4 The Atom & Electronic
Configuration
Bohr Model of the Atom
Bohr suggested that an electron moving in an orbit can only have
certain amounts of energy not an infinite number of value. This
energy is quantised.
The energy that an electron needs in order to move in a particular orbit
depends on the radius of the orbit.
An electron in an orbit further away from the nucleus requires higher
energy than an electron nearer the nucleus.
If the energy of the electron is quantised then the radius of the orbit
must also be quantised. there are a restricted number of orbits.
Ionisation Energy
Definition The 1st ionisation energy of an element is the
energy change for the conversion of 1mol of gaseous
atoms into 1 mol of gaseous ions
i.e the energy change for the process
M(g)  M+(g) + e
Similarly the 2nd ionisation energy is the energy change for the process
M+(g)
 M2+(g) + e
e.g. 1st ionisation energy
2nd ionisation energy
H = 1310 KJ/mol
He = 2370 KJ/mol
5250 KJ/mol
The second electron is always harder to remove than the
1st
x
2p
x
Helium contains 2 electrons and 2 protons. When 1
electron is removed there are still 2 positive charges
in the nucleus  more attraction on the remaining
electron which then requires more energy to remove.
Lithium has 3 electrons and 3 protons
1st I.E. = 519 KJ/mol
2nd I.E. = 7300 Kj/mol
3rd I.E. = 11800 Kj/mol
Notice the large increase in ionisation energy between the 1st and
2nd and then a smaller jump between the 2nd and 3rd
x
x
3p
x
The outer electron is easier to remove as it is in a shell further
away from the nucleus
Carbon has 6 electrons 2 in first shell. 4 in second shell
and 6 protons in the nucleus (electronic configuration 2,4)
x
x
x
6p
x
x
x
The 4 electrons in the second shell are easier to remove
than the 2 in the first shell as they are further from the
nucleus.
However if we plot a graph of
ionisation energy against
electron removed
14000
12000
1
Line 2
Line 3
Line 4
Line 5
Line 6
Line 7
10000
8000
6000
4000
2000
0
1,
2
3
4
The first 2 electrons are relatively easy to remove, the 2nd
two slightly harder and the last 2 much harder. The first 4
are in the outer shell the last 2 are in the inner shell
Notice the ease of removal is not a smooth trend from 1 4 . There is a slight jump in ionisation energy between the
2nd and 3rd electron. This suggests that the 1& 2
electrons being removed are in a slightly lower energy
level than the 3 & 4
5
6
From this we conclude that the 2nd shell consists of 2
subshells, one slightly closer to the nucleus than the
other
1s
2s
x x
x
6p
x
x x
2p
We now write the electronic configuration as 1s2 2s2 2p2
The first 2 electrons (1,2) removed are in a 2p orbital
The next 2 electrons (3,4) removed are in 2s orbital
The last 2 electrons (5,6) removed are in a 1s orbital
There is also a third type of orbital called a d orbital. The
first element with a d orbital is scandium (the first
transition element)
ALL orbitals can hold 2 electrons.
The first shell has only 1 s orbital
The second shell has 1 s orbital and 3 p orbitals
The 3rd shell has 1 s orbital, 3 p orbitals and 5 d orbitals
 The first shell can hold 2 electrons
The second shell can hold 8 electrons
The third shell can hold 18 electrons
We can now write electronic configurations as follows
Li 3electrons 1s2 2s1
C 6 electrons 1s2 2s2 2p2
Na 11 electrons 1s2 2s2 2p6 3s1
Note that the outer shell contains the same no of electrons
as the group number. Carbon group 4 has 4 electrons in
the 2nd shell
Finding the electronic configuration of ions
Li 1s2 2s1
Li+ 1s 2
It has lost 1 electron and now has a filled shell and is stable
Be 1s2 2s2
Be2+ 1s2
it has lost 2 electrons and and now has a filled shell and is
stable
Write the configuration of the following elements
N
Mg
Al
Cl
1s22s22p3
1s22s22p63s2 1s22s22p63s23p1
Al3+
F-
Na+
1s22s22p6
1s22s22p6
1s22s22p6
1s22s22p63s23p5
Elements in groups 1 and 2 are filling s orbitals and are called s block
Element in groups 2-8 are filling p orbitals and are called p block
The transition elements are filling d orbitals and are called d block
When we write a configuration we ‘fill’ orbitals from the lowest energy to
the highest. i.e. electrons are added to the inner 1s orbital first
followed by 2s then 2p etc. This is called the aufbau principal (to
build up) An atom with all electrons in the lowest possible energy
levels is said to be in its ground state
When writing configurations for the transition elements the 4s orbital is
filled before the 3d (the 4s orbital is actually lower in energy than the
3d)
So the configuration of Sc is 1s22s22p63s23p64s23d1
It is the first transition metals and has 1 d electron. (It is easy to find the
number of d electrons – just count from scandium across the row)