isotopes (page 35):

• hydrogen: protium, ¹₁H (1 proton, 99.985% of hydrogen atoms); deuterium, ²₁D (1 proton and 1 neutron, 0.015%); tritium, ³₁T (1 proton and 2 neutrons, only in traces);
• carbon: ¹²₆C, ¹³₆C and ¹⁴₆C, with 6, 7 and 8 neutrons;
• chlorine: ³⁵₁₇Cl and ³⁷₁₇Cl, with 18 and 20 neutrons.

Therefore, all the isotopes of a given element show same chemical behaviour.

NCERT Chemistry Part I (Class 11), Section 2.2.4, page 35

The reason for their broadly similar chemistry is their matching electron arrangements in the same charge state (page 35). The quoted statement is an approximation: different isotope masses can change reaction rates and equilibrium constants, especially for hydrogen versus deuterium (IUPAC Gold Book, isotope effect). Natural isotope mixtures are why atomic masses are averages, and why Dalton’s idea of identical atoms had to change.

Drawbacks of Rutherford’s model

Rutherford’s atom looks like a small solar system, and the electrostatic force between electron and nucleus, kq₁q₂/r^², has the same form as gravity between the sun and a planet (NCERT Section 2.2.5, page 36). But there is a crucial difference. A body moving in a circle is accelerating even at constant speed, because its direction keeps changing, and Maxwell’s electromagnetic theory says that an accelerating charge radiates energy. Planets are uncharged; electrons are not. So the electron should lose energy, spiral inwards and fall into the nucleus.

Thus, the Rutherford model cannot explain the stability of an atom.

NCERT Chemistry Part I (Class 11), Section 2.2.5, page 36

NCERT says the fall would take only 10^⁻⁸ s (page 36). The standard classical estimate for the hydrogen atom is shorter still, about 10^⁻¹¹ s (beyond the NCERT text); either way, atoms would not last. In exams, quote NCERT’s 10^⁻⁸ s. A spiralling electron would also give out a continuous range of frequencies, whereas atoms give line spectra (beyond NCERT Section 2.2.5). Electrons cannot simply stand still either, since attraction would then pull them into the nucleus (page 37). The model also says nothing about how the electrons are arranged round the nucleus or what their energies are (page 37). Bohr’s model, which followed, tackled both problems.

Solved examples

Common mistakes to avoid

Quick revision summary

• Thomson (1898): uniform sphere of positive charge with embedded electrons; mass spread evenly.
• Rutherford’s α-scattering: most pass through, a few deflected, about 1 in 20 000 bounce back. So the atom is mostly empty, with a tiny, dense, positive nucleus (about 10^⁻¹⁵ m against 10^⁻¹⁰ m for the atom).
• Nuclear model: electrons orbit the nucleus, held by electrostatic attraction.
• Z = number of protons (= electrons in a neutral atom); A = Z + number of neutrons.
• Isotopes: same Z, different A, broadly similar chemistry. Isobars: same A, different Z.
• Rutherford’s model fails: an orbiting (accelerating) electron should radiate and fall into the nucleus, and the model says nothing about electron energies.

Questions students ask

What are the main atomic models in order?

The main atomic models, in order, are Dalton’s indivisible atom (1808), Thomson’s plum pudding model (1898 in NCERT, often dated 1904), Rutherford’s nuclear model (1911), Bohr’s model (1913) and the quantum mechanical model (1926), used today. Each earlier model fell to a result it could not explain: electrons ended the indivisible atom, α-particle scattering ended Thomson’s, atoms’ stability and line spectra ended Rutherford’s, and the electron’s wave nature ended Bohr’s.

What is the difference between Thomson’s and Rutherford’s models of the atom?

Thomson’s model spreads the atom’s positive charge and mass evenly through a sphere about 10^⁻¹⁰ m in radius, with the electrons embedded in it, while Rutherford’s model packs all the positive charge and nearly all the mass into a nucleus about 10^⁻¹⁵ m in radius, with the electrons moving round it through mostly empty space. Thomson’s atom could turn α-particles only slightly, so it could not explain the few that bounced back from gold foil; Rutherford’s dense nucleus could. Neither model fixed the energies of the electrons or explained line spectra, which Bohr’s model tackled in 1913.

Why did Rutherford use gold foil in his scattering experiment?

Rutherford’s team used gold because it is the most malleable metal, so it can be beaten into a foil only about 100 nm thick, a few hundred atoms, thin enough for most α-particles to pass straight through and for each large deflection to come from a single nucleus. A gold nucleus also has a large charge, +79e, and is about 50 times as heavy as an α-particle, so it can turn an α-particle through a large angle, even back, while hardly recoiling. Geiger and Marsden found that large-angle scattering grew with the atomic mass of the foil’s metal, and so with its nuclear charge, as the nuclear model predicts.

What are the limitations of Thomson’s model of the atom?

The main limitation of Thomson’s model is that it cannot explain Rutherford’s α-particle scattering: with its charge and mass spread evenly, no part of the atom could turn a fast α-particle through a large angle, yet about 1 in 20 000 bounced back from gold foil. Nor could it explain the line spectra that atoms emit. Its one success was explaining why an atom is electrically neutral.

Are Thomson’s and Rutherford’s models in the JEE Main 2027 and NEET (UG) 2026 syllabuses?

Thomson’s and Rutherford’s models are not named in the Chemistry Unit 2 (Atomic Structure) text of the JEE Main 2027 or NEET (UG) 2026 syllabus, which starts from the nature of electromagnetic radiation, but Rutherford’s model is in both Physics syllabuses: Unit 18, Atoms and Nuclei, lists the ‘alpha-particle scattering experiment’ and ‘Rutherford’s model of atom’. Thomson’s model is named in neither. Atomic number, mass number and isotopes are used all through both exams, so the basics of NCERT Section 2.2 are still needed.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Write the complete symbol for the atom with (i) Z = 17, A = 35; (ii) Z = 92, A = 233; (iii) Z = 4, A = 9 (NCERT Exercise 2.4, page 69).

   Show answerHide answer

   (i) ³⁵₁₇Cl; (ii) ²³³₉₂U; (iii) ⁹₄Be.

2. Q2. An element with mass number 81 contains 31.7% more neutrons than protons. Assign its atomic symbol (NCERT Exercise 2.42, page 71).

   Show answerHide answer

   Let protons = p; neutrons = 1.317p. Then p + 1.317p = 81, so p = 81/2.317 ≈ 35.0. Z = 35 is bromine: ⁸¹₃₅Br, with 46 neutrons. The supplied 31.7% is approximate: 46 is actually about 31.4% greater than 35; the nearest consistent integer assignment is bromine-81.

3. Q3. An ion with mass number 37 carries one unit of negative charge and has 11.1% more neutrons than electrons. Find its symbol (NCERT Exercise 2.43, page 71).

   Show answerHide answer

   Let protons = p; electrons = p + 1; neutrons = 1.111(p + 1). Then p + 1.111(p + 1) = 37, so 2.111p = 35.889 and p = 17.0. Z = 17 is chlorine: ³⁷₁₇Cl⁻, with 18 electrons and 20 neutrons.