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Lesson 3 of 7 · Textbook 8.2

The proton number names the element

One number, and no two elements are allowed to share it. Almost everything in this section follows from that single rule.

Reading the proton number

At present there are 118 elements that have been discovered — either naturally occurring or created in a laboratory.

The number written above the chemical symbol of an element in the periodic table is the proton number. It is the number of protons in an atom of that element. Helium's proton number is 2, so a helium atom contains two protons.

Reading a periodic table box

2 He Helium 4 proton number relative atomic mass
Same layout Iz used in his notes for iron: 26 above, Fe in the middle, 56 below.
The rule the whole section rests on Elements are arranged in the periodic table in order of increasing number of protons. No two elements can have the same number of protons in their atoms. The number of protons is what differentiates one element from another.
Element Protons Electrons Neutrons
hydrogen110
helium222
lithium334

Because they have different numbers of protons, they have unique atomic structures — and so different physical and chemical properties. Lithium is a solid, helium fills party balloons, and hydrogen is stored as a liquid in tanks.

Notice which column is doing the work Hydrogen has 0 neutrons and helium has 2 — but that is not what makes them different elements. Swap the neutron counts and hydrogen would still be hydrogen. It is the proton column that names the element, every time.

How the table came to be

Russian chemist Dmitri Mendeleev is widely known as the father of the periodic table. He devised a table containing the 63 elements known to exist at the time, and — crucially — left gaps for elements that had not yet been discovered.

His table had some inconsistencies. Iodine used to be placed before tellurium. Decades later another scientist found that arranging elements by increasing proton number resolved them, which is why tellurium now appears before iodine.

There have been more than a thousand different versions of the periodic table since Mendeleev's first draft. Work continues — attempts have even been made to use artificial intelligence to identify relationships between elements and develop alternative versions.

Making new elements

There are no naturally occurring elements with more than 92 protons on Earth. So how were elements 113, 115, 117 and 118 discovered?

Scientists collide atoms of two elements at high speed in a particle accelerator. The masses of the two elements used should add up to the mass of the new element.

Nihonium — the first element discovered in Asia Japanese physics professor Kosuke Morita and his team created element 113 using zinc atoms (30 protons) and bismuth atoms (83 protons). Note that 30 + 83 = 113. It took more than four trillion collisions over nine years. They named it nihonium after “Nihon”, the Japanese name for Japan; its symbol is Nh.

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