CBSE Class 9 Chemistry Structure of the Atom Notes Set 10

Electronic Configuration

1. Maximum electrons in a shell = 2n squared: K 2, L 8, M 18, N 32. The outermost shell holds at most 8 (2 for K), and a new shell starts only after the one before is complete up to 8 for the first 20 elements.
2. Valency is the number of electrons lost, gained or shared to complete the outer shell. Na (2, 8, 1) has valency 1, Mg (2, 8, 2) has valency 2, Cl (2, 8, 7) has valency 1 and Ne has valency 0.
3. Atomic number Z = number of protons = number of electrons in a neutral atom. Mass number A = protons + neutrons. The notation is mass number on top and atomic number below the symbol.

Isotopes, Isobars and Radioactivity

1. Average atomic mass = sum of (isotopic mass x fractional abundance). This is why many atomic masses are not whole numbers, for example chlorine is 35.5 u.
2. Radioactive nuclei are unstable and emit alpha (helium nucleus, mass -4 and charge -2), beta (electron, atomic number +1) and gamma rays (no change). Gamma is the most penetrating and alpha the least.
3. Half-life is the time for half the nuclei to decay. After n half-lives the fraction left is (1/2) to the power n.
4. Uses of isotopes: cobalt-60 for cancer treatment, iodine-131 for thyroid problems, carbon-14 for dating. Fusion of hydrogen into helium powers the Sun.