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Summary Chemistry AS-Level OCR Chapter 5-6
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A summary of Chapter 5-6 for Chemistry A OCR AS-Level.
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Chapter 5: Electrons and Bonding5.1 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
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