3.1    Sub-shells and Atomic Orbitals

Principle quantum shell

 

• Electrons are arranged outside the nucleus in energy levels or principle quantum shell, n.

 

• The principal quantum shells are numbered according how far are they from the nucleus

 

 

• The lowest energy level, n = 1 is closest to the nucleus, the energy level n = 2 is further out, and so on

• The electrons in energy level further away from the nucleus have more energy and held less tightly by the nucleus.

 

• Electrons do not move in fixed circular paths, they occupy a space called the atomic orbitals

 

• The total number of electrons that can occupy any principal shell is 2n², where n is the principal quantum

 

Quantum sub-shells

 

• The principal quantum shells, apart from the first, are split into sub-shells. Each principle quantum shell contains a different number of sub-shells.

 

• The first energy level contains one sub-shell, the second energy level contains two and so on

 

• The sub-shells are distinguished by letters s, p, d, f and so on

 

• The energy of  electrons in the sub-shells increases in the order s < p < d < f

The impossibility of drawing orbits for electrons

 

• Heisenberg Uncertainty Principle says, you cannot know with certainty where an electron is and where it is going next

 

• This makes it impossible to draw out an orbit or pathway in which the electrons move

 

Atomic orbitals

 

• An atomic orbital is a region of space around the nucleus where the probability of finding a particular electron is maximum (>95%).

 

• The sub-shells are split further into orbitals where the electrons are placed

• The number of orbital in each sub-shell depends on the sub-shells.

1. s – one orbital {s}
2. p – three orbitals {px, py, pz}
3. d – five orbitals {dxy, dyz, dxz, dx²-y², dz²}

 

• Orbitals having the same energy are called degenerate orbitals. For example, px, pyand pz are degenerate orbitals

 

• The concept of orbitals arises from the fact that an electron has dual It behaves as a particle as well as a wave.

 

• In the nth principal quantum shell, there are n sub-shells, n² orbitals and a maximum of 2n² electrons

 

The s orbital

• All the s orbitals are spherical and non-directional.

 

• The shaded region represents the region in which the chance of finding the s electron is more than 95%.

 

• The size of the s orbital increases in the order 1s < 2s < 3s <4s

The p orbital

• All the p orbitals are dumb-bell shaped and directional.

• p orbitals are only available from the second principal quantum shell and onwards

 

• There are 3 types of p orbitals, px, py and pz. All 3 different types of p orbitals are perpendicular to each other along the x, y and z axes

 

• Going to a higher energy level, the ‘lobes’ of the p orbital become longer