Formula for Hydrogen?

I admit it. I love TV game shows. Last Friday I watched one of my favourite shows while eating my (late) lunch. I was even moderately successful at answering some of the questions, until the Host asked The Chaser what the chemical formula for hydrogen was. This led to the following exchange:
Chaser: H
Host: Incorrect
Contestants: H one (we will assume they meant H₁)
Host: Incorrect. The correct answer is H two (we will assume he meant H₂)

So, who was right?

Let's take the Host's "correct" answer first.
The Earth's atmosphere contains small amounts of diatomic molecules of hydrogen gas. "Di" means two and "atomic" refers to atoms so hydrogen gas in the atmosphere is made up of molecules in which 2 atoms of hydrogen are bonded together. When we make hydrogen gas in the laboratory we are making these H₂ molecules. So it seems that the Host got it right ..... except ..... the question didn't ask for the formula of hydrogen gas found in the atmosphere!

So let's turn our attention to the Contestants' response.
Is H₁ a plausible chemical formula for hydrogen?
Not really. If there is only 1 atom of an element in the chemical formula, the "1" is trivial and not included in the formula, so H₁ is the same as H which was the Chaser's response!

So, was the Chaser right?
Is H a valid chemical formula for hydrogen?
Hydrogen is a strange atom. It has 1 proton in its nucleus, and 1 electron "orbiting" that nucleus. In fact, this 1 so-called "valence electron" is a feature common to all Group 1 metals (alkali metals), but other properties of hydrogen suggest it is more like a non-metal than a metal. This similarity to the Group 1 metals led to the prediction that it should be possible to create metallic hydrogen. This would be a solid in which the hydrogen atoms (protons in effect) would be held in a 3-dimensional array with delocalised electrons acting as the metallic bonds holding the array together. This metallic hydrogen would, in theory, be an excellent conductor, indeed it would be a "superconductor", which is why the race has been on to create it!

A chemical formula of a covalent molecule tells us how many atoms of each element are covalently bonded together, H₂ has 2 atoms of hydrogen with a covalent bond between them.
But the chemical formula for a 3-dimensional metallic array refers to the ratio of atoms of each element, if only 1 element is present in a metallic array, like that of sodium metal, then the chemical formula is just the symbol for the element, Na, in this case, or H if you are referring to metallic hydrogen.
So, H is a valid chemical formula for metallic hydrogen, if it exists.
But does metallic hydrogen exist?

In January 2017, researchers at Harvard University announced that they had produced metallic hydrogen in the laboratory using immense pressure. So metallic hydrogen, H, can exist.

Back to the game show.
The Host was right, H₂ is the chemical formula for gaseous hydrogen in the atmosphere.
The Chaser was right, H is the chemical formula for metallic hydrogen.
The Contestants were almost right: Chemists don't write H₁ they just write H.

There is a moral to this story.
Be careful when writing questions. The question should not be ambiguous unless you are prepared to accept multiple different answers that are correct.
Be even more careful when answering test and exam questions. If you need to make assumptions to answer the question you MUST state what those assumptions are when you write your answer.

Reference:

1. Ranga P. Dias, Isaac F. Silvera. Observation of the Wigner-Huntington Transition to Metallic Hydrogen. Science, 2017 DOI: 10.1126/science.aal1579

Tutorials Relevant to this Post

Periodic Table of the Elements
Elements and Compounds
Metals and Nonmetals
Molecular Formula

Naming Covalent Compounds
Empirical Formula and Molecular Formula
Trends in Group 1 Elements
Metallic Bonding

Suggested Study Questions

1. Use the Periodic Table of the Elements to find the chemical symbol for each of the following atoms:
• hydrogen
• helium
• carbon
• nitrogen
• oxygen
• chlorine
2. Write a molecular formula for each of the following diatomic gas molecules:
• hydrogen
• nitrogen
• oxygen
• chlorine
3. Give the number of atoms of each element present in the molecular formulae below:
• H₂O
• H₂O₂
• CO
• CO₂
• NH₃
• NO
• NO₂
• N₂O₂
4. Let M represent an atom of an element. Circle the elements below for which the molecular formula of the element at room temperature and pressure could be represented by M
• helium
• sodium
• oxygen
• iron
• gold
• neon
• chlorine
• nitrogen
• hydrogen
5. For the description of each molecule below, write the molecular formula
• one carbon atom and four hydrogen atoms
• one nitrogen atom and three chlorine atoms
• two nitrogen atoms and one oxygen atom
• one nitrogen atom and five oxygen atoms
• two chlorine atoms and two oxygen atoms
• one carbon atom, one hydrogen atom and three chlorine atoms
6. Given the name of each molecule below, write the molecular formula:
• hydrogen chloride
• carbon monoxide
• carbon dioxide
• sulfur dioxide
• sulfur trioxide
• sulfur dichloride
7. Consider the list of compounds with a possible molecular formulae below. Circle the incorrect formulae and justify your answer:
• water, 2HO
• carbon monoxide, C₁O₁
• hydrogen peroxide, H2O₂
• sulfur trioxide: SO₂
• ammonia, NH3
• hydrogen sulfide, H₂S
• carbon dioxide, C₂O
• sulfur dichloride, S₁Cl₂
8. From the list below, circle the elements that belong to Group 1 of the Periodic Table of the Elements:
• sodium
• helium
• oxygen
• lithium
• chlorine
• nitrogen
• carbon
• potassium
• calcium
9. Draw a table with the headings "metal" and "nonmetal". Place each of the following elements in the correct column:
• hydrogen
• helium
• calcium
• carbon
• nitrogen 
• potassium
• oxygen
• chlorine
• sodium
10. From the list below, circle the elements that would exist at room temperature and pressure as an array of "atoms" help together by delocalised electrons:
• hydrogen
• carbon
• sodium
• lithium
• nitrogen
• chlorine
• iron
• gold
• oxygen

Lithium for a Longer Life

Lithium is the 25^th most abundant element in the Earth's crust, with approximately 20mg of lithium present in every kilogram of crustal material. It is a soft, silvery-white metal that belongs to Group I (alkali metals). It is so highly reactive that when it is cut in air it will quickly corrode before your eyes. In the presence of water, lithium reacts to form hydrogen gas and lithium hydroxide in aqueous solution. Because it is so reactive, lithium does not occur free in nature, it only appears naturally in compounds.

Lithium-6 and lithium-7 were among the three elements synthesized in the Big Bang according to cosmological theory, the other two elements being hydrogen and helium. Lithium is present in cooler, less massive brown dwarf stars but is destroyed in hotter red dwarf stars, so its presence in the stars' line emission (atomic) spectra can be used to differentiate between these two kinds of stars in the so-called 'lithium test'.

Trace amounts of lithium ions are present in the oceans. The total lithium content of seawater is estimated to be 230 billion tonnes, and is present in concentrations of about 0.2 parts per million.
Lithium is also present in trace amounts in plants and animals. Vertebrates contain lithium in concentrations between 21 and 763 parts per billion.

Lithium salts, such as lithium carbonate, have been to shown to be useful as mood-stabilizing drugs. Therapeutically useful amounts of lithium are between 1.0 and 1.2 millimolar, which is only slightly lower than the toxic amount of 1.5 millimolar.

Scientists at the Friedrich Schiller University Jena have just demonstrated that a regular uptake of lithium could lead to a longer life. The scientists studied the impact of lithium in a concentration that is regularly found in ordinary tap water by analyzing the mortality rate in 18 adjacent Japanese municipalities in relation to the amount of lithium contained in tap water from the respective regions. They found that the mortality rate was considerably lower in those municipalities with more lithium in the drinking water.

Reference
Kim Zarse, Takeshi Terao, Jing Tian, Noboru Iwata, Nobuyoshi Ishii, Michael Ristow. Low-dose lithium uptake promotes longevity in humans and metazoans. European Journal of Nutrition, 2011; DOI: 10.1007/s00394-011-0171-x

Further Reading
Elements and Compounds
Metals and Non-metals
Trends in Group I
Electron Configuration
Naming Ionic Compounds
Writing Ionic Formulae
Parts per Million Concentration
Molarity Concentration
Emission (Atomic) Spectra
Isotopes
Relative Atomic Mass

Study Questions

1. What is the atomic number of lithium?
2. What is the simple electron configuration for an atom of lithium?
3. What is the expected charge on a lithium ion? Explain your answer.
4. Write the formula for each of these compounds:
   
   • lithium hydroxide
   • lithium carbonate
5. Write a balanced chemical equation to represent the reaction between lithium metal and water.
6. For each of the following isotopes of lithium, give the number of protons and the number of neutrons present:
   
   • lithium-6
   • lithium-7
7. Given that the relative atomic mass of lithium is 6.941, and assuming lithium-6 and lithium-7 are the only isotopes of lithium present, calculate the abundance of each isotope.
8. Convert the following to concentrations in molL^-1 (M):
   
   • 0.2ppm
   • 1.2millimolar
   • 1.5millimolar
     
   • 21ppb
   • 763ppb
9. If the total lithium content of seawater is estimated to be 230 billion tonnes, and is present in concentrations of about 0.2 parts per million, what is the mass of seawater present on Earth?
10. In the 'lithium test' for stars, what spectral lines do you expect to see in brown dwarf stars that will not be present in red dwarf stars?