The atomic number of an element is the number of protons in the nucleus of its atoms. This number is what makes each element unique — change the number of protons and you have a completely different element. Hydrogen has 1 proton, helium has 2, lithium has 3, and so on. The atomic number also tells you the number of electrons in a neutral atom, since protons and electrons must balance.
Look at a periodic table and notice that the elements are arranged in order of increasing atomic number. Pick several elements and practice identifying their atomic number, then predicting how many protons and electrons a neutral atom of each has.
You have learned that atoms contain protons, neutrons, and electrons. But what makes a gold atom different from an iron atom or a carbon atom? The answer comes down to one number: the atomic number, which is simply the count of protons in an atom's nucleus.
Every element on the periodic table has its own unique atomic number. Hydrogen, the simplest element, has an atomic number of 1 — every hydrogen atom has exactly 1 proton. Carbon has an atomic number of 6, so every carbon atom has 6 protons. Gold has an atomic number of 79. There are no exceptions: if an atom has 79 protons, it is gold. If it has 78 protons, it is platinum. If it has 80, it is mercury. The proton count is the identity card of every element.
This idea has a powerful consequence. You can change the number of neutrons in an atom and it remains the same element (you just get a different isotope, which you will learn about soon). You can add or remove electrons and it remains the same element (you get an ion with an electrical charge). But if you change the number of protons, you have transformed one element into another entirely. In fact, that is essentially what happens in nuclear reactions — protons are added or removed, turning one element into a different one.
For a neutral atom — one with no overall electrical charge — the atomic number also tells you the electron count. Since each proton contributes a +1 charge and each electron contributes a -1 charge, a neutral atom must have equal numbers of both. So a neutral oxygen atom (atomic number 8) has 8 protons and 8 electrons. This makes the atomic number doubly useful: it identifies the element and, for neutral atoms, immediately tells you how many electrons are present.
When you look at a periodic table, the elements are arranged in order of increasing atomic number, starting with hydrogen at 1 and continuing through more than 100 known elements. This ordering is not random — it reflects the fundamental structure of atoms and creates patterns in how elements behave, which you will explore as you learn about the periodic table's rows and columns.