Edexcel A Level Chemistry:复习笔记1.2.3 Orbitals

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

1.1-Atomic-Structure-Orbitals 

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

1.1-Atomic-Structure-Summary

 

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

     

1.1-Atomic-Structure-Electron-Spin

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

     

1.1-Atomic-Structure-Electron-Configuration-1

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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