Orbitals (Edexcel AS Chemistry): Revision Note

Orbital Shapes

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

Filling Orbitals

• Electrons can be imagined as small spinning charges which rotate around their own axis in either a clockwise or anticlockwise direction

  • The spin of the electron is represented by its direction

  • The spin creates a tiny magnetic field with N-S pole pointing up or down

Electrons can spin either in a clockwise or anticlockwise direction around their own axis

• Electrons with the same spin repel each other which is also called spin-pair repulsion

  • Therefore, electrons will occupy separate orbitals in the same subshell first to minimise this repulsion and have their spin in the same direction

  • They will then pair up, with a second electron being added to the first p orbital, with its spin in the opposite direction

• This is known as Hund's Rule

  • E.g. if there are three electrons in a p subshell, one electron will go into each p_x, p_y and p_z orbital

Electron configuration: three electrons in a p subshell

• The principal quantum number indicates the energy level of a particular shell but also indicates the energy of the electrons in that shell

  • A 2p electron is in the second shell and therefore has an energy corresponding to n = 2

• Even though there is repulsion between negatively charged electrons, they occupy the same region of space in orbitals

• An orbital can only hold two electrons and they must have opposite spin - the is known as the Pauli Exclusion Principle

• This is because the energy required to jump to a higher empty orbital is greater than the inter-electron repulsion

• For this reason, they pair up and occupy the lower energy levels first