# Atomic Structure

Atom is the smallest particle of an element that may or may not exist independently.
E.g. Iron (Fe), calcium (Ca) and hydrogen (H).

Molecule is the smallest particle of an element or compound that exists independently.
E.g. Oxygen (O2), nitrogen (N2) and water (H2O).
Mole is the collection of 6.022140857 × 1023 particles.
1 mole = Avagadro's number = 6.022140857 × 1023.
1 mole atom = 6.022140857 × 1023 atoms.
1 mole molecule = 6.022140857 × 1023 molecules.

## Constituents of the Atom

Constituents of the atom are proton, neutron and electron.

### 1. Proton

• Discovered by Goldstein and named by Ernest Rutherford.

### 3. Electron

• Discovered by J.J. Thompson and named by Johnstone Stoney.
Proton Neutron Electron
Found inside nucleus inside nucleus outside nucleus
Relative charge +1 0 -1
Absolute charge + 1.6 x 10-19 coulomb
+ 4.8 x 10-10 esu
0 - 1.6 x 10-19 coulomb
- 4.8 x 10-10 esu
Relative mass 1.00763 1.00863 0.000543
Absolute mass 1.673 x 10-24 grams
1.673 x 10-27 kg
1.675 x 10-24 grams
1.675 x 10-27 kg
9.1 x 10-28 grams
9.1 x 10-31 kg
p = No.of protons
n = No.of neutrons
e = No.of electrons
Atomic number (Z) = p = e
Mass number (A) = p + n
Atomic symbol = X
Representation:
${}_{Z}{}^{A}X$ .
In
${}_{\mathrm{15}}{}^{\mathrm{31}}P$ ,
Z = 15,
A = 31,
n = Z - A = 31 - 15 = 16
Isotopes has same atomic number but different mass number.
E.g.
${}_{7}{}^{12}N$ ;
${}_{7}{}^{14}N$ ;
${}_{7}{}^{16}N$ .
Isobars has same mass number but different atomic number.
E.g.
${}_{6}{}^{14}C$ ;
${}_{7}{}^{14}N$ .
Isotones has same number of neutrons.
E.g.
${}_{15}{}^{31}P$ ;
${}_{16}{}^{32}S$ .