

Period 1 HYDROGEN and
HELIUM
Doc
Brown's Chemistry
Advanced Level Inorganic Chemistry Periodic Table
Revision Notes
Part
3. Survey of Period 1: hydrogen H to helium He
Period 1, the
shortest period of the periodic table, only consists of two elements,
hydrogen and helium. Although the properties of helium match
those of the other Noble Gases, hydrogen does not really fit into any
group. A brief summary of their properties is given below. Technically
they are s block elements but they bare little resemblance to the group
1 and 2 metals, but see discussion of hydrogen and comparison with group
1 and group 7/17 elements.
For non-A level
students
KS4 Science GCSE/IGCSE
Periodic Table notes links
INORGANIC Part 3 Period 1 page sub-index:
3.1
Period 1 H-He * 3.1 Hydrogen * 3.2 Helium * 3.3 Period
1 Summary * 3.4 Where do heavier elements come
from?
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
3.1
Survey of
Period
1: H to He (2 elements, Z = 1 to 2)
Z
= 1. Hydrogen
H not in a Group
-
The
structure of the element:
-
Physical properties:
-
Colourless gas, less
dense than air; mpt -259oC (14K), bpt -253oC
(20K).
-
Poor conductor of heat/electricity
in any physical state.
-
Group, electron configuration
and oxidation states:
-
Not in any
group strictly
speaking*; e.c. [1] or 1s1
-
Oxidation
states: (+1) e.g. in HCl, H2O and (-1) with electropositive
metals e.g. sodium hydride, NaH or Na+H-.
-
*
It has been displayed at the top of
Gp 1 Alkali Metals or Gp 7 (17) Halogen in the
Periodic Table in the past, for the following reasons ..
-
Gp 1: Forms H+ ion and in alkali metal vapour there
is a tiny fraction of M2 molecules.
-
BUT, you can
hardly argue it has any real metallic properties physically
since it is a non-metallic gas and
chemically it doesn't/can't react with water like alkali
metals to form an alkali and hydrogen!
-
When ignited
it will react rapidly with
oxygen/chlorine if initiation energy is supplied e.g. heat/uv
light but the resulting compounds are very different,.
-
Water, H2O, is a
covalent neutral molecule as opposed to the ionic group 1 oxide
-
Hydrogen chloride is an acidic
covalent molecule dissolving in water to form a strongly acid
solution, whereas group 1 metal chlorides are ionic, M+Cl-,
and dissolve to form neutral solutions with water.
-
So under 'normal' laboratory
conditions hydrogen doesn't really have any metallic character
either physically or chemically.
-
Gp 7(17):
Hydrogen is non metallic in character and occurs as the
gaseous H2 molecule, being colourless fits in with
increase in colour intensity down the Halogen group. It
similarly chemically combines with a valency of 1, forming covalent
simple molecular compounds with
non-metals e.g. hydrogen chloride, HCl, methane, CH4
etc. as do halogens. It also forms ionic compounds with the most electropositive
metals e.g. sodium hydride, Na+H- where
the hydride ion parallels the halide ion and sodium hydride
exist as white crystalline solid just like sodium chloride, Na+Cl-
('salt'). So hydrogen, at least shows some non-metal
character.
-
However, it
hardly matches the halogen in reactivity e.g. it aught to be
more reactive than fluorine and more importantly, its maximum
oxidation state is +1 and doesn't have the extensive chemistry
of halogens below fluorine. Fluorine only has an oxidation state
of -1 in compounds and has no +1 compound, since it has the
highest electronegativity of any element (4.0, hydrogen is 2.2).
-
From chlorine downwards,
all the halogens exhibit a huge range of
compounds with oxidation states of +1, +3, +5 and +7 (max.
possible) and even +4 ox. state oxides. The oxides are acidic
and the oxyanions acts as strong oxidising agents. Hydrogen
oxide is water, is neutral and acts as relatively weak oxidising
agent.
-
So, although
it physically and chemically it fits in a bit more with Group 7/17 than
Group 1,
I don't think it really fits convincingly in any group,
and many modern
periodic tables show it on its own at the top on period 1.
I'm afraid I'm irritated when its put at the top of Group 1 or 7/17
in the periodic table, but its a personal view!
-
Reaction of element with oxygen:
-
Reaction of element with chlorine:
-
Reaction of hydrogen
with metals
-
Reaction of chloride
with water:
-
Other comments:
-
In chemistry,
its 'proton form' is the basis of the concept of Bronsted-Lowry
acids and bases.
-
An acid is
a proton donor e.g. hydrogen chloride.
-
A base is a
proton acceptor e.g. ammonia.
-
Section 3.4 shows how the heavier elements
are built up from hydrogen isotopes via nuclear reactions.
-
Links to other pages
on this site:

TOP OF PAGE
3.2 Z
= 2.
Helium He in Group 0/18
-
The
structure of the element:
-
Physical properties:
-
Colourless gas, less
dense than air; mpt -272oC (1K), bpt -269oC
(4K)
-
Poor conductor of heat/electricity
in any physical state.
-
Group, electron configuration
(and oxidation states):
-
Reaction with anything:
-
Other comments:
-
Last element in the
period, as the outer principal quantum level 1 (shell 1) is full to the
maximum number of electrons allowed, conferring extra chemical stability
on the atom.
-
The 1st ionisation
energy of helium (2372 kJmol-1) is nearly twice that
of hydrogen (1312 kJmol-1) because the effective
nuclear attraction charge is doubled for the same 1s quantum
level (+1 to +2).
-
Links to other pages
on site:
3.3 Summary of Period
1: H to He
(elements 1 to 2)
-
Maximum of two elements
in period 1 as there is a maximum of two electrons allowed in the 1st
principal quantum level (1s) and only an s-orbital energy sub-level is
allowed.
-
Both elements are
non-metallic gases, but a considerable difference in chemical
reactivity!
-
Hydrogen, as
outlined above, has quite an extensive chemistry ie combines with
all non-metals except the noble gases and can also combine with some
reactive metals eg the group 1 alkali metals.
- Helium, with one full shell only
(outer = inner !) has the highest ionisation energy of any element and
is chemically the most stable and least reactive of any element in the
periodic table and has no meaningful chemistry.
TOP OF PAGE
3.4 Where
do the heavier elements come from?
-
There are 92 naturally
occurring elements on planet Earth.
-
All the 91 naturally
occurring elements after hydrogen up to 92U, uranium, were formed in
stars by nuclear fusion reactions.
-
Extremely
high temperatures are needed in stars to give the atomic nuclei enough kinetic
energy to overcome the very powerful positive nucleus-nucleus repulsion forces
and fuse together in a 'fruitful collision'.
-
Examples of stellar nuclear
fusion reactions
building up the heavier elements from hydrogen and helium are shown below.
key: AZX,
A = mass
number, Z = atomic/proton number, X
= element symbol
From hydrogen, helium is
formed e.g. the sequence ...
11H + 10n
==> 21H (hydrogen-2, 'heavy hydrogen', 'deuterium')
21H + 10n
==> 31H (hydrogen-3, 'tritium')
31H + 11H
==> 32He + 10n
32He + 10n
==> 42He (the most common isotope of helium)
From helium the heavier
elements are formed as bigger and bigger nuclei fuse together.
e.g. 2 42He ==>
[84Be] which is highly unstable and
rapidly changes, on
impact with a 3rd helium nucleus, into a carbon nucleus,
[84Be] + 42He
==> 126C
and from carbon-12, oxygen-16
and neon-20 are formed e.g. via
42He + 126C
==> 168O
2 126C ==> 2010Ne
+ 42He
and so on, until
even
small amounts of 23892U are eventually formed
but require the highest of temperature e.g. in a super-nova explosion of
giant stars a lot bigger than our sun!
Many isotopes of
elements after lead, 82Pb are unstable. After uranium, 92U,
the vast majority of the isotopes of
the elements of atomic number 93+ are inherently unstable. They will not
have survived even if they were formed billions of years ago in the Sun,
and
retained or formed in the initial 'spin-off' material that formed the
'very early' Earth. However, the
advent of nuclear reactors has enabled up to kg quantities of e.g.
plutonium, 94Pu (used in nuclear reactors and weapons) and americium, 95Am
(used in smoke alarms) to be produced.
Cyclotrons, particle bombardment linear accelerators, have enabled 'super-heavy'
elements up to Z = 118? to be 'synthesised', but only a few atoms at a
time (The Russia-US space race seems to have been partly replaced by 'who
can synthesize the biggest atom'.
See also
GCSE-AS
nuclear fusion - radioactivity page AND
A Level Periodic Table Part 1 History
WHAT NEXT?
INORGANIC Part 3 Period 1 page sub-index:
3.1
Period 1 H-He * 3.1 Hydrogen * 3.2 Helium * 3.3 Period
1 Summary * 3.4 Where do heavier elements come
from?
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
TOP OF PAGE
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