Lesson 3 of 12 · 8 min
Atomic number, isotopes and isobars
NCERT §2.2.3; §2.2.4
The sodium in the street lamp, the strontium that paints a rocket red and the hydrogen in the lab tube: how do you count what is inside each of them?
The lesson in notes
In short
Atomic number Z = number of protons in the nucleus, which equals the number of electrons in a neutral atom (H: Z = 1; Na: Z = 11).
Protons and neutrons together are nucleons; mass number A = protons + neutrons, so neutrons = A − Z whether the species is neutral or an ion.
An atom is written with A as a left superscript and Z as a left subscript on the symbol, e.g. ⁸⁰₃₅Br has 35 protons, 35 electrons and 45 neutrons.
For an ion, compare protons and electrons first: 16 protons, 16 neutrons and 18 electrons is ³²₁₆S²⁻, an anion carrying the two extra electrons as its charge.
Isotopes have the same Z and different A, so they differ only in neutron count. Hydrogen: protium ¹H (99.985%), deuterium ²H or D (0.015%), and tritium ³H, found in trace amounts.
Other isotopes: carbon with 6, 7 and 8 neutrons (¹²C, ¹³C, ¹⁴C) and chlorine with 18 and 20 neutrons (³⁵Cl, ³⁷Cl).
Isobars have the same A and different Z, such as ¹⁴₆C and ¹⁴₇N.
Chemical behaviour is set by the electrons, which the proton count fixes; neutrons hardly affect it, so isotopes of an element react alike.