Journey Inside the AtomClass 9 Science Important Points

Atoms are composed of three subatomic particles: protons ($p^+$) with a +1 charge, neutrons ($n^0$) with a 0 charge, and electrons ($e^-$) with a -1 charge. Protons and neutrons are in the nucleus, while electrons orbit it.

J. J. Thomson proposed the 'plum pudding' model, suggesting an atom is a sphere of uniform positive charge with negatively charged electrons embedded within it, similar to plums in a pudding.

Rutherford's experiment involved firing positively charged alpha particles at a thin gold foil. Most passed through, but some were deflected, proving that the atom is mostly empty space with a tiny, dense, positively charged nucleus.

Based on his experiment, Rutherford proposed a planetary model where electrons orbit a central, positive nucleus. However, this model could not explain why atoms are stable.

Niels Bohr proposed that electrons move in fixed circular paths called shells or energy levels (K, L, M, N) around the nucleus. Electrons in these shells do not radiate energy, which explains atomic stability.

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

The mass number (A) is the total number of protons and neutrons in the nucleus, collectively called nucleons. The formula is $A = (\text{Number of protons}) + (\text{Number of neutrons})$.

An element is represented by the notation ${ }_{Z}^{A} \text{X}$, where X is the element symbol, A is the mass number, and Z is the atomic number. For example, carbon is written as ${ }_{6}^{12} \text{C}$.

Electrons fill shells in order (K, L, M...). The maximum number of electrons a shell can hold is given by the formula $2n^2$, where 'n' is the shell number (1 for K, 2 for L, etc.).

Valency is the combining capacity of an atom. Atoms tend to achieve a stable configuration with 8 electrons in their outermost (valence) shell by gaining, losing, or sharing electrons.

Isotopes are atoms of the same element that have the same atomic number (same number of protons) but different mass numbers (different numbers of neutrons). For example, hydrogen has isotopes Protium (${ }_{1}^{1} \text{H}$), Deuterium (${ }_{1}^{2} \text{H}$), and Tritium (${ }_{1}^{3} \text{H}$).

Isobars are atoms of different elements that have different atomic numbers but the same mass number. For example, Argon (${ }_{18}^{40} \text{Ar}$) and Calcium (${ }_{20}^{40} \text{Ca}$) are isobars.

The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, based on their abundance. This is why atomic masses are often not whole numbers, like Chlorine's mass of $35.5 \text{ u}$.

J. J. Thomson discovered the electron in 1897. Ernest Rutherford discovered the nucleus in 1911. James Chadwick discovered the neutron in 1932.