Electron Structure

Electron Structure

1. Electrons and shells

   • Shell 1holds 2 e-
     • Shell 2 holds 8 e-
       • Shell 3 holds 18 e-
         • Shell 4 holds 32 e-
   • Shells = energy levels
     • As shell no. increases, energy increases
       • Energy lvl/ Shell no. = Principal Quantum Number, n
   • Current model of e- based on idea that e- has properties of wave and particle (wave-particle duality)
     • Around atom e- only fit into energy levels defined by wave nature of e-
   • Max no. of e- in shell n = 2n^2

2. Atomic Orbitals

   • Region around nucleus that can hold up to 2 e- with opposite spins
     • Models visualise as a region in space where there is a high probability of finding 1 e-
   • e- can be thought of as negative-charge cloud with shape of orbital (a.k.a electron cloud)
   • Diff. types of orbitals
     • s-
       • p-
         • d-
           • f-
             • Each shell from n=4 has 7 f-orbitals
           • Each shell from n=3 has 5 d-orbitals
         • Electron cloud has shape of dumb-bell
           • There are 3 separate p-orbitals at right angles to each other (p_x, p_y, p_z)
             • As shell no. increases, the further p-orbital is from nucleus
       • Electron cloud has shape of sphere
         • Each shell from n=1 contains 1 s-orbital
     • Each type has diff. shape
   • Sub-shells
     • Within a shell, orbitals of the same type are grouped together as sub-shells
       • Each new shell gains a new type of orbital
         • No. of orbitals increases with each new type of orbital: s-1, p-3, d-5, f-7
           • 2 e- fit into each orbital, so no. of e- in each sub-shell increases: s-2, p-6, d-10, f-14

3. Filling Orbitals

   • Fill in order of increasing energy
     • Sub-shells have slightly diff. energy lvls
       • Within each shell, new type of sub-shell added has higher energy
         • order when n=2: 2s,2p
         • order when n=3: 3s,3p,3d
         • order when n=4, 4s,4p,4d,4f
         • 3d sub-shell has higher energy lvl than 4s sub-shell, so fills first: order = 3p,4s,3d
   • e- repel one each other, and have property called spin (up or down); 2 electrons in an orbital must have opposite spins to help counteract repulsion
   • Orbitals have the same energy within sub-shell, so one e- occupies each orbital b4 pairing starts to prevent any repulsion until there is no further orbital available at the same energy lvl

4. Electron Configuration

   • Shows how sub-shells are occupied by e-
     • E.g. Li: 1s^2 2s^1
   • Can be expressed more simply in terms of previous noble gas + outer electron sub-shells
     • This notation is useful to emphasise similarities in electron configurations of outer shell
   • Should be shown in shell order not order of filling

5. Electron Configuration of Ions

   • Periodic table can be divided into blocks according to their highest energy lvl
     • s-block: highest energy e- in s-sub-shell (Group 1 + 2)
     • p-block: highest energy e- in p-sub-shell (Group 3-8)
     • d-block: highest energy e- in d-block (Transition metals)
   • When forming ions, highest energy sub-shells lose/ gain e-
   • For d-block elements, 4s sub-shell at lower energy lvl than 3d sub-shell, so filled first.
     • Energy of 4s and 3d sub-shells = v. close
       • Once filled, 3d energy lvl falls below 4s
         • So, 4s sub-shell fills and empties b4 3d sub-shell

Images and files in the map