CBSE Class 10 Chemistry Atomic Structure Notes Set 09

Structure of the Atom

An atom has a tiny, dense, positively charged nucleus containing protons and neutrons, with electrons moving around it in shells. Protons carry +1 charge, electrons carry −1 charge and neutrons carry no charge. Almost all the mass of the atom is in the nucleus, while most of the atom is empty space.
1. Atomic number Z = number of protons = number of electrons in a neutral atom.
2. Mass number A = protons + neutrons. Neutrons = A − Z.
3. Isotopes: same Z, different A (C-12 and C-14). Isobars: same A, different Z (Ca-40 and K-40). Isotones: same number of neutrons.

Electron Arrangement

Electrons occupy shells K, L, M, N. A shell numbered n can hold at most 2n² electrons: 2, 8, 18, 32. The outermost shell never holds more than 8 electrons (2 for the first period). The electrons in the outermost shell are valence electrons and decide the valency and chemical behaviour of the element.
Example: sodium (Z = 11) is 2, 8, 1; chlorine (Z = 17) is 2, 8, 7; argon (Z = 18) is 2, 8, 8.

Atomic Models in Brief

1. Dalton: atoms are indivisible particles.
2. Thomson: atoms contain electrons embedded in a positive sphere.
3. Rutherford: most alpha particles pass straight through a gold foil, so the atom is mostly empty with a small positive nucleus.
4. Bohr: electrons revolve only in certain stationary orbits without losing energy.
5. Chadwick: discovered the neutron; Moseley: established atomic number.

Atomic Weight, Molecule and Mole

The atomic weight of an element is the average mass of its atoms compared with 1/12 of the mass of a carbon-12 atom. It is usually fractional because most elements are mixtures of isotopes (for chlorine 75% Cl-35 and 25% Cl-37 give 35.5). A molecule is the smallest particle of a substance that can exist independently and keep its properties.
One mole of any substance contains 6.022 × 10²³ particles (Avogadro's number) and has a mass equal to its atomic or molar mass in grams.
Number of moles = mass in g / molar mass = number of particles / 6.022 × 10²³.
For gases at STP one mole occupies 22.4 litres.

Symbols, Valency and Formulae

A symbol is a short form of the name of an element (Na for sodium from natrium, Fe for iron from ferrum). Valency is the combining capacity of an atom: the number of electrons it loses, gains or shares. To write a formula, write the symbols with their valencies and cross them over, then simplify. Example: aluminium (3) and oxygen (2) give Al₂O₃; calcium (2) and nitrogen (3) give Ca₃N₂.
Molecular weight is the sum of the atomic weights of all atoms in a formula, for example H₂SO₄ = 2 + 32 + 64 = 98.