Mass number, atomic number and isotopes — AQA GCSE Physics
Test yourself on Mass number, atomic number and isotopes with AQA GCSE practice questions.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Mass number, atomic number and isotopes explained
Every proton carries a relative charge of +1 and every electron a relative charge of -1.
Read the full explanation
In an atom the two counts are identical, so the positive and negative charges cancel exactly and the atom has no overall charge. Neutrons play no part in this balance, because they are uncharged. A carbon atom has six protons and therefore six electrons, total charge zero. Take one electron away and the balance is broken: there are now more protons than electrons and the particle is a positive ion. Add an electron and it becomes a negative ion. So an object only becomes charged when electrons move, never because protons have moved or been created.
All atoms of a particular element have the same number of protons.
The number of protons is what defines an element. Every carbon atom anywhere has six protons; anything with a different number of protons is simply not carbon. Neutrons may vary, giving isotopes of the same element, and electrons may be lost or gained, giving ions, but neither changes what the element is. Change the proton number and you have changed the element, which is exactly what happens in alpha and beta decay. This is the reason examiners award the mark for identifying two atoms as the same element only when the answer compares proton numbers, or the equivalent atomic numbers, and it is also why electron counts are ignored in those answers.
The number of protons in an atom of an element is called its atomic number.
The atomic number, given the symbol Z, is simply a count of the protons in the nucleus. It is written as the lower of the two numbers in the nuclide symbol, so in ¹²₆C the 6 is the atomic number and tells you the nucleus holds six protons. Because a neutral atom has as many electrons as protons, the atomic number also gives the electron count for a neutral atom, although not for an ion. The periodic table is ordered by atomic number, so knowing Z is enough to name the element. Being a count, the atomic number has no unit, and it changes only when the nucleus itself gains or loses protons.
The total number of protons and neutrons in an atom is called its mass number.
The mass number, symbol A, counts the particles in the nucleus: protons plus neutrons together. It is written as the upper number in the nuclide symbol, so ⁵⁶₂₆Fe has a mass number of 56 and an atomic number of 26. Subtracting gives the number of neutrons: 56 - 26 = 30. Electrons are not counted, because their mass is tiny compared with a proton or a neutron, which is why the mass number is a good guide to the mass of the whole atom. Like the atomic number, it is a count and has no unit. In alpha decay the mass number falls by four; in beta and gamma emission it does not change at all.
Atoms can be represented as shown in this example: Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
A nuclide symbol carries three pieces of information: the chemical symbol, the mass number above it and the atomic number below it, as in ¹⁴₆C. Isotopes are atoms of the same element, so they share the atomic number, but they hold different numbers of neutrons and therefore have different mass numbers. Carbon-12 and carbon-14 both have six protons; one has six neutrons and the other eight. Since the electron arrangement is unchanged, isotopes of an element behave identically in chemical reactions but differ in mass and in nuclear stability, which is why carbon-14 is radioactive while carbon-12 is not. Examiners ignore any mention of electron numbers in the definition.
Atoms turn into positive ions if they lose one or more outer electron(s).
Outer electrons are held least tightly, so they are the ones that can be pulled away, by rubbing two surfaces together or by ionising radiation. When an atom loses one, the protons in the nucleus are unchanged but there is now one fewer electron, so positive charge outweighs negative and the particle is a positive ion with a charge of +1. Lose two and the charge is +2. Protons never move between objects, which is why a mark scheme awards nothing for any answer that mentions transferring positive charge, and nothing for the phrase positive electrons. When a cloth is rubbed on a plastic rod, the cloth becomes positive because it has lost electrons to the rod.
Students should be able to relate differences between isotopes to differences in conventional representations of their identities, charges and masses.
Read a nuclide symbol as three separate questions. The lower number, the atomic number, names the element, so ³⁵₁₇Cl and ³⁷₁₇Cl are both chlorine. The upper number, the mass number, gives protons plus neutrons, so those two have 18 and 20 neutrons: same element, different isotope, different mass. Any charge written after the symbol refers to electrons, not protons, so ²⁴₁₂Mg²⁺ still has twelve protons but only ten electrons. Put together, a change in the lower number means a different element, a change in the upper number alone means a different isotope, and a charge means an ion. Neutral isotopes of an element always have identical electron counts.
Your focus
- Determine the number of electrons in a neutral atom from its atomic number.
- Explain why an atom has no overall electrical charge, referring to equal and opposite charges.
- Predict the overall charge of a particle that has eleven protons and ten electrons, and name it.
Show all 21 objectives
- Identify which of several atoms are the same element by comparing their proton numbers.
- Explain why gaining or losing an electron does not change which element an atom is.
- Explain why a nucleus that emits a beta particle becomes a different element.
- Determine the number of protons in a nucleus from a given nuclide symbol.
- Identify an element from its atomic number using a periodic table.
- Explain why the atomic number gives the number of electrons in a neutral atom but not in an ion.
- Calculate the number of neutrons in a nucleus from its mass number and atomic number.
- Determine the mass number of an atom given the numbers of protons and neutrons it contains.
- Explain why electrons are not included in the mass number.
- Describe what is meant by isotopes of an element in terms of protons and neutrons.
- Identify which of several nuclide symbols represent isotopes of the same element, and justify the choice.
- Calculate the number of neutrons in carbon-12 and carbon-14 and explain why both are still carbon.
- Explain how alpha radiation turns the atoms it passes into positive ions, and why that makes it the most strongly ionising radiation.
- Determine the charge on an ion that has lost two electrons, and write it using the correct notation.
- Explain why a cloth rubbed against a plastic rod becomes positively charged, referring only to the movement of electrons.
- Determine the numbers of protons, neutrons and electrons from a nuclide symbol, including one carrying a charge.
- Compare two nuclide symbols and decide whether they represent isotopes, ions or different elements.
- Explain why chlorine-35 and chlorine-37 behave identically in chemical reactions.
Mass number, atomic number and isotopes exam tips
Marking Points
- one mark for stating that the number of electrons equals the number of protons
- one mark for stating that the charges on a proton and an electron are equal and opposite
- one mark for concluding that the charges cancel so the atom has no overall charge
- one mark for stating that neutrons are uncharged and so do not affect the balance
- one mark for stating that every atom of an element has the same number of protons
- one mark for stating that this number is the atomic number of the element
- one mark for identifying two atoms as the same element by comparing their proton numbers only
- one mark for stating that a different number of protons means a different element
- one mark for the atomic number being the number of protons in the nucleus
- one mark for reading the atomic number as the lower number in the nuclide symbol
- one mark for stating that in a neutral atom the atomic number also gives the number of electrons
- one mark for using the atomic number to name the element from a periodic table
- one mark for the mass number being the total number of protons and neutrons
- one mark for calculating the number of neutrons as mass number minus atomic number
- one mark for reading the mass number as the upper, larger number in the nuclide symbol
- one mark for stating that the mass number is a count and so has no unit
- one mark for isotopes being atoms of the same element, with the same number of protons
- one mark for stating that they have different numbers of neutrons
- one mark for the equivalent wording, the same atomic number but a different mass number
- one mark for reading the nuclide symbol correctly, mass number above and atomic number below
- one mark for an electron being removed from the outer energy level of the atom
- one mark for stating that there are now more protons than electrons in the particle
- one mark for stating that the ion has an overall positive charge, of +1 for each electron lost
- one mark for stating that only electrons are transferred, never protons
- one mark for comparing the lower numbers to show two atoms are the same element
- one mark for comparing the upper numbers and calculating the different numbers of neutrons
- one mark for stating that a charge written after the symbol shows electrons lost or gained, not protons
- one mark for stating that neutral isotopes of an element have identical numbers of electrons
Examiner Tips
- 💡Say equal and opposite charges, then say they cancel; both steps are usually needed for two marks.
- 💡Check whether the question says atom or ion before you equate electrons and protons.
- 💡Neutrons deserve a mention only to say they carry no charge.
- 💡When asked which atoms are the same element, quote the proton numbers and say they match; the mark scheme ignores electrons.
- 💡Say same atomic number as an alternative form of the same answer.
- 💡Remember that this is why alpha and beta decay produce a new element, while gamma emission does not.
- 💡Remember the smaller number is the atomic number; the larger cannot be smaller than it.
- 💡Write Z when you need to keep track in a nuclear equation, so the columns stay clear.
- 💡Do not add a unit to the atomic number; it is a count, not a measurement.
- 💡Write out A - Z = number of neutrons before you substitute; it prevents the reversed subtraction.
- 💡Check that your neutron count is not negative and not larger than the mass number.
- 💡Nucleon number means the same thing as mass number if a question uses that word.
- 💡A two-mark definition needs both halves: same protons and different neutrons.
- 💡Naming an isotope as carbon-14 tells you the mass number, so the neutron count is 14 minus the atomic number.
- 💡Do not mention electrons in an isotopes definition; it wins nothing and can cost clarity.
- 💡Start the answer with the electrons that moved, then state the resulting imbalance, then the charge.
- 💡Say which object gained the electrons as well as which lost them; both are usually needed.
- 💡Use the word outer, because it explains why those electrons in particular are the ones removed.
- 💡Annotate the symbol in the margin: protons, neutrons, electrons, before you answer anything.
- 💡Give the neutron count as a subtraction shown in your working, so the method earns credit.
- 💡For an ion, adjust only the electron count and leave the two nuclear numbers alone.
Common Mistakes
- matching the number of electrons to the mass number instead of the atomic number
- saying neutrons balance the charge of the protons
- stating that an atom is neutral because it contains neutrons
- offering an ion as an example of a neutral atom
- identifying an element by its mass number rather than its proton number
- concluding that two atoms with different numbers of neutrons must be different elements
- using the number of electrons to decide which element an atom is
- claiming that an ion of an element is a different element
- reading the mass number as the atomic number in a nuclide symbol
- defining the atomic number as the total number of protons and neutrons
- using the atomic number to count the electrons in an ion
- assuming the atomic number cannot change during radioactive decay
- including the electrons in the total when working out the mass number
- subtracting the wrong way round, giving a negative number of neutrons
- attaching a unit such as grams to the mass number
- confusing the mass number of one isotope with the relative atomic mass printed in the periodic table
- defining isotopes by a difference in the number of electrons, which the mark scheme ignores
- claiming isotopes of an element are different elements
- saying the atomic number differs between isotopes
- writing the mass number in the lower position of the nuclide symbol
- explaining a positive charge by the transfer of protons or of positive charge, which scores zero
- writing about positive electrons, which also scores zero
- saying an atom that gains an electron becomes positively charged
- writing more protons than electrons without saying that electrons were lost, which is ignored unqualified
- reading a charge of 2+ as two extra protons in the nucleus
- comparing mass numbers and concluding the atoms must be different elements
- forgetting to subtract the atomic number when counting neutrons from a nuclide symbol
- assuming every isotope written with a large mass number must be radioactive