IV. Atomic Numbers
How Do We Make A List Nature’s 92 Elements?
The 92 different kinds of atoms, and the 92 elements that they make, have different colours, weights, smells, and hardness. Most elements are solids, but many are gases and two are liquids (bromine and mercury). How can we make a list of the elements, and what property do we use to sort them?
Your school class might have a list that is read out every morning during roll call. How is that organised? Alphabetically! If we listed the elements alphabetically, then we would put actinium first, and zirconium last. Do you think that this would be a logical way to list the elements?
Instead we might line the class up in order of students’ weights. Little Johnny Featherweight would be first in line, and big Chuck Heavyweight would be last.
Likewise, in 1869 the brilliant Russian scientist, Dmitri Mendeleev, decided to list the elements in order of their atoms’ weights– starting with the element with the lightest atoms, hydrogen, and going to the element with the heaviest atoms, uranium.
Scientists then give each element a number based on its place in this list, starting with the first and going to the last. This is called the element’s Atomic Number. What is the Atomic Number of the lightest element, hydrogen? 1. Helium is the 2nd lightest element. So its Atomic Number is 2. What must the Atomic Number of the heaviest element, uranium? 92.
Each element has its own Atomic Number, and each Atomic Number belongs to just one element. If you know an element’s Atomic Number, then you also know its name, and vice versa. It’s a bit like each prisoner having his own number, and the guard knows who each number belongs to.
The Atomic Number of an element is its place in the list from that with the lightest to that with the heaviest atoms.
Sidebar: In 1913 English scientist, Henry Moseley, discovered that the Atomic Number was also equal to the number of little particles called protons found in the nucleus (centre) of each atom. The modern definition is now updated to show this discovery.
More about that later.
3 ways to call an element
Every element has its own name, shorthand symbol and Atomic Number. They are unique to it, so knowing just one of these will tell us what element it is.
And if we know any one of these we then know the other two. For example, if we talk about oxygen, we also know that its symbol is O, and its Atomic Number is 8. Instead, an Atomic Number of 7 must refer to nitrogen, with a symbol of N. The symbol Li refers to lithium, so we also know that its Atomic Number is 3.
From the element parade below, you can work out what aluminium’s Atomic Number and symbol are. Scroll down for the answer.
How about the name and symbol for the element with the Atomic Number 79?
Can you discover the name and Atomic Number of the element with the symbol Sr?
Ans: The symbol for aluminium is Al, and its Atomic Number is 13.
Ans: The element with Atomic Number 79 is gold, with symbol Au.
Ans: The element with the symbol Sr is strontium, with an Atomic Number of 38