The Periodic Table · Grade 9

What does the atomic number count, and why does it fix an element's identity?

The atomic number counts the protons in the nucleus, and nothing else. The number of protons is what decides which element it is: an atom with six protons is carbon, however many neutrons it carries. When the table was ordered by proton number instead of by mass, exactly the places where the mass order looked wrong were put right.

Learning objectives

Three particles

A proton carries a positive charge, sits in the nucleus and has a mass of one unit. A neutron has no charge, also sits in the nucleus, and has roughly the mass of a proton. An electron carries a negative charge, moves outside the nucleus, and its mass is negligible — about a two-thousandth of a proton's.

Two conclusions follow immediately. Almost all of an atom's mass is in the nucleus, and almost all of its volume is outside it.

In a neutral atom the number of electrons equals the number of protons, so the charges cancel. An ion is an atom that has lost or gained electrons, so the two numbers no longer match.

Two numbers in every box

The atomic number is the number of protons. The mass number is protons plus neutrons. The difference between them is the number of neutrons, and that is all the arithmetic required here.

Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 and carbon-14 are both carbon because both have six protons; they differ in mass, not in identity.

So two isotopes share one box in the table. The box is sorted by proton number, and that is the same for both. It is also why the atomic mass printed in the table is not a whole number — it is a weighted average of the isotopes found in nature.

What the atomic number fixed

There were three places in Mendeleev's table where ordering by mass gave an odd result. To keep the properties in the right column he had to swap the order of those elements, without being able to say why.

Once it emerged that the elements are ordered by proton number, those swaps stopped being exceptions. They became the correct order, and it was the ordering by mass that had been the approximation.

This is a familiar pattern in science: the pattern was found first, and the explanation arrived afterwards and corrected precisely the places where the pattern had looked broken.

Worked examples

  1. An atom has atomic number 17 and mass number 35. How many protons, neutrons and electrons?

    1. Protons = atomic number = 17
    2. Neutrons = 35 minus 17 = 18
    3. In a neutral atom, electrons = protons = 17

    Answer: 17 protons, 18 neutrons, 17 electrons. This is chlorine

  2. Is an atom with 6 protons and 8 neutrons still carbon?

    1. Identity is set by proton number alone
    2. 6 protons means carbon
    3. 8 neutrons instead of 6 changes the mass

    Answer: Yes. That is carbon-14, an isotope of carbon

  3. An ion has 11 protons and 10 electrons. What is its charge and which element is it?

    1. 11 protons means sodium
    2. There is one electron fewer than protons
    3. A single positive charge

    Answer: A sodium ion with a charge of 1+

Common mistakes

Thinking mass decides which element it is
Two isotopes differ in mass and are the same element. What decides is the number of protons, which is why the table is sorted by it.
Adding instead of subtracting to find neutrons
The mass number already includes the protons. Neutrons are the difference, and an answer larger than the mass number gives the error away.
Assuming electrons always equal protons
That holds only in a neutral atom. In an ion the numbers differ, and the difference between them is exactly the charge.

What to remember

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