Orbitals

Orbitals

1. Evidence for shells

   • Ionisation energy
     • First ionisation energy - the energy needed to remove 1 electron from each atom in 1 mole of geseous atoms to form 1 mole of gaseous a+ ions.
     • x(g) --> x+(g) + e-
     • Factors affecting ionisation energy
       • Atomic radius - greater radius, smaller nuclear attraction for outer electrons.
       • Nuclear charge - greater nuclear charge, greater attractive force on outer electrons.
       • Electron shielding - inner shells repel outer shells, more inner shells, larger shielding effect, smaller nuclear attraction to outer electrons.
     • Electrons in the outer shell are removed first as they have the least nuclear attraction so need less ionisation energy.
   • Successive ionisation energies
     • A measure of the energy required to remove each electron in turn.
     • As each electron is removed, there is less repulsion between electrons so shells are drawn in, nuclear attraction increases, more ionisation energy is needed.

2. Shells and orbitals

   • 1st shell - 2 electrons 2nd shell - 8 electrons 3rd shell - 18 electrons 4th shell - 32 electrons.
   • Each orbital holds 2 electrons with opposite spins.
   • S-orbital - spherical shape, each shell contains 1 s-orbital, holding 2 electrons.
   • P-orbital - 3D dump-bell shape, each shell contains 3 p-orbitals, holding 6 electrons.
   • D-orbital - each shell contains 5 d-orbitals, holding 10 electrons.
   • F-orbitals - each shell contains 7 f-orbitals, holding 14 electrons.

3. Sub-shells and electron configuration

   • A sub-shell is a group of the same type of atomic orbitals within a shell with the same energy level.
   • Electron configuration is the arrangement of electrons in an atom.
   • Electrons added one at a time, lowest available energy level is filled first, each energy level must be full before the next.
   • Sub-shells are made up of the same orbitals

4. Energy levels

   • Sub-shell energies increase in order (s,p,d,f.)

5. Electrons and the periodic table

   • 4s has a lower energy level than 3d. 4s fills before 3d. 4s electrons are lost before 3d electrons.
   • Oxygen is the 4th element in the 2p block - 1s2,2s2,2p4.
   • Electron configuration simplified by previous noble gas configuration followed by the last electron config.
   • When + ions are formed, electrons are removed from the highest energy orbital. When - ions are formed, electrons are added to the highest energy orbital.