Electron Configuration

• Energy increases as the shell number increases.
• The shell number is called the principal quantum number.
• Subshells are labelled with letters (s, p, d, f).
• • Shell 1 holds 2 electrons.
  • Shell 2 holds 8 electrons.
  • Shell 3 holds 18 electrons.
  • Shell 4 holds 32 electrons.
• Shells are made up of atomic orbitals.
• An atomic orbital is a region around the nucleus that can hold up to 2 electrons, with opposite spins.
• Orbitals can hold no more than 2 electrons, so the number of electrons in an orbital is always 2.
• Different types of orbitals (s-, p-, d-, and f-) each have different shapes.
• S-orbitals are spherical.
• Every shell contains an s-orbital.
• There are three p-orbitals perpendicular to one another.
• The bigger the shell number, the bigger the radius of its s-orbital.
• The p-orbital is the shape of a dumbbell/infinity symbol.
• There are three p-orbitals at right angles to each other.
• Each shell from n=2 contains 3 p-orbitals.
• The bigger the shell number, the further the p-orbital is from the nucleus.
• Each shell from n=3 contains 5 d-orbitals.
• Orbitals of the same type are grouped together as sub-shells.
• Electrons prefer lower energy levels so they will occupy orbitals with lowest possible energy first.
• Sub-shells have different energies due to their distance from the nucleus.
• When electrons fill up an orbital, it becomes full (100%).
• Why do two electrons in an orbital have opposite spin?

  To counteract repulsion.
• The Aufbau principle states that when filling up electron configurations, electrons go into the lowest available energy level first.
• Hund's rule states that if there is more than one empty orbital at the same energy level, then all electrons will be paired except for those in the last half filled sub-level which will remain unpaired until all other orbitals are fully occupied.
• The ground state is the most stable electronic configuration of an atom which has the lowest amount of energy.
• The writing of an electron configuration based on electrons entering the lowest energy level available is known as the Aufbau Principle.
• Electrons occupy sub-shells in order of increasing energy levels.
• S block elements have the highest energy electron in an s orbital.
• P block elements have the highest energy electron in a p orbital.
• F block elements have the highest energy electron in an f orbital.
• D block elements have the highest energy electron in a d orbital.
• The transition metals fill the 4s subshell before the 3d subshell, but they also lose electrons from the 4s first rather than from the 3d subshell.
• The electron configuration of H is $1s^2$
• The electron configuration of N is $1s^22s^22p^3$
• The electron configuration of Ar is $1s^22s^22p^63s^23p^6$
• The electron configuration of Br- is $1s^22s^22p^63s^23p^64s^23d^104p^6$
• There are 2 exceptions to the Aufbau Principle are Chromium and Copper as their configurations are more energetically favourable.
• The electron configuration can also be represented using the orbital spin diagrams. Each box represents an atomic orbital. The boxes are arranged in order of increasing energy from lower to higher. The electrons are represented by opposite arrows to show the spin of the electrons.

  
• State what is meant by an atomic orbital.

  An atomic orbital is a region around the nucleus that can hold up to 2 electrons with opposite spins.
• What is the electron configuration of a calcium atom?

  $1s^22s^22p^63s^23p^64s^2$