Shapes of Orbitals (Edexcel International A Level Chemistry)

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The Shape of Orbitals

Orbitals

  • We have seen before that the total number of electrons that each subshell in each orbital can hold are:
    • 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

Atomic Structure Orbitals, downloadable AS & A Level Chemistry revision notes

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

Atomic Structure Summary, downloadable AS & A Level Chemistry revision notes

 An overview of the shells, subshells and orbitals in an atom

Populating 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

Atomic Structure Electron Spin, downloadable AS & A Level Chemistry revision notes

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 px, py and pz orbital

    Atomic Structure Electron Configuration 1, downloadable AS & A Level Chemistry revision notes

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

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Philippa

Author: Philippa

Expertise: Chemistry

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.