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Every element has a number: hydrogen is 1, carbon is 6, iron is 26, gold is 79. That number, the atomic number, isn’t just a position in a list. It’s the single most important fact about an element, because it tells you how many protons are in the nucleus of every atom of that element. Change it, and you have a different element entirely.
Definition
Atomic number (Z) = the number of protons in the nucleus of an atom.
It’s usually written as a subscript before the element symbol, for example ₆C for carbon, or at the top of each box on the periodic table.
Why protons define the element
Atoms contain protons, neutrons and electrons. Why is it the proton count that decides which element an atom is?
- Electrons can change. Atoms gain or lose electrons in chemical reactions to form ions. A sodium atom that loses an electron becomes a sodium ion, Na⁺, but it’s still sodium. See what is an ion?
- Neutrons can vary. Atoms of the same element can have different numbers of neutrons. Carbon-12, carbon-13 and carbon-14 all have 6 protons but 6, 7 and 8 neutrons. They’re isotopes of carbon, and they behave almost identically in chemical reactions.
- Protons are fixed in chemistry. Ordinary chemical reactions involve only electrons; they never change the nucleus. So the proton count stays constant through any chemical change.
There’s also a deeper reason. The number of protons sets the positive charge of the nucleus, and that charge decides how many electrons a neutral atom holds and how strongly they’re attracted. That, in turn, determines the atom’s electron arrangement and therefore its chemical behaviour. Protons ultimately control chemistry, even though chemistry itself happens among the electrons.
What atomic number tells you
For a neutral atom, the atomic number gives you both:
- the number of protons (by definition)
- the number of electrons (equal to the number of protons, so the charges balance)
It doesn’t tell you the number of neutrons. For that, you need the mass number (A), the total of protons and neutrons:
Number of neutrons = A − Z
See mass number explained and protons, neutrons and electrons.
Worked examples
Example 1: Oxygen has atomic number 8. How many protons and electrons does a neutral oxygen atom have?
8 protons and 8 electrons.
Example 2: An atom has 20 protons, 20 neutrons and 18 electrons. What is it?
Atomic number = 20, so it’s calcium. It has 2 fewer electrons than protons, so it’s the ion Ca²⁺.
Example 3: Chlorine-37 has atomic number 17. How many neutrons does it have?
Neutrons = 37 − 17 = 20.
Example 4: An ion has 16 protons and 18 electrons. Identify it.
Z = 16, so it’s sulfur. Two extra electrons give a 2− charge: S²⁻ (sulfide).
How atomic number was discovered
In Dmitri Mendeleev’s periodic table of 1869, elements were ordered mainly by atomic mass. That mostly worked, but there were awkward exceptions. For example, tellurium (atomic mass about 127.6) is heavier than iodine (about 126.9), yet iodine clearly belongs with the halogens (fluorine, chlorine, bromine) and tellurium with oxygen, sulfur and selenium. Mendeleev swapped them based on their properties, trusting chemistry over mass. See the history of the periodic table.
The explanation came in 1913, from the young English physicist Henry Moseley. He fired electrons at samples of different elements and measured the X-rays they emitted. He found that the square root of the X-ray frequency increased in perfectly regular steps from one element to the next. Each step corresponded to one more unit of positive charge in the nucleus.
Moseley’s work showed that:
- The true ordering principle of the periodic table is nuclear charge (atomic number), not mass.
- Tellurium (52) really does come before iodine (53), justifying Mendeleev’s swap.
- There were gaps at atomic numbers 43, 61, 72 and 75, corresponding to elements not yet discovered (later found to be technetium, promethium, hafnium and rhenium).
Tragically, Moseley was killed in 1915 during the First World War, aged 27. Many scientists believe he would otherwise have received a Nobel Prize.
Atomic number and the periodic table
The modern periodic table is arranged in order of increasing atomic number. Each element has exactly one more proton than the element before it. Because the number of electrons also increases by one each time, the electron arrangements repeat in patterns, producing the periodic trends in properties. See why is the periodic table shaped like that? and s, p, d and f blocks.
There are no gaps: every whole number from 1 (hydrogen) to 118 (oganesson) corresponds to a known element. Explore each one on the interactive periodic table or find individual element pages such as carbon.
Can atomic number change?
Not in chemistry, but yes in nuclear processes:
- Alpha decay: the nucleus loses 2 protons and 2 neutrons. Z decreases by 2. For example, uranium-238 (Z = 92) decays to thorium-234 (Z = 90).
- Beta-minus decay: a neutron changes into a proton, emitting an electron. Z increases by 1. Carbon-14 (Z = 6) decays to nitrogen-14 (Z = 7).
- Positron emission and electron capture: a proton becomes a neutron. Z decreases by 1.
- Nuclear reactions in accelerators: new, heavier elements are made by fusing nuclei. See how new elements are made.
In each case, changing Z means one element has literally turned into another. The alchemists’ dream of turning lead into gold is possible in principle, but only with nuclear physics, and in tiny, uneconomic amounts. See alpha, beta and gamma radiation.
Atomic number vs other numbers
| Quantity | Symbol | What it counts | Changes in chemical reactions? |
|---|---|---|---|
| Atomic number | Z | protons | no |
| Mass number | A | protons + neutrons | no |
| Number of electrons | — | electrons | yes (ions form) |
| Relative atomic mass | Aᵣ | weighted average mass of the isotopes | no |
Why it matters in everyday chemistry
Atomic number underlies almost every calculation and prediction in chemistry. It tells you how many electrons to place when writing an electron configuration, which group and period an element sits in, and therefore how it’s likely to react. When you see that sodium is element 11, you can work out that it has one outer electron and will readily form Na⁺ ions, without ever having seen sodium react.
A quick self-test: if an element has atomic number 17, what do you know without looking anything else up? It has 17 protons, a neutral atom has 17 electrons, and it sits in group 17 with seven outer electrons.
Key takeaways
- Atomic number (Z) is the number of protons in the nucleus.
- It defines the element: every atom with 6 protons is carbon, whatever its neutrons or electrons.
- In a neutral atom, the number of electrons equals the atomic number.
- Henry Moseley showed in 1913 that nuclear charge, not atomic mass, orders the periodic table.
- Chemical reactions never change Z; nuclear reactions and radioactive decay can.
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