Electrons & Orbitals
Shells
- The arrangement of electrons in an atom is called the electron configuration
- Electrons are arranged around the nucleus in principal energy levels or principal quantum shells
- Principal quantum numbers (n) are used to number the energy levels or quantum shells
- The lower the principal quantum number, the closer the shell is to the nucleus
- So, the first shell which is the closest to the nucleus is n = 1
- The higher the principal quantum number, the greater the energy of the shell and the further away from the nucleus
- The lower the principal quantum number, the closer the shell is to the nucleus
- Each principal quantum number has a fixed number of electrons it can hold, as follows:
- n = 1 : up to 2 electrons
- n = 2 : up to 8 electrons
- n = 3 : up to 18 electrons
- n = 4 : up to 32 electrons
Subshells
- The principal quantum shells are split into subshells which are given the letters s, p and d
- Elements with more than 57 electrons also have an f shell
- The energy of the electrons in the subshells increases in the order s < p < d
- The order of subshells appear to overlap for the higher principal quantum shells as seen in the diagram below:
Electrons are arranged in principal quantum shells, which are numbered by principal quantum numbers
Orbitals
- Subshells contain one or more atomic orbitals
- Orbitals exist at specific energy levels and electrons can only be found at these specific levels, not in between them
- Each atomic orbital can be occupied by a maximum of two electrons
- This means that the number of orbitals in each subshell is as follows:
- s : one orbital (1 x 2 = total of 2 electrons)
- p : three orbitals ( 3 x 2 = total of 6 electrons)
- d : five orbitals (5 x 2 = total of 10 electrons)
- f : seven orbitals (7 x 2 = total of 14 electrons)
- The orbitals have specific 3-D shapes
s orbital shape
- The s orbitals are spherical in shape
- The size of the s orbitals increases with increasing shell number
- E.g. the s orbital of the third quantum shell (n = 3) is bigger than the s orbital of the first quantum shell (n = 1)
p orbital shape
- The p orbitals have a dumbbell shape
- Every shell has three p orbitals except for the first one (n = 1)
- The p orbitals occupy the x, y and z axes and point at right angles to each other, so are oriented perpendicular to one another
- The lobes of the p orbitals become larger and longer with increasing shell number
Representation of orbitals (the dot represents the nucleus of the atom) showing spherical s orbitals (a), p orbitals containing ‘lobes’ along the x, y and z axis
- Note that the shape of the d orbitals is not required
An overview of the shells, subshells and orbitals in an atom