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Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · 1.9 Check the specification (PDF) (opens in a new tab)
An atom has a central nucleus containing and . The number of protons fixes the atom's identity: every chlorine atom, for example, has the same number of protons. If the proton number changed, the atom would become a different element.
The neutron number is allowed to vary. Atoms can therefore belong to the same element while having slightly different nuclei.
Isotopes are different atoms of the same element that contain the same number of protons but different numbers of neutrons in their nuclei.
Both parts of that description matter:
The word isotope describes a version of an element, not a different element or a different subatomic particle.
Chlorine-35 and chlorine-37 are isotopes of chlorine. Each nucleus contains 17 protons, so both atoms are chlorine. Chlorine-35 has 18 neutrons, whereas chlorine-37 has 20 neutrons.
Chlorine-35 and chlorine-37 are the same element because both have 17 protons, but they are different isotopes because their nuclei contain different numbers of neutrons.
Their names contain different mass numbers because the mass number is the total number of protons and neutrons in the nucleus. The important isotope comparison, however, is simply that the proton number stays fixed while the neutron number changes.
Changing the number of neutrons does not change the positive charge of the nucleus because neutrons have no charge. A neutral atom still has one electron for every proton, so neutral isotopes of the same element also have the same number of electrons.
Keep this distinct from ion formation: an isotope differs because of its neutrons, whereas an ion has formed because an atom or group of atoms has gained or lost electrons.
Isotopes are atoms of the same element with:
Same proton number + different neutron number → isotopes.
Different proton number → different elements, not isotopes.
Different neutron numbers give the isotopes different mass numbers:
Neutrons have no charge, so changing their number does not itself make an ion. Isotopes and ions must not be confused.
To decide whether two atoms are isotopes, compare both particle counts: the proton numbers must be the same and the neutron numbers must be different.
Do not say that isotopes have different numbers of protons. Changing the proton number changes the element.
When explaining different mass numbers, refer to different numbers of neutrons in the nucleus—not different numbers of electrons.
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Isotope
An atom of an element that has the same number of protons as other atoms of that element but a different number of neutrons in its nucleus.
Proton
A positively charged subatomic particle found in the nucleus; its number determines the element.
Neutron
An uncharged subatomic particle found in the nucleus.
Nucleus
The small central part of an atom that contains its protons and neutrons.
Mass number
The total number of protons and neutrons in the nucleus of an atom.
Put your knowledge into practice — try past paper questions for Chemistry
Isotope
An atom of an element that has the same number of protons as other atoms of that element but a different number of neutrons in its nucleus.
Proton
A positively charged subatomic particle found in the nucleus; its number determines the element.
Neutron
An uncharged subatomic particle found in the nucleus.
Nucleus
The small central part of an atom that contains its protons and neutrons.
Mass number
The total number of protons and neutrons in the nucleus of an atom.