Class 9 Science Chapter 8: Journey Inside the Atom — Notes, Examples & Practice

Atoms contain three main particles. The electron ($e^-$), discovered by J.J. Thomson in 1897 using cathode rays, has a negative charge (−1) and a tiny mass — about $\dfrac{1}{1836}$ of a proton's.

The proton ($p^+$) has a positive charge (+1) and a mass of about 1 u (atomic mass unit). Positively charged 'canal rays' (studied by E. Goldstein in 1886) led to its identification. The neutron ($n$), discovered by James Chadwick in 1932, has no charge and a mass of about 1 u.

Protons and neutrons sit together in the nucleus (they are called nucleons); electrons move around it. A neutral atom has equal numbers of protons and electrons, so the charges cancel.

Once the electron was known, Thomson proposed (1904) that an atom is a sphere of positive charge with electrons embedded in it, like seeds in a watermelon (often called the 'plum pudding' model). The positive and negative charges are equal, so the atom is neutral.

This model explained why atoms are neutral and that electrons can be removed from them.

Its limitation: it could not explain the results of Rutherford's gold-foil experiment, which showed that the positive charge is not spread out at all.

In 1911 Ernest Rutherford's team fired fast, positively charged alpha (α) particles at a very thin gold foil. Observations: (1) most α-particles passed straight through; (2) some were deflected by small angles; (3) a very few (about 1 in 12,000) bounced almost straight back.

Conclusions: (1) most of the atom is empty space; (2) the positive charge occupies a very small volume; (3) nearly all the mass and all the positive charge are concentrated in a tiny, dense nucleus — about $10^5$ times smaller than the atom. Electrons revolve around the nucleus in circular paths.

Limitation: according to classical physics, an electron moving in a circle should keep radiating energy and spiral into the nucleus in a fraction of a second, so atoms would be unstable. Real atoms are stable, so the model was incomplete.

Niels Bohr (1913) proposed that electrons move only in certain allowed orbits, called shells or energy levels, labelled K, L, M, N (n = 1, 2, 3, 4) from the nucleus outward. While in an allowed shell, an electron does not radiate energy, so the atom is stable.

Electrons can jump to a higher shell by absorbing energy and fall back by emitting it as light of a definite colour. This is why each element gives its own colour in a flame test — sodium yellow, copper blue-green.

Filling rules (for the first 18 elements): the maximum number in shell n is $2n^2$ (K = 2, L = 8, M = 18), the outermost shell holds at most 8 electrons, and shells fill in order — inner first. The electrons in the outermost shell are valence electrons. Valency is the combining capacity: the number of electrons an atom gains, loses or shares to complete its outer octet (or duplet). With 1–3 valence electrons, valency = number of valence electrons; with 5–7, valency = 8 − valence electrons. Full shells (He, Ne, Ar) give valency 0.

Atomic number (Z) = number of protons. It identifies the element: every carbon atom has 6 protons. In a neutral atom it also equals the number of electrons. Mass number (A) = protons + neutrons, so neutrons $= A - Z$. We write ${}^{A}_{Z}\text{X}$, e.g. ${}^{23}_{11}\text{Na}$ has 11 protons, 11 electrons and 12 neutrons.

Isotopes are atoms of the same element (same Z) with different mass numbers (different numbers of neutrons): ${}^{1}_{1}\text{H}$, ${}^{2}_{1}\text{H}$, ${}^{3}_{1}\text{H}$; ${}^{12}_{6}\text{C}$ and ${}^{14}_{6}\text{C}$; ${}^{35}_{17}\text{Cl}$ and ${}^{37}_{17}\text{Cl}$. They have the same chemical properties but different physical properties. Uses: cobalt-60 in cancer treatment, iodine-131 for thyroid problems, uranium-235 in nuclear reactors, carbon-14 for dating old remains.

Isobars are atoms of different elements with the same mass number: ${}^{40}_{18}\text{Ar}$ and ${}^{40}_{20}\text{Ca}$. The average atomic mass of an element with isotopes is a weighted mean: chlorine is about 75% Cl-35 and 25% Cl-37, so $0.75 \times 35 + 0.25 \times 37 = 35.5$ u.