About 200 years ago, the famous scientist John Dalton realised he could work out the mass of one kind of atom compared to another kind through chemical experiments. For example, he discovered that 1 gram of hydrogen gas reacts with 8 grams of oxygen gas to make water. Dalton didn’t know how many atoms had reacted, or how many molecules were made, so he couldn’t work out the actual mass of a single atom or molecule in grams. But he was interested to know much an oxygen atom weighed compared to a hydrogen atom. That is, the ratio of their masses.
Dalton was tricked because he thought that a water molecule was made of just one hydrogen atom and one oxygen atom, making its formula HO.
This would’ve meant that an equal number of hydrogen and oxygen atoms had reacted, and an equal number were in the water. Because the water molecules were made from 1 gram of hydrogen combined with 8 grams of oxygen, he concluded that an oxygen atom must be 8 times heavier than a hydrogen atom.
But when scientists realised that water is H2O, and that there are twice as many hydrogen as oxygen atoms, they knew that an oxygen atom must be 16 times heavier than a hydrogen atom. Putting it the other way around, the ratio of a hydrogen atom to an oxygen atom is 1:16.
A similar experiment between hydrogen and nitrogen to make (bump) ammonia, NH3, showed that a nitrogen atom is 14 times heavier than a hydrogen atom. And combining hydrogen with carbon to make methane, CH4, showed that a carbon atom is 12 times heavier than a hydrogen atom.
This compares the 3 atoms’ masses to hydrogen’s.
H : C : N : O
1 : 12 : 14 : 16
In 1803 Dalton suggested that these numbers be called the element’s Atomic Weight, or more accurately, Atomic Mass.
By giving the lightest atom- hydrogen- an Atomic Mass of 1, all the other atoms are bigger than 1.
He could have chosen, say, oxygen to have a value of 1 and compared everything else with it instead, but that would require dividing all the other numbers by 16, 134 making these other Atomic Masses, fractions that are less than one. Making hydrogen’s mass 1 is the simplest choice, and scientists like simple. They’re now worked out the Atomic Masses of all kinds of atoms, so each element has 2 numbers belonging to it- its Atomic Number showing its place in the list of atoms, and now its Atomic Mass showing how heavy it is compared to hydrogen.
You can see all elements’ Atomic Masses at the bottom of their squares in the Periodic Table. Notice that while an element’s Atomic Number is always a whole number, its Atomic Masses isn’t, and we’ll explain that in another video.
Later scientists changed the definition of Atomic Mass to a comparison with carbon taken as 12, but this doesn’t change the numbers much because the ratios are almost identical.
When a molecule is made from atoms, its mass is just the sum of its Atomic Masses. This is called its Molecular Mass, not surprisingly. What’s the Molecular Mass of water, H2O? It contains 2 hydrogen atoms, each with an Atomic Mass of 1, and one oxygen atom with an Atomic Mass of 16. So add them up and its Molecular Mass comes to 2 x 1 + 16 = 18.
How about CO2? It’s made of one carbon atom with an Atomic Mass of 12, and two oxygen atoms each with an Atomic Mass of 16. So its Molecular Mass is 12 + 2 x 16 = 44.
Of course you won’t find Molecular Masses on the Periodic Table, because they’re the combination of different Atomic Masses. Each square can only belong to one kind of atom, so you’ll only find Atomic Masses here.
But that’s all you need because you can put them together to work out any Molecular Mass you want.