Doc Brown's Chemistry Advanced Level Inorganic Chemistry Periodic Table Revision Notes

Period 1 HYDROGEN and HELIUM

 Part 3. Survey of Period 1: hydrogen H to helium He

3.1 Survey of Period 1 elements: H to He (2 elements, Z = 1 to 2)

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.

Z = 1. Hydrogen H not really in a Group

• The structure of the element:

  • Non-metal existing as a diatomic molecule, H₂, with a single covalent bond.

• Physical properties:

  • Colourless gas, less dense than air; mpt -259^oC (14K), bpt -253^oC (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 ..

  • Comparison with Group 1 elements

    • 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

    • (M⁺)₂O^2-, which forms a strong alkali in water.

    • 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.

  • Comparison with Group 7/17 elements

    • 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 similar to the halogens.

    • 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: 

  • Explosive reaction when ignited in an air mixture, a jet of the gas burns with very pale blue flame to form the colourless covalent liquid molecule, water.

    • 2H_2(g) + O_2(g) ==> 2H₂O_(l) 

• Reaction of element with chlorine: 

  • Explosive if the mixture is heated or subjected to uv light. The colourless covalent gas hydrogen chloride is formed.

    • H_2(g) + Cl_2(g) ==> 2HCl_(g) 

• Reaction of hydrogen with metals

  • Hydrogen will combine with alkali metals (M) to form a series of Group 1 hydrides

    • H_2(g) + 2M_(s/l) ==> 2MH_(s) 

    • These are white ionic compounds.

    • Hydrides are formed with other reactive metals but their chemical structure is more complex, with different degrees of ionic/covalent character.

• Reaction of chloride with water:

  • Hydrogen chloride gas dissolves to form the strong hydrochloric acid (100% ionised).

    • HCl_(g) + H₂O_(l) ==> H₃O⁺_(aq) + Cl^-_(aq) 

• Other comments on hydrogen:

  • 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.

    • NH_3(g) + HCl_(g) ==> NH₄⁺Cl^-_(s)

    • The hydrogen chloride donates a proton to the ammonia molecule.

  • Section 3.4 shows how the heavier elements are built up from hydrogen isotopes via nuclear reactions.

• Links to other pages on this site: 

  • Preparation of hydrogen gas

  • Test for hydrogen gas

  • Acid-Base theory

  • Use of hydrogen in ammonia-fertiliser production

  • Use of hydrogen in margarine production

TOP OF PAGE

3.2 Z = 2. Helium He in Group 0/18

• The structure of the element:

  • Non-metal existing as single atoms, He, sometimes described as 'monatomic molecules'.

• Physical properties: 

  • Colourless gas, less dense than air; mpt -272^oC (1K), bpt -269^oC (4K)

  • Poor conductor of heat/electricity in any physical state.

• Group, electron configuration (and oxidation states): 

  • Gp0/18 Noble gas; e.c.  [2] or 1s²

  • There is no stable oxidation states (other than 0 for the element itself!) because of the exceptionally stable full outer shell of electrons, so no compounds!

• Reaction with anything: 

  • None! Far too stable electron configuration which also explains why it prefers to exist as single atoms.

• Other comments on helium

  • 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: 

  • GCSE LEVEL notes on Group 0 Noble Gases

  • Advanced Level Chemistry Notes on Group 0/18 Noble Gases - Helium

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 i.e. combines with most non-metals except the noble gases and can also combine with some reactive metals e.g. 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.

3.4 Where do the heavier elements come from?

(this section is also dealt with on my 'history of the periodic table' page)

• 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: ^A_ZX, 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 in smaller sun's up to iron, element 26.

From Co, element 27 onwards, even up to small amounts of ²³⁸₉₂U, are eventually formed but require the highest of temperatures of all  e.g. in a super-nova explosion of giant stars, whose temperatures are far greater 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 >92 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 Advanced Level Periodic Table Part 1 History