Writing the Formula of an Inorganic Salt

Organic chemistry deals with compounds of carbon.
Inorganic chemistry deals with compounds of any other element.
One of the earliest known classes of inorganic compounds were salts.
Salts are binary, ionic compounds, that is, salts are made up of two ions:

• a positively charged ion called a cation
• a negatively charged ion called an anion 

The International Union of Pure and Applied Chemistry (IUPAC) is establishing rules to help us name, and write the formula, for compounds.
In an earlier tutorial we looked at how we name inorganic salts, in this tutorial we look at how to write the formula of an inorganic salt.

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

Oxygen Gas From Comets

Rosetta was a European Space Agency (ESA) Mission, launched in 2004 with the goal of capturing comet 67P/Churyumov-Gerasimenko in 2014 and to accompany it into the interior solar system. Onboard was an instrument known as ROSINA (Rosetta Orbiter Sensor for Ion and Neutral Analysis) which combined two mass mass spectrometers to study the composition of  the comet's corona.
In 2015, researchers from the Center for Space and Habitability (CSH) at the University of Bern analysed the comet's gases and made an unexpected discovery, traces of molecular oxygen (O_2(g)) were detected! It turned out that molecular oxygen was the fourth most abundant gas in the comet's atmosphere after water, carbon monoxide and carbon dioxide!
Where did this molecular oxygen come from? Surely if it was formed very early, at about the same time as the Solar System, it would have reacted with other substances by now because molecular oxygen is very reactive. We could expect compounds containing oxygen like carbon dioxide and water, but not molecular oxygen.
This puzzle was only solved in 2017.

Caltech Professor Konstantinos P. Giapis studies chemical reactions involving high-speed ions colliding with semiconductor surfaces as a means to create faster computer chips and larger digital memories for computers and phones. He thought the same thing was happening in the comet.

First, as the comet is heated by the sun, water vapor is released from the icy comet..
Next, ultraviolet light from the sun causes the water molecules to become ionised.
Then the ionized water molecules are pushed back towards the surface of the comet by the sun's wind.
After that the ionized water molecules hit oxygen containing compounds on the surface of the comet like rust and sand.
Finally the molecules pick up another oxygen atom from these surfaces to form molecular oxygen.

This gives us a possible mechanism by which molecular oxygen could be produced in space without the need for living things, and, could change the way we search for signs of life on planets beyond our solar system.


Reference
California Institute of Technology. "Chemical engineer explains oxygen mystery on comets." ScienceDaily. ScienceDaily, 8 May 2017. .

Further Reading:
Pure Substances and Mixtures

Elements and Compounds

Molecular Formula

Molecular Formula for Covalent Compounds

Physical and Chemical Changes

Mass and Moles in a Chemical Reaction

Mass Spectroscopy for Isotopes

Mass Spectroscopy for Structural Determination


Suggested study Questions:

1.  Which of the following are pure substances?
   • molecular oxygen
   • water
   • carbon monoxide
   • carbon dioxide
   • rust
2.  Which of the following substances are mixtures?
   • molecular oxygen
   • water
   • carbon monoxide
   • carbon dioxide
   • rust
3. Which of the following substances are elements?
   • molecular oxygen
   • water
   • carbon monoxide
   • carbon dioxide
   • rust
4. Which of the following substances are compounds?
   • molecular oxygen
   • water
   • carbon monoxide
   • carbon dioxide
   • rust
5.  Write the molecular formula for each of the following:
   • molecular oxygen
   • water
   • carbon monoxide
   • carbon dioxide
6.  Write chemical equation for the first reaction that occurs on the comet's surface.
7.  Explain how a water molecule could be ionized.
8.  Explain why molecular oxygen is considered to be a reactive molecule. 
9.  Assume that a comet has a mass of 10¹⁴ kg and that it is composed only of water. Calculate the maximum mass of molecular oxygen that could be formed if the entire comet was vaporized.
10.  Explain how a mass spectrometer can be used to identify elements and compounds in space.

  

The First Helium Compounds

Can helium form compounds?
Helium, He, is the first Group 18 element or Noble Gas element.
In the nucleus of a helium atom there are 2 protons, and surrounding the nucleus there are just 2 electrons. These 2 electrons complete the first energy level and require large amounts of energy to remove. For this reason, an atom of helium is very stable and does not undergo chemical reactions on Earth.
But what if you tried to react helium, a non-metal, with a highly reactive metal, such as sodium, at extremely high pressures (greater than 113 GPa)?
Utah University Chemists predicted that under these conditions helium and sodium would form a compound. Then, high pressure synthesis in a diamond anvil actually produced a stable compound with the formula Na₂He.

On the left hand side is a "ball and stick" model of Na₂He in which the purple balls represent sodium and the white balls represent helium.You can see that the structure is cubic.
On the right hand side is a polyhedral representation of Na₂He in which sodium atoms form the cubes (Na₈). Half of these cubes are occupied by helium atoms, and these are shown as grey boxes. But the other half of the cubes are occupied by 2 electrons and these are shown in the diagram as red spheres.
The chemists are predicting that other compounds of helium may also be possible, such as Na₂HeO.

Reference:
 Utah State University. "Up, up and away: Chemists say 'yes,' helium can form compounds: Helium and sodium form stable compound at high pressure." ScienceDaily. www.sciencedaily.com/releases/2017/02/170206111848.htm (accessed February 12, 2017).

Further Reading:
Introduction to the Modern Periodic Table 
 Bohr Model of the Atom 
Evidence for Electron Configuration of an Atom
Subshell Electronic Configuration

Suggested Study Questions:

1. What is the atomic number for
   • helium
   • sodium
2.  How many electrons are present in an atom of
   • helium
   • sodium
3.  Write the simple electronic configuration for an atom of each element below:
   • helium
   • sodium
4.  Write the electron configuration for an atom of each element below using subshell notation:
   • helium
   • sodium
5. Write an equation for the loss of an electron from a gaseous atom of sodium. 
6. Give the simple electronic configuration for the ion of sodium produced above.
7. Give the electronic configuration of the sodium ion produced above in terms of subshells.
8. Explain why the first ionisation of helium is so much higher than the first ionisation energy of sodium.
9. Explain why sodium readily forms compounds.
10. Explain why it is extremely difficult to produce helium compounds, and why it has required such enormous pressures in order to produce the first helium compound.

Phlogiston Theory

Before the modern chemical ideas of atoms and elements, phlogiston theory was a widely held belief.
According to phlogiston theory, matter consisted of three essential essences:

• sulfur (terra pinguis - the essence of inflammability, which was to become known as phlogiston)
• mercury (terra mercurialis - the essence of fluidity)
• salt (terra lapida - the essence of fixity and inertness)

 Phlogiston theory was an early attempt to to try to explain what happened when things were burnt or combusted. In phlogiston theory, substances were made up of a "calx" (or residue) combined with phlogiston (the essence of inflammability). When a substance was burnt (combusted), phlogiston was released, and the residue (calx) was left behind. Even the rusting of iron could be explained using phlogiston theory, because the "iron" would lose its phlogiston during the rusting process and leave behind the "calx" or residue. One of the problems with the phlogiston theory is that metals should lose mass when they burn, owing to the loss of phlogiston.

In the 18th century, Antoine-Laurent de Lavoisier, the man who is considered to be the father of modern of modern chemistry, conducted a series of combustion experiments. In these experiments he carefully weighed the substances to be combusted as well as the products of combustion, and found that the weight of the products of combustion was greater than the weight of the substance before combustion. He also demonstrated that when a substance corrodes in a sealed container, the gain in weight of the substance is equal to the loss in weight of the air in the container.
This was the beginning of the downfall of the phlogiston theory, but it was the beginning of the modern chemistry when Lavoisier generalized that if the weights of all substances involved in a chemical reaction are considered then there is no overall loss or gain in weight.

Further Reading:
http://www.ausetute.com.au/elemhist.html
http://www.ausetute.com.au/elements.html
http://www.ausetute.com.au/atomichist.html
http://www.ausetute.com.au/wriiform.html
http://www.ausetute.com.au/namiform.html
http://www.ausetute.com.au/namcform.html
http://www.ausetute.com.au/balcheme.html

Suggested Study Questions

1. Lavoisier burnt sulfur. When sulfur burns in air, it reacts with oxygen to form sulfur dioxide. Write a word equation for this reaction.
2. Write the chemical formula for each of the following:
   • sulfur
   • oxygen gas
   • sulfur dioxide gas
3. Write a balanced chemical equation for the combustion of sulfur to produce sulfur dioxide gas.
4. If Lavoisier had weighed out 32 grams of sulfur and then burnt it in air, the sulfur dioxide he collected would have a mass of 64 grams. What mass of oxygen would have been reacted with the sulfur? 
5. Explain why a substance such as sulfur appears to gain mass when it is combusted.
6. When wood is burnt, the mass of the ashes left behind is actually less than the mass of wood you started with. How do you explain this loss of mass?
7. Magnesium is a metal that combusts readily in oxygen gas to form magnesium oxide.
   • Write a word equation for the combustion of magnesium to form magnesium oxide,
   • Write a balanced chemical equation for this reaction.
8. If 20.16 grams of magnesium oxide is produced as a result of the combustion of 12.16 grams of magnesium, how much oxygen gas was consumed during the reaction?
9. Imagine you are living in the 18th century and that you are a firm believer in the phlogiston theory. How would you explain to Lavoisier how metals gain mass when they burn?