INORGANIC Part 4 Period 2
survey sub-index : Period 2 Survey of
individual elements : 3. lithium :
4. Beryllium
:
5. Boron :
6. Carbon :
7. Nitrogen
:
8. Oxygen :
9. Fluorine
:
10. Neon * 4.2 Period 2
element trends and explanations of physical properties
* 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae &
reactions
Advanced
Level Inorganic Chemistry Periodic Table Index *
Part 1
Periodic Table history
* Part 2
Electron configurations, spectroscopy,
hydrogen spectrum, ionisation energies *
Part 3
Period 1 survey H to He *
Part 4
Period 2 survey Li to Ne *
Part 5 Period 3 survey Na to Ar *
Part 6
Period 4 survey K to Kr and important trends
down a group *
Part 7
s-block Groups 1/2 Alkali Metals/Alkaline Earth
Metals *
Part 8
p-block Groups 3/13 to 0/18 *
Part 9
Group 7/17 The Halogens *
Part 10
3d block elements & Transition Metal Series
*
Part 11
Group & Series data & periodicity plots
Part 4
Survey of Period
2:
Li across to Ne (8 elements, Z = 3 to 10)
4.2
Period 2 trends and explanations of selected physical
properties
Element |
Lithium |
Beryllium |
Boron |
Carbon |
Nitrogen |
Oxygen |
Fluorine |
Neon |
1st ionization energy (kJ mol-1) |
513 |
900 |
801 |
1086 |
1402 |
1314 |
1681 |
2081 |
Atomic metallic or covalent
radius (pm, /1000 for nm) |
152 (met) |
111 (met) |
88 (cov) |
77 (cov) |
70 (cov) |
66 (cov) |
64 (cov) |
51 (cov) |
Electronegativity (Pauling
scale) |
0.98 |
1.57 |
2.04 |
2.55 |
3.04 |
3.44 |
3.98 |
4.84 |
Melting Point (K) |
454 |
1551 |
2573 |
3820 |
63 |
55 |
54 |
24 |
Boiling Point (K) |
1620 |
2760 |
3932 |
5100 |
77 |
90 |
85 |
27 |
Relative electrical
conductivity |
0.150 |
0.250 |
<0.001 |
0.001 |
<0.001 |
<0.001 |
<0.001 |
<0.001 |
Density (g cm-3) |
0.5 |
1.8 |
2.3 |
2.3 |
<0.1 |
<0.1 |
<0.1 |
<0.1 |
The data is plotted
below with explanations and comments (Z = proton/atomic number
for the x axis)
Above each period graph
is the corresponding periodicity graph for the whole of the
periodic table |


(1)
Variation of 1st Ionisation enthalpy across Period 2
ΔH/kJmol-1
for the process X(g)
==> X+(g) + e-
The energy required to remove the
most loosely bound electron (kJmol-1) from the
gaseous atoms at 298K/1atm.
The peaks correspond with the Group 0/18 Noble Gases at the end
of a period and the troughs with the Group 1 Alkali Metals at
the start of a period.
As you go across the
period from one element to the next, the positive nuclear charge
is increasing by one unit as the atomic/proton number increases
by one unit and the charge is acting on electrons in the same
principal quantum level. The effective nuclear charge can be
considered to be equal to the number of outer electrons (this is
very approximate and NOT a rule) and this is increasing from
left to right as no new quantum shell is added i.e. no extra
shielding. Therefore the outer electron is increasingly more
strongly held by the increasing positive charge of the nucleus
and so, increasingly, more energy is needed remove it.
So, for Period 2,
the Group1 Alkali Metal (lithium, lowest Z) has the lowest 1st
ionisation energy and the Group 0/18 Noble Gas (neon, highest Z)
has the highest 1st ionisation energy value and most values
follow the general trend of increasing from left to right across
period 2.
However there are
two anomalies in the atomic number versus 1st ionisation
energy graphs for period 2.
It should be first
pointed out that these anomalies are due to the complex
behaviour of the quantum levels in multi-electron systems - do
not expect any perfect trends in chemistry, thanks to quantum
physics!
(i) A decrease from Be [1s22s2]
to B [1s22s22p1]
Box diagram of 2s2p orbitals 
==>  
The anomalously low value for
boron is considered to be due to the first time a 2p electron is
shielded by the full 2s sub-shell and, more importantly, the 2p
electron is a bit
further away on average from the nucleus than the 2s electrons
(so less strongly bound). The effect overrides the effect of increasing proton number i.e. the
increase in positive nuclear charge from Be to B. However, after
the kink, the continued increase in nuclear charge ensures the
Period 2 trend for the 1st ionisation energy continues as
expected until oxygen, the 2nd anomaly.
(ii) A decrease from N [1s22s22p3]
to O [1s22s22p4]
Box diagram of 2s2p orbitals 
==>  
Prior to the 4th 2p electron, the
other three p electrons occupy separate p sub-orbitals (Hund's
Rule of maximum multiplicity) to minimise repulsion between
adjacent orbitals. The anomalously low value for oxygen is
considered to be due to the effect of the first pairing of
electrons in the p orbitals producing a repulsion effect
that overrides the effect of increasing proton number (increase
in positive
nuclear charge). From the 'kink', the Period 2 trend for the 1st
ionisation energy continues as expected from oxygen to neon with
increase in nuclear charge.
You see the same anomalous pattern
in Period 3
See
also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions
and 6.4
Important element trends down a Group |


(2)
Variation of atomic
radius across period 2
Can be defined as volume within
which 95% of the electron charge exists on a time averaged
basis.
The peaks correspond with the Group 1
Alkali Metals at the start of a period and the troughs with the
Group 0 Noble Gases at the end of a period.
It generally decreases from left to
right across a period, as the actual and effective nuclear
charge increases within the same principal quantum level with
increase in proton number, pulling the electron cloud closer to
the nucleus without any increase in shielding. The argument is
almost identical to that for increasing ionisation energy.
So, for Period 2, the Group1 Alkali Metal (lithium, lowest Z)
has the largest atomic radius and the Group 7/17 Halogens &
Group 0/18 Noble Gas (fluorine & neon, highest Z's) have the
smallest atomic radii (there is some uncertainty in the
noble gas radii).
See
also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions and
6.4
Important element trends down a Group |


(3)
Variation of electronegativity
across Period 2 (Pauling scale)
The power of an atom, in terms of an
electric field effect, to attract electron charge towards it, in
the context of a pair of electrons of a covalent bond linking it
to another different atom.
The peaks correspond to the Group 7
Halogens/Group 0 Noble Gases at the end of a period and the troughs'
correspond to the most electropositive Group 1 Alkali Metals at the
start of a period.
It generally increases from left to right across a period, as
the actual and effective nuclear charge increases within the
same principal quantum level, pulling the bonding electron cloud
(bonding pair of electrons) closer to the nucleus (see 1st IE
arguments) i.e. increase in proton charge without increase in
shielding. The argument is almost identical to that for
increasing ionisation energy.
So, for Period 2,
the Group1 Alkali Metal (lithium, lowest Z) has the lowest
electronegativity and the Group 7 Halogen & Group 0/18 Noble Gas
(fluorine & neon, highest Z's) have the highest
electronegativities
(there is some uncertainty in the noble gas
electronegativities).
In the context of a
bond between two different elements, the element with the
greater electron pulling power acquires a partial negative charge
and the other less electronegative element a partial positive
charge.
So, in the covalent
bond Mδ+-Xδ-, X has the greater
electronegativity e.g. the polar bond Cδ+-Fδ-
in covalent CF4.
This has major
consequence on the type of bonding from ionic oxides and
chlorides to non-metallic covalent oxides and chlorides. If the
difference is large an ionic bond results. e.g. Li+
F-
See
also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions
and 6.4
Important element trends down a Group |

(4)
Variation of melting
points and boiling points across Period 2
Trends in melting/boiling point can
be complicated because of significant differences in the
structure of the element.
The melting points and boiling
points tend to peak in the middle of Periods 2 and 3
(Groups 3/13 and 4/14) and the lowest values at the end of the
period - the Noble Gases.
Generally you are
moving from a low melting, but still quite high boiling,
metallic lattice of lithium in Group1 of moderately strong
bonding with one outer delocalised valence electron ==> a
much higher melting/boiling metallic lattice with 2 outer
electrons for beryllium.
For groups 3/4 boron/carbon (B/C) you have a very high melting
giant covalent lattices of a strong 3D or 2D network of strong
covalent bonds. The mpts and bpts are even higher than the
preceding metals because 3/4 outer valence electrons are
involved in the bonding.
From Group 5 onwards there is a
dramatic fall as the elements now consist of low melting small
covalent molecules (N2, O2, F2
and Ne) only held together by weak inter-molecular forces
(transient dipole - induced dipole interactions).
See
also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions
and 6.4
Important element trends down a Group |

(5)
Variation of relative
electrical conductivity across Period 2
Not surprisingly, the highest
values correspond to the metals at the start of the period with
the greatest number of outer electrons that can be delocalised.
Increases
dramatically from left to right for Groups 1-2 as the metallic
lattice contains 1-2 mobile delocalised electrons involved in
electrical conduction.
From Group 3 to 0 the element structure changes to giant
covalent lattice or simple molecular structures with no free
delocalised electrons within the structure to convey an electric
current.
Although the graphite allotrope of
carbon conducts electricity via the delocalised electrons in the
linked hexagons of carbon atoms, it is still a very poor
electrical conductor compared to metals (diamond is virtually an
insulator).
See
also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions
and 6.4
Important element trends down a Group |


(6) Variation of density across
Period 2
The peaks correspond to the
metals in the middle of the period with the strongest bonding in
the solid.
The density increases from
lithium to beryllium as the atomic radii decrease and the
bonding gets stronger with 1 ==> 2 bonding electrons
(delocalised outer valency electrons in the metal lattice).
However, they are relatively low densities compared to most
metals.
Boron and silicon have a low
density, typical of non-metallic covalent solids.
Nitrogen, oxygen, fluorine and
neon are small covalent molecules and have very low densities
being gaseous at room temperature because only weak
intermolecular forces act between them. Even the densities of
the liquid are quite low, again typical of low atomic number
non-metals. See also 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae
& reactions
and 6.4
Important element trends down a Group |
WHAT NEXT?
See also
4.1 Period 2 Survey of the individual elements,
5.1
Period 3 survey of elements,
5.2 Period 3 element trends & explanations of
physical properties,
Period 3 element trends in bonding,
structure, oxidation state, formulae & reactions, 6.1
Survey of Period 4 elements, 6.2 Period 4
element trends in physical properties, 6.3
Period 4 element trends in bonding, formulae
and oxidation state and
6.4
Important element trends down a Group
INORGANIC Part 4 Period 2
survey sub-index : Period 2 Survey of
individual elements : 3. lithium :
4. Beryllium
:
5. Boron :
6. Carbon :
7. Nitrogen
:
8. Oxygen :
9. Fluorine
:
10. Neon * 4.2 Period 2
element trends and explanations of physical properties
* 4.3 Period 2
element trends in bonding, structure, oxidation state, formulae &
reactions
Advanced
Level Inorganic Chemistry Periodic Table Index *
Part 1
Periodic Table history
* Part 2
Electron configurations, spectroscopy,
hydrogen spectrum, ionisation energies *
Part 3
Period 1 survey H to He *
Part 4
Period 2 survey Li to Ne *
Part 5 Period 3 survey Na to Ar *
Part 6
Period 4 survey K to Kr and important trends
down a group *
Part 7
s-block Groups 1/2 Alkali Metals/Alkaline Earth
Metals *
Part 8
p-block Groups 3/13 to 0/18 *
Part 9
Group 7/17 The Halogens *
Part 10
3d block elements & Transition Metal Series
*
Part 11
Group & Series data & periodicity plots
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