# Summary

- Atoms are the building blocks of elements. They are the smallest part of an element that react chemically
- Atoms are divisible into still smaller particles called sub-atomic particles. They are electron, proton and the neutron.
- Thomsons plumb-pudding model was discarded and Rutherford's model came into being. His model did not stand the test of time as it was having a flaw. According to his theory any revolving electron in circular orbit will radiate energy and finally collapse into the nucleus. Such a situation did not exist. Hence his theory was modified by Boh'r.
- Bohr's model was able to explain the spectra of hydrogen.
- Bohr postulated that electron moves around the nucleus in circular orbits. Only certain orbits can exist and each orbit correspond to a specific energy.
- As an orbit is a clearly defined path and this path can be defined only if we know the exact position and velocity of an electron simultaneously
- Boh'r model of the hydrogen atom not only ignores the dual behaviour of electron but also contradicts Heisenberg's uncertainity principle.
- Schrodinger proposed an equation incorporating de Broglie concept of wave -particle duality and which is in conformity with Heisenbergs uncertainity principle.
- Solution to Schrondinger equation gave us the quantum numbers
- The quantum numbers are, principal (n) azimuthal (l) magnetic (ml) and spin (ms)
- Electron distribution of an atom containing a number of electrons is divided into shells.
- Shells are thought to be divided into sub-shells
- If two orbitals have the same n+l values the orbital with the lower value of 'n' has the lower energy .
- According to Pauli exclusion principle; no two electrons in an atom can have the same set of four quantum numbers
- Hund's rule states that pairing of electrons take place only after vacant orbitals are filled with at least one electron.