Structure of Atoms
Bohr theory
Presented by Neil Bohr.
- Electrons of an atom can have only very distinct energy values. Therefore, the electrons are restricted to specific “energy levels” or “stationary states”.
- Electrons in these energy levels rotate about the nucleus in fixed orbits, without radiating or absorbing any energy.
- When an electron receives energy, it moves from a lower to a higher energy level.
- Emission of radiant energy (electromagnetic radiation) is due to the movement of the electrons from the higher level to the former lower level of energy.
Bohr Theory can’t be used to describe the energy characteristics of atoms containing many electrons. Consequently, in the Modern Atomic Theory, four quantum numbers were introduced to describe the energy levels.
Quantum numbers
Principal quantum number
The number given in Bohr’s original stationary states corresponds to the
principal quantum number (
Secondary quantum number
Represents the various secondary sub-levels within the main energy level.
Relates to the shape of the electron cloud. Denoted by
Magnetic quantum number
Represents the direction of maximum extension in space of the electron cloud in
the sub shells p, d, and f. which has the dumb - bell shape. Denoted by
Spin quantum number
Represents the electron spin.
Principles
Pauli’s exclusion principle
No two electrons in a single atom can have all four-quantum numbers the same.
Aufbau Principle
Sub-levels with the lowest energy are filled before those with higher energy.
Hund’s Rule
The sublevels p, d, and f are occupied by single electrons before any pairing of electrons with opposed spins take place.
Atomic bonding
Primary (strong) bonds
Form when valence electrons are present (outer shell not completely filled).
Ionic
Forms between highly electropositive elements (metals) and highly electronegative elements (non-metals). Non-directional.
Covalent
Forms within electronegative elements that are close to each other in the periodic table. Directional.
Metallic
Forms in metals. Arrangement of positive ion cores in a sea of electrons. Non-directional.
Secondary (weak) bonds
Form when there are no valence electrons (outer shell filled completely).
Van Der Waal’s
Forms between inert gases and between symmetric molecules. Forms because of the dipolarity caused inside molecules. Non-directional.
Hydrogen
Forms between polar covalently bonded molecules containing
Properties of bonds
| Property | Order |
|---|---|
| Hardness, Melting point, Crystalline percentage | Ceramics > Metals > Polymers |
| Directionality | Covalent > Ionic > Metallic |
| Brittleness | Ionic > Covalent > Metallic |