NCERT Chemistry Class 11 Shapes of Orbital Important Notes – Unit 2
Chemistry notes for Class 11: The s Orbital
An orbital with l = 0 (angular momentum quantum number, which describes the shape of an orbital) and thus, m= 0 (a magnetic quantum number which describes the orientation of orbital in space around the nucleus) is called s orbital. Since m has one value this means the s orbital can have only one orientation and the sphere can be defined completely by a single value i.e. radius. So, the s orbital are spherically symmetrical about the nucleus. The size of s orbital depends on the value of the principal quantum number. Therefore 1s orbital is smaller than 2s orbital, but both are spherical in shape.
Shapes and size of s orbitals
The symmetry of s orbitals along three axes x,y,z
Between two regions of the high probability of electrons is a spherical node, it is the region where the probability of finding electrons is zero.
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The p Orbital
The maximum value of l is n – 1, so the only levels with n = 2 or higher have a p orbital. 2p orbital is closest to the nucleus. Unlike s orbitals, p orbitals have different spatial orientations. If n =2, the values of l= 2-1 i.e. 1. So, the values of m will be -1, 0, +1, i.e. there are three values which suggest that there are three p orbitals because a number of m values give the number of orbitals. The p orbital has two regions or lobes of the high probability of finding electrons, one on either side of the nucleus. This makes the orbital look like two pears joined together at their narrow ends. The whole arrangement appears like a dumb- bell. The nucleus lies at the nodal plane of this dumbbell shaped orbital.
While identical in size, shape, and energy, the three p orbitals differ in orientations. The px orbital lies along the x-axis, py orbital lies on y-axis and pz orbital lies along z-axis. Similar to the pattern of s orbital, the a3p orbital is bigger than 2p orbital and 4p orbital is bigger than 3p orbital and so forth.
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The d Orbital
For n = 3, l can have three values, 0, 1, 2. We have discussed the shape and orientations of orbitals when n = 1 and n = 2. When n =3, then l= 3-1 i.e. 2. This means m -2, -1, 0, +1, +2, hence there will be five d orbitals. The five d orbitals possess equal energies and differ only in their orientations in space.Four of the five orbitals have four lobes (a cloverleaf shape) with two mutually perpendicular nodal planes between them and nucleus at the junction of the lobes.
Three of these orbitals lie in the xy, xz and yz planes, with their lobes between the axes, and are called dxy, dxz, and dyz orbitals. A fourth, dx2 – y2orbital, also lies in the xy plane, but its lobes are along the axes. The fifth d orbital, the dz2, has two major lobes along the z-axis, and a donut-shaped region in the center. An electron in the d orbital spends equal time in all of its lobes.
Orbitals with higher values
When n = 4, then l = 3, this means m = -3, -2, -1, 0, 1, 2, 3. Hence there are seven f orbitals. Each f orbital has a complex, multi-lobed shape with several nodal planes.
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