Periodic Table Revision Notes Part 3 Survey of Period 1 H to He Advanced Level Inorganic Chemistry at Doc Brown's Chemistry Clinic docbasa2ptable3

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The Periodic Table

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

Advanced Level Inorganic Chemistry Revision notes

(e.g. UK Advanced Level Chemistry GCE-AS-A2-IB US K12 grades ~11-12)

GCSE Periodic Table notes * EMAIL query?comment

Part 3 Period 1 page sub-index * 3.1 Period 1 - Hydrogen * 3.2 Helium * 3.3 Period 1 Summary * 3.4 Where do heavier elements come from?

Advanced Periodic Table Index * Part 1 A brief Periodic Table history * the modern Periodic Table * Part 2 Electronic structure of atoms : Spectroscopy and the H spectrum : Ionisation energies * Part 7 s-block metals Groups 1-2 Alkali/Alkaline Earth Metals * Part 10 3d-block Sc-Zn and Transition Metals * Part 11 Group and Series data summaries and links to periodicity plots

ABBREVIATIONS:
- bpt = boiling point, e.c. = electron arrangement/configuration, Pd = period of the Periodic Table, Gp = group of the Periodic Table, max. = maximum, mpt = melting point, ox. state = oxidation state or oxidation number.
INFORMATION on the NOTES
1. The oxidation states quoted are numerically equal to the element's valency. The oxidation state of an element in its normal stable state is 0. The information given assumes this and any reference to the oxidation states of the element refers to its value in compounds.
2. The periodicity of melting/boiling points, atomic radii, 1st ionisation energy and electronegativity and other data for the elements are also tabulated and graphically shown on a separate web pages:
  - Z=1-20 Periods 1-3 + start Pd4, Z=1-38 Periods 1-4 + start Pd5, Z=1-92 Periods 1-6 + start of Pd 7

Survey of Period 1: H to He (elements 1 to 2)

3.1. Hydrogen H

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 ..
  - 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
        
        (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.
  - 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').
    - 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, it does not really fit 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 in the periodic table.
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 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:
- 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 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 gas : Test for gas : Acid-Base theory :
- Use in ammonia-fertiliser production or margarine production :

3.2. Helium He

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; no stable oxidation states (other than 0 for the element itself!) 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:
- 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.
Links to other pages on site:
- GCSE notes on Group 0 Noble Gases :

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!

3.4 Where do the heavier elements come from?

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 carbon,

[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 GCSE-AS radioactivity page.

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