In 1913, Niels Bohr proposed a model of the atom. He proposed that the electrons in an atom could only be in certain orbits, or energy levels, around the nucleus. Refinement of Bohr theory led to the modern theory of atomic structure based on quantum mechanics.

Bohr's model is based on particle theory.

As wave-particle duality which says that all micromatter particle exhibit dualitya new model is proposed.

Debroglie p = h/lambda

Uncertainty principle delta(p)* delta(x) >=h/4pi

The quantum mechanics model

Principal quantum number-Shell,Azimuthal quantum number-sublevel,Magnetic quantum number-orbital, spin quantum number

The orbits are called as shells. The energy level of orbits or shells increases as they increase in distance from the nucleus of the atom. The orbits or shells are represented by numbers as 1,2,3,4,5,6 or 7. They are represented by letters as K,L,M,N,O,P,Q.

Sublevel of an Orbit

The energy levels, or orbits or shells are further divided into sublevels, or subshells. These subshells are designated by letters: s for the first possible sublevel, p for the second possible sublevel, d for the third, f for the fourth, g for the fifth, and from here on they simply go in alphabets.

The number of sublevels of each energy level is equal to the number of the energy levels. This means energy level 1, the K shell will have only one sub levels ? s sublevel. The energy level 2, the L shell will have 2 sub levels ? s and p.


Orbitals

Sublevels have further divisions called orbitals. Electrons are found in these orbitals. Each orbital contains two electrons.

?s? sublevel has only one orbital. ?p? sublevel has 3 orbitals. ?d? sublevel has 5 orbitals. ?f? sublevel has 7 orbitals.

The two electons in each orbital spin in different directions.


Shape of Orbitals
1. Spherical shape for s.
2. Dumbbell shape for orbitals of p.
3. Four-lobed shape for orbitals of d.
4. Complex shape for all orbitals of higher sublevels.

Pauli's exclusion principle: No two electrons can have all four same quantum numbers

Electrons occupy the lowest energy sublevels that are available. This is known as ?aufbau? order or principles.


Hund?s rule says that, for any set of orbitals of equal energy say p orbitals of orbit 2, there is one electron is each orbital before the second electron enters or occupies an orbital.

The energy level of some sublevels at higher orbits is less than the some sublevels at lower orbitals. This fact is to be kept in mind when electron configuration is determined for any atom. The increasing order of energy levels of sublevels is:

1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f*, 5d, 6p, 7s, 5f*, 6d, 7p, 8s

 

 

Visible lines in H atom
spectrum are called the
BALMER series.

Atomic Spectra
When one heats up a gas, it emits light of various wavelengths. For monatomic gases formed of only one kind of atom the emission spectra contains only light of particular wavelengths, and for hydrogen the spectra obeys a very simple relation:

1/lambda = RH[(1/nf²) -(1/ni²) ]

where ni and nf are positive non-zero integers with ni > nf and RH is a constant called Rydberg's constant:
RH = 1.097 x 10^7 m - 1. (2)


One of the first such series of lines discovered which obey the rule was found by Balmer, which corresponded to nf = 2 and is in the visible light region.

 

certain phenomena of light can be explained when light is considered as composed of particle. Certain other phenomena can be explained by considering light as a wave.

Similarly micromatter like electrons have particle characteristics and wave characteristics.

 

 

Shape of Orbitals
1. Spherical shape for s.
2. Dumbbell shape for orbitals of p.
3. Four-lobed shape for orbitals of d.
4. Complex shape for all orbitals of higher sublevels.

 

 

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