The proton number (Z) is the number of protons; the nucleon number (A) is the number of protons plus neutrons, so neutrons = A − Z. Isotopes are atoms of the same element with the same proton number but different nucleon numbers; they have the same chemical properties but different physical properties.
This page covers a single Form 4 content standard: proton number, nucleon number and isotopes. It is one of the most calculation-friendly standards in SPM Chemistry, once you can read standard atomic notation, you can count protons, neutrons and electrons in seconds, and explain isotopes with confidence.
Proton number and nucleon number
Two numbers define an atom:
- The proton number (also called the atomic number, symbol Z) is the number of protons in the nucleus. It identifies the element, every atom of a given element has the same proton number.
- The nucleon number (also called the mass number, symbol A) is the total number of protons and neutrons in the nucleus.
From these two numbers you can find everything else:
- Number of neutrons = A − Z (nucleon number − proton number).
- Number of electrons = number of protons = Z (in a neutral atom).
The standard atomic notation writes the nucleon number as a top-left superscript and the proton number as a bottom-left subscript before the symbol, for example the notation for sodium with A = 23 and Z = 11. Read the top number as protons + neutrons and the bottom number as protons.
Isotopes
Isotopes are atoms of the same element (so the same proton number) that have different nucleon numbers because they contain different numbers of neutrons. Common examples include the three isotopes of hydrogen (with 0, 1 and 2 neutrons), carbon-12, carbon-13 and carbon-14, and chlorine-35 and chlorine-37.
Because isotopes have the same number of electrons and the same electron arrangement, they have the same chemical properties, they react in exactly the same way. But because they have different numbers of neutrons, they have different masses, and therefore slightly different physical properties such as density, melting point and boiling point.
Key idea. Same proton number → same element and same chemical properties. Different neutron number → different mass and different physical properties. Chemistry is decided by electrons; mass is decided by protons and neutrons.
Isotopes have important uses: carbon-14 is used in radiocarbon dating, cobalt-60 in cancer treatment and sterilising equipment, and iodine-131 in diagnosing and treating thyroid problems. You are not required to memorise a long list, one or two correct examples are enough.
Worked example
Question. An atom of chlorine has the notation with nucleon number 37 and proton number 17. State the number of protons, neutrons and electrons in this atom, and explain why it has the same chemical properties as chlorine-35.
Step 1, Read the two numbers. Nucleon number A = 37; proton number Z = 17.
Step 2, Find the protons and electrons. Number of protons = Z = 17. In a neutral atom, number of electrons = number of protons = 17 electrons.
Step 3, Find the neutrons. Number of neutrons = A − Z = 37 − 17 = 20 neutrons.
Step 4, Explain the chemistry. Chlorine-37 and chlorine-35 both have 17 protons and 17 electrons, so they have the same electron arrangement. Chemical properties depend on the electron arrangement, so the two isotopes react in the same way.
Answer. 17 protons, 20 neutrons, 17 electrons. Chlorine-37 has the same chemical properties as chlorine-35 because both have 17 electrons in the same arrangement; they differ only in the number of neutrons (and hence in mass).
Practice question
Question. An atom has a proton number of 19 and a nucleon number of 39. (a) Name the element and state the number of neutrons. (b) A second atom of the same element has a nucleon number of 41. State the relationship between the two atoms and the number of neutrons in the second atom.
Answer. (a) Proton number 19 is potassium; number of neutrons = 39 − 19 = 20 neutrons. (b) The two atoms are isotopes (same proton number, different nucleon numbers). The second atom has 41 − 19 = 22 neutrons. They have the same chemical properties but different masses.
Exam tip
Turn every notation question into three quick steps: read Z (protons, and also electrons in a neutral atom), read A (nucleons), then subtract to get neutrons (A − Z). The single most common error is to give the nucleon number as the number of neutrons, it is protons plus neutrons, so you must subtract. For isotope questions, always answer in two halves: state that the chemical properties are the same (same electron arrangement) and the physical properties are different (different mass because of different neutron numbers). Naming that split earns the marks in full.
Isotopes and relative atomic mass
The existence of isotopes explains why many elements have a relative atomic mass that is not a whole number. Naturally occurring chlorine, for example, is a mixture of chlorine-35 and chlorine-37, and its relative atomic mass is about 35.5, a value between the two nucleon numbers, closer to 35 because chlorine-35 is the more abundant isotope. The relative atomic mass is therefore the average mass of the atoms of an element, taking into account how much of each isotope is present. You do not need to carry out a full weighted-average calculation here, but understanding that a non-whole-number relative atomic mass is a clue that the element exists as a mixture of isotopes is exactly the kind of link the exam rewards, and it connects this standard forward to the mole and formula calculations in the next chapter.
Where this fits
This standard follows directly from atomic structure and subatomic particles in the Matter and the Atomic Structure chapter. Revise it with the revision notes and practise counting particles with the worked examples. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers check that you never confuse nucleon number with neutron number, and that your isotope answers always cover both the “same chemical” and “different physical” halves.
Quick recap
- Proton number Z = number of protons (= electrons in a neutral atom); it identifies the element.
- Nucleon number A = protons + neutrons; neutrons = A − Z.
- Isotopes: same proton number, different nucleon numbers (different neutrons).
- Isotopes have the same chemical properties but different physical properties.
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