Atomic Terms

 

Atomic terms

a) Atomic Number (Z): The atomic number of an element is the number of protons contained in the nucleus of the atom of that element.

b) Nucleons: Protons and neutrons are present in a nucleus. So, these fundamental particles are collectively known as nucleons.

c) Mass Number (A): The total number of protons and neutrons i.e,  the number of nucleons present in the nucleus is called the mass number of the element.

d) Nuclide: Various species of atoms in general. A nuclide has specific values of atomic number and mass number.

IUPAC notation of an atom (nuclide)

Let X be the symbol of the element. Its atomic number be Z and mass number be A. Then  the element can be represented as  

e) Isotopes: Atoms of the element with same atomic number but different mass number e.g. ₁H¹, ₁H², ₁H³. There are three isotopes of hydrogen.

f) Isobars: Atoms having the same mass number but different atomic numbers, e.g. ₁₅P³² and ₁₆S³² are called isobars.

g) Isotones: Atoms having the same number of neutrons but different number of protons or mass number, e.g. ₆C¹⁴, ₈O¹⁶, ₇N¹⁵ are called isotones.

h) Isoelectronic: Atoms, molecules or ions having same number of electrons are isoelectronic  e.g. N₂,CO, CN^–.

i) Nuclear isomer: Nuclear isomers (isomeric nuclei) are the atoms with the same atomic number and same mass number but with different radioactive properties.

Example of nuclear isomers is

Uranium –X (half life 1.4 min) and

Uranium –Z (half life 6.7 hours)

The reason for nuclear isomerism is the different energy states of the two isomeric nuclei.

Other examples are

₃₀Zn⁶⁹  ₃₀Zn⁶⁹

(T_1/2 = 13.8 hr) (T_1/2 = 57 min)

  ₃₅Br⁸⁰   ₃₅Br⁸⁰

(T_1/2 = 4.4 hour) (T_1/2 = 18 min)

j) Isosters : Molecules having  same number of atoms and also same number of electrons are called isosters.

E.g., (i) N₂  and CO

ii) CO₂ and N₂O

iii) HCl  and F₂

k) Atomic mass unit: Exactly equal to 1/12 of the mass of ₆C¹² atom

1 amu = 1.66 ´10^–27 kg  = 931.5 MeV