An atom is the basic unit of matter. It is made up of sub-atomic particles. Scientists developed different atomic models to explain the structure of the atom.
Sub-Atomic Particles
Particle
Charge
Mass (approx.)
Location
Electron
–1
Very small
Outside nucleus
Proton
+1
1 amu
Nucleus
Neutron
0
1 amu
Nucleus
Atomic Models
1. Thomson’s Model
Atom is a positively charged sphere.
Electrons are embedded like plums in pudding.
Could not explain stability of atom.
2. Rutherford’s Model
Atom has a small, dense, positively charged nucleus.
Electrons revolve around the nucleus.
Failed to explain why electrons do not fall into the nucleus.
3. Bohr’s Model
Electrons revolve in fixed circular paths called shells or orbits.
Each shell has a fixed energy.
Explains stability of atom.
Bohr’s Shells
Shells are named as K, L, M, N.
Maximum electrons in a shell = 2n² (where n = shell number)
Shell
Max Electrons
K (n=1)
2
L (n=2)
8
M (n=3)
18
Atomic Number (Z)
Number of protons in an atom.
Determines the identity of an element.
In a neutral atom: Protons = Electrons
Mass Number (A)
Total number of protons + neutrons.
Formula: A = Z + N
Isotopes
Atoms of the same element with same atomic number but different mass numbers.
Example: Hydrogen (¹H, ²H, ³H)
Uses of Isotopes
C-14: Carbon dating
Co-60: Cancer treatment
I-131: Thyroid treatment
Isobars
Atoms of different elements with same mass number but different atomic numbers.
Example: Calcium-40 and Argon-40
Valency
Combining capacity of an atom.
Depends on electrons in the outermost shell.
Electron Distribution (Example)
Oxygen (Z = 8) K shell = 2 electrons L shell = 6 electrons