Atomic Number, Elements and the Periodic Table

Atoms and ions can be grouped into categories called elements, and these elements are listed in a table called the periodic table. However, before we can understand what elements and the periodic table are, we first have to learn about something called atomic number.

Atomic number is the number of protons in an atom or ion

As discussed previously, every atom or ion has a structure in its centre called the nucleus, which is a ball of protons and neutrons. The number of protons and neutrons in the nucleus varies from one atom or ion to the next.

The number of protons in the nucleus of an atom or ion is called its atomic number. For example, if an ion has seven protons, then its atomic number is seven.

Atomic number is also known as proton number.

An element is a group of atoms and ions with the same atomic number

Atoms and ions are grouped into categories called elements based on their atomic number. Every element has an atomic number, and all atoms and ions with that atomic number are atoms and ions of that element.

For example, the element magnesium has an atomic number of 12. This means that any atom that has an atomic number of 12 is a magnesium atom, and any ion that has an atomic number of 12 is a magnesium ion.

Note that the number of neutrons does not affect which element something is. For example, some magnesium atoms have 12 neutrons, others have 13, and others have 14. However, they are all still magnesium atoms because they all have 12 protons (in other words, they all have an atomic number of 12).

Similarly, the number of electrons does not affect which element something is. In an atom, the number of electrons is always the same as the number of protons, but in an ion the number of electrons could be anything as long as it is not the same as the number of protons. This makes no difference to which element it is - the element depends only on the number of protons.

The elements are listed in the periodic table

The periodic table is a table which lists all of the known elements. The elements are listed in order of increasing atomic number.

The periodic table. It is made up of boxes arranged in rows and columns. Each box contains information about one element.

The periodic table.

The first element in the periodic table is hydrogen, which has an atomic number of 1. The next element is helium, with an atomic number of 2, followed by lithium, which has an atomic number of 3.

The table continues in this way all the way up to the element with an atomic number of 118, oganesson. So far, no ions or atoms have been discovered with more than 118 protons. If such atoms or ions are discovered in the future, then new elements will be named and added to the periodic table.

The periodic table allows us identify which element any atom or ion is as long as we know how many protons it has. For example, if we know that an ion has 37 protons, then we simply look in the periodic table for the element with an atomic number of 37, which turns out to be rubidium. This tells us that it is a rubidium ion.


Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.


What is atomic number? What is it also known as?


What is an element?


What is the periodic table?


In what order are the elements in the periodic table listed?


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