Summary Chemistry AS-Level OCR Chapter 5-6

Chapter 5: Electrons and Bonding
5.1 Electron structure
Shells
o Shells are regarded as energy level
o Energy increases as shell numbers increases
o Shell number or energy level number is called the principal quantum number n
 Maximum number of electrons in the first 4 shells:
Shell number Number of electrons
n

1 2

2 8

3 18

4 32
o Number of electrons = 2n^2

Atomic orbitals
 Shells are made of atomic orbitals
 Atomic orbital - A region around the nucleus that can hold up to 2 electrons, with
opposite spins
 Model visualise as a region in space where there is a high probability of finding an
electron
 Electrons can be though as negative charge cloud with the shape of the orbital, referred
as electron cloud
o Orbital can hold one or two electrons, no more
o Different types of orbitals: s-, p-, d- and f- orbitals
o Each type has different shape

Types of orbitals
 s-orbitals
o Electron cloud is within the shape of a sphere
o As with orbitals, s-orbitals can hold one-two electrons
 Each shell from n = 1 contains one s-orbitals
 The greater the shell number n, the greater the radius of its s-orbital
 p-orbitals
o Electron cloud is within a shape of a dumbbell
o There are 3 separate p-orbitals at right angle to one another
o These orbitals are referred to as px, py and pz
 Each shell contain n = 2 containing three p-orbitals
 The greater the shell number n, the further the p-orbital is from the
nucleus
 d-orbitals and f-orbitals
o Each shell from n = 3 contains five d-orbitals
o Each shell from n = 4 contains seven f-orbitals
 Sub -shells
o New type of orbital is added for each additional shell

, o Within a shell, orbitals of the same type are grouped together - sub-shells
Shell Number of Subshells Number of electrons in subshell
orbitals present
s/p/d/f

1 1 1s 2

2 1/3 2s + 2p 2+6

3 1/3/5 3s + 3p + 3d 2 + 6 + 10

4 1 / / 7 4s + 4p + 4d + 4f 2 + 6 + 10 + 14
o Each new shell gains a new type of orbital
o The number of orbital increase with each new type of orbital
 s,1 p,3 d,5 f,7
o Two electrons fit into each orbital, so the number of electrons in each subshell
also increase
 s,2 p,6 d,10 f,14

Filling of orbitals
Orbitals fill in order of increasing energy
 Subshells that make up shells have slightly different energy levels
 Within each shell, new type of subshells has a higher energy
o In the n = 2 shell, order of filling is 2s, 2p
o In the n = 3 shell, order of filling is 3s, 3p, 3d
o In the n = 4 shell, order of filling is 4s, 4p, 4d, 4f
 Highest energy level in third shell overlaps with the lowest level in fourth shell
o 3d subshell is at a higher energy level than 4s
o 4s subshell therefore fills before 3d
Electron pair with opposite spins
 Represent the electron in box model
o Electrons are negatively charged and repel
o They have property called spin, either up or down
o They are shown as arrows
o Two electrons in an orbital must have opposite spins, which help to counteract
the repulsion between the negative charges of the 2 electrons
Orbitals with the same energy are occupied singly first
 Within a subshells, the orbitals have the same energy
 One electron occupies each orbitals before pairing
o Prevent any repulsion between paired electrons until there is no further orbital
available at the same energy
o Fill each orbital with one electron first, then add the left-overs electron in
Electron configuration of atoms
 Electron configuration should be shown in shell order rather than order of filling
o 1s - 2s - 2p - 3s - 3p - 4s - 3d - 4p
o The 3d would still come before the 4s
 This configuration can be expressed more simply
o We can changed the first part of the configuration by having a noble gas and
then plus the outer electron subshells