In the pre-16 model of the atom, students are introduced to electrons occupying energy levels. All electrons within the energy level have the same energy. This idea is modified and developed through introducing students to orbitals. 

Each atomic energy level contains a number of orbitals. An orbital is an area of space around the nucleus in which we have a high probability (90%) of finding an electron. Each orbital can contain a maximum of 2 electrons, with opposite spins.

An orbital can be defined by three factors:

• its energy level
• its shape (desginated with a letter: s, p ,d or f)
• its degenercy (the number of orbitals with the same energy)

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For the sake of visualisation, the orbitals are often drawn in 3D using Cartesian axes of x, y and z. The labelling of the orbits reflects their orientation in space. 

S - orbitals are spherical in shape.

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P-orbitals are labelled p_x, p_y and p_z. They have two lobes with a 'node' at the nucleus. 

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D-orbitals are labelled d_xy, d_xz, d_yz, d_z² and d_x²-y². Their shapes are harder to descibe but can be seen in the diagrams. 

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Ordering the orbitals

Within an energy level, the relative energies of the orbitals are as follows: s [less than] p [less than] d

As energy level increases, the size of the orbitals increases and they become more diffuse ie. a 1s orbital is smaller than a 2s orbital which is smaller than a 3s orbital etc.

The diagram below shows the ordering of the orbitals. The exact order of the 4s and 3d orbitals are a source of much debate and we will explore this in the coming pages. 

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The underlying explanation for the shape and arrangement of orbitals lies within quantum mechanics and goes beyond the scope of the course.