Structure of Atom

In 1897, J.J. Thomson performed cathode ray tube experiments and discovered the electron — a negatively charged particle much lighter than an atom. This was the first evidence that atoms are not indivisible.

Cathode rays travelled from the negative electrode (cathode) to the positive electrode (anode) in vacuum, and were deflected by electric and magnetic fields. Thomson showed the rays were streams of negatively charged particles.

The electron has a mass of about 9.1 × 10⁻³¹ kg — nearly 2000 times lighter than a hydrogen atom. The charge on an electron is −1.6 × 10⁻¹⁹ C.

In 1886, E. Goldstein used a perforated cathode in a discharge tube and discovered canal rays (also called anode rays). These rays travelled from the anode toward the cathode.

Canal rays were deflected by electric and magnetic fields in the direction opposite to cathode rays, showing they were positively charged. The lightest positive ray was obtained with hydrogen gas — this positive particle is called the proton.

The proton has a mass of about 1.67 × 10⁻²⁷ kg (approximately 1 u) and a charge of +1.6 × 10⁻¹⁹ C, equal in magnitude but opposite in sign to the electron.

After discovering both electrons and the presence of positive charge in atoms, J.J. Thomson proposed his atomic model in 1904.

Thomson suggested that an atom is a positively charged sphere with electrons embedded inside it, like plums in a pudding or seeds in a watermelon. The atom is overall neutral because the negative charge of the electrons balances the positive charge of the sphere.

This model is sometimes called the "plum pudding model" or the "raisin bread model." It was useful as an early attempt, but it could not explain the results of Rutherford's later scattering experiment.

In 1909, Ernest Rutherford, along with Hans Geiger and Ernest Marsden, performed the famous gold foil experiment. A thin gold foil was bombarded with fast-moving alpha particles (positively charged helium nuclei).

Observations: (1) Most alpha particles passed straight through the foil without any deflection. (2) Some alpha particles were deflected at small angles. (3) A very few alpha particles — about 1 in 20,000 — bounced straight back.

Rutherford was surprised by the back-scattering. He famously said it was "as if you fired artillery shells at tissue paper and they came back and hit you."

Based on his experiment's results, Rutherford proposed a nuclear model of the atom in 1911.

Key features: (1) An atom has a very small, dense, positively charged nucleus at its centre. (2) The nucleus contains nearly all the mass of the atom. (3) Electrons revolve around the nucleus in circular orbits at high speed. (4) Most of the atom is empty space — the electrons are very far from the nucleus relative to atomic size.

The nuclear model explains all three observations from the experiment. The empty space explains most particles passing through. The concentrated nucleus explains the rare back-scattering.

Rutherford's model had a major flaw. According to classical electromagnetic theory, a charged particle (electron) undergoing circular motion (acceleration) should continuously emit radiation and lose energy. This would cause the electron to spiral inward and collapse into the nucleus within a tiny fraction of a second.

But atoms are stable — they do not collapse. Rutherford's model could not explain this stability.

Also, if electrons could spiral at any radius, they should emit a continuous spectrum. But atoms actually emit specific, discrete spectral lines (like hydrogen's visible spectrum lines). Rutherford's model could not explain the discrete nature of these spectral lines.

In 1913, Niels Bohr proposed an improved model to address the drawbacks of Rutherford's model, specifically for the hydrogen atom.

Key postulates: (1) Electrons revolve around the nucleus only in certain allowed circular orbits called shells or energy levels. (2) As long as an electron stays in a fixed shell, it does not emit or absorb energy. (3) Energy is emitted or absorbed only when an electron jumps from one shell to another. The shells are designated K, L, M, N... or numbered 1, 2, 3, 4...

Bohr's model successfully explained the hydrogen atom spectrum and introduced the concept of fixed shells, which is still used in Class 9 and Class 10 chemistry to understand electronic configuration, valency, and chemical bonding.

In 1932, James Chadwick discovered the neutron — an electrically neutral particle inside the nucleus with a mass very close to that of the proton (≈ 1 u).

The neutron was the last of the three fundamental subatomic particles to be discovered. Its existence explained why nuclei of the same element can have different masses (isotopes) — they have the same number of protons but different numbers of neutrons.

Neutrons have no charge, so they are not deflected by electric or magnetic fields and were harder to detect than protons and electrons.

Atomic number (Z) is the number of protons in the nucleus of an atom. It uniquely identifies the element. In a neutral atom, atomic number also equals the number of electrons.

Mass number (A) is the total number of protons and neutrons (collectively called nucleons) in the nucleus. So: number of neutrons = A − Z.

For example, sodium has Z = 11 and A = 23. So sodium has 11 protons, 11 electrons (neutral atom), and 23 − 11 = 12 neutrons.

Electrons fill the shells of an atom in a fixed sequence. The maximum number of electrons each shell can hold is given by 2n² where n is the shell number.

K shell (n=1): maximum 2 electrons. L shell (n=2): maximum 8 electrons. M shell (n=3): maximum 18 electrons (but at Class 9 level, the M shell is treated as holding a maximum of 8 electrons for the first 18 elements).

Electrons fill the innermost shell first and move outward. The outermost occupied shell is called the valence shell.

Valency is determined by the number of electrons in the outermost shell (valence electrons). Atoms with 1, 2, or 3 valence electrons tend to lose them (valency = 1, 2, 3). Atoms with 5, 6, or 7 valence electrons tend to gain electrons (valency = 3, 2, 1 respectively).

Atoms with 4 valence electrons can share electrons (valency = 4). Atoms with 8 valence electrons (like noble gases) have a full outer shell and valency = 0 — they are chemically inert.

Sodium (2,8,1) has 1 valence electron → valency 1. Oxygen (2,6) has 6 valence electrons → needs 2 more → valency 2. Chlorine (2,8,7) → needs 1 more → valency 1.

Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different number of neutrons). Since chemical properties depend on electron configuration, and isotopes of an element have the same number of electrons, isotopes have nearly identical chemical properties.

Hydrogen has three isotopes: protium (¹H, 0 neutrons), deuterium (²H, 1 neutron), and tritium (³H, 2 neutrons). All three are hydrogen with Z = 1.

Chlorine has two main isotopes: ³⁵Cl (17 protons, 18 neutrons) and ³⁷Cl (17 protons, 20 neutrons). Their natural abundance gives chlorine an average atomic mass of 35.5 u.

Radioactive isotopes have important applications: cobalt-60 is used in cancer treatment at hospitals across India; iodine-131 is used for thyroid diagnosis; carbon-14 is used in archaeological dating.

Isobars are atoms of different elements that have the same mass number but different atomic numbers. Since their atomic numbers differ, they have different numbers of protons and electrons, so they belong to different elements with different chemical properties.

Example: calcium-40 (Z = 20, A = 40) and argon-40 (Z = 18, A = 40) are isobars. Both have mass number 40, but calcium is a reactive metal while argon is a noble gas — very different elements with very different properties.

Subatomic particles: electron (−1 charge, ≈0 u), proton (+1 charge, 1 u), neutron (0 charge, 1 u).

Thomson: plum pudding. Rutherford: nuclear model (tiny +ve nucleus, electrons orbit outside). Bohr: fixed shells (K, L, M, N).

Atomic number Z = protons = electrons (neutral). Mass number A = protons + neutrons. Neutrons = A − Z.

Shell capacity: K=2, L=8, M=8 (first 18 elements). Valence electrons → valency.

Isotopes: same Z, different A → same element. Isobars: different Z, same A → different elements.