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ATOMIC STRUCTURE · CHAPTER 2 OF 12
What are isotopes and ions?
Two carbon atoms can have different masses and still be carbon. An atom can also lose an electron and become charged. The key is knowing which particle has changed.
By the end, you’ll be able to…
- use atomic number and mass number to count protons, neutrons and electrons
- explain what makes two atoms isotopes of the same element
- explain how losing electrons forms positive ions
- describe how electrons change energy levels by absorbing or emitting electromagnetic radiation.
Using atomic number and mass number
In Chapter 1, you learned that an element is identified by its number of protons. You also learned how to read the atomic number and mass number beside an element’s symbol.
Now we’ll use those numbers to work out how many protons, neutrons and electrons an atom contains — and explore what happens when the number of neutrons or electrons changes.
* A nucleon is a particle in the nucleus: a proton or a neutron. Mass number is a whole-number particle count, not a mass measured in grams.
Read the two numbers
Hover over a tile to explore it. Click or tap 14, 6 or C to keep its explanation open.
14 is the mass number. It counts all the protons and neutrons in the nucleus: 6 protons + 8 neutrons = 14. Electrons are not included.
6 is the atomic number. It tells you there are six protons. Their number identifies the element. A neutral carbon atom also has six electrons outside its nucleus.
C is the chemical symbol for carbon. Every carbon nucleus contains six protons. Carbon isotopes can have different numbers of neutrons, but they are all carbon.
* Pink + circles: protons. White circles: neutrons. This is an enlarged nucleus, not the whole atom. The faint circle indicates the nucleus’s approximate extent, not a physical wall. Electrons are not shown. Not to scale.
How do we count the particles?
You can now use the atomic number and mass number you met in Chapter 1 to calculate particle numbers. Start with the protons. Subtract the atomic number from the mass number to find the neutrons. For a neutral atom, the number of electrons equals the number of protons.
The relationship
Count the particles inside a nucleus
Choose a nucleus. Predict its neutron count, then select “Count the particles”. Select the same button to reset.
Carbon-12
There are 12 nucleons altogether. Six are protons. How many are neutrons?
12 − 6 = 6 neutrons
Carbon-12 has equal numbers of protons and neutrons. Not every nucleus does.
How many electrons would its neutral atom have?
Six electrons. A neutral atom has equal numbers of protons and electrons. Electrons are outside the nucleus and are not shown here.
Carbon-14
There are 14 nucleons altogether. Six are protons. How many are neutrons?
14 − 6 = 8 neutrons
Carbon-12 and carbon-14 both have six protons. Their different neutron counts make them isotopes of carbon.
How many electrons would its neutral atom have?
Six electrons, just like neutral carbon-12. Different neutron counts do not change the electron count of a neutral carbon atom.
Sodium-23
There are 23 nucleons altogether. Eleven are protons. How many are neutrons?
23 − 11 = 12 neutrons
The same method works for sodium. Its eleven protons identify the element.
How many electrons would its neutral atom have?
Eleven electrons. Eleven negative electrons balance the charge of eleven positive protons.
Aluminium-27
There are 27 nucleons altogether. Thirteen are protons. How many are neutrons?
27 − 13 = 14 neutrons
More particles, same method. Subtract the atomic number from the mass number.
How many electrons would its neutral atom have?
Thirteen electrons. Neutrality depends on equal numbers of protons and electrons, not equal numbers of protons and neutrons.
* Simplified 2D nucleus models. The faint circle indicates the approximate extent of each nucleus, not a physical wall. Particle positions are illustrative, not to scale. Moving the symbols into counting groups is not a real nuclear reaction. No particles are created or destroyed. Electrons are outside the nucleus and are not shown.
To calculate neutrons:
A neutral carbon-14 atom has mass number 14 and atomic number 6.
neutrons = mass number − atomic number
neutrons = 14 − 6
8 neutrons
To calculate neutrons:
A neutral sodium-23 atom has mass number 23 and atomic number 11.
neutrons = mass number − atomic number
neutrons = 23 − 11
12 neutrons
To calculate neutrons:
A neutral chlorine-37 atom has mass number 37 and atomic number 17.
neutrons = mass number − atomic number
neutrons = 37 − 17
20 neutrons
Your turn: count the particles
This is a neutral aluminium atom. Choose its correct particle counts.
Same element, different neutrons
Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers.
Carbon-12, carbon-13 and carbon-14 all have six protons. Their neutron counts differ, so their masses differ. Their nuclei have the same positive charge because each contains six protons.
Change the neutrons, keep the carbon
Switch isotopes. Watch the extra neutrons appear while the six protons and six electrons stay in place.
6 protons · 6 neutrons · 6 electrons
Mass number: 12 · Atomic number: 6
Nuclear charge: +6 · Overall atomic charge: 0
6 protons · 7 neutrons · 6 electrons
Mass number: 13 · Atomic number: 6
Nuclear charge: +6 · Overall atomic charge: 0
6 protons · 8 neutrons · 6 electrons
Mass number: 14 · Atomic number: 6
Nuclear charge: +6 · Overall atomic charge: 0
What stayed the same?
The six protons identify carbon. All three neutral atoms also have six electrons. Their nuclei have the same charge; their masses differ because they have different neutron counts.
* This compares three different atoms; clicking does not show one atom changing into another. Pink + circles: protons. White circles: neutrons. Blue − circles: electrons. Highlighted white circles mark extra neutrons. Positions are kept fixed for comparison; the shell model is not to scale.
Isotope does not mean radioactive. Carbon-12 and carbon-13 are stable; carbon-14 is radioactive. We will explore unstable nuclei and radioactive decay in Chapter 4.
Would it still be carbon?
A neutral carbon-12 atom is compared with a neutral atom containing seven protons and seven neutrons.
Decide whether they are isotopes of the same element.
What happens when an atom loses an electron?
An ion is an atom, or group of atoms, with an overall electrical charge. Here we are looking at single-atom ions.
When a neutral atom loses an outer electron, it loses negative charge. More protons than electrons remain, so the atom becomes a positive ion. Losing one electron gives a relative charge of +1; losing two gives +2.
The nucleus stays the same during this ion formation. The atomic number and mass number do not change, and it remains the same element.
One electron leaves. What changes?
Select “Lose one electron” to watch a neutral lithium atom become a positive ion. Select “Neutral atom” to reset.
* Each blue − circle is an electron; pink + circles are protons and white circles are neutrons. Energy must be supplied to remove an electron. This animation shows only the change in particle counts, not the energy transfer or a literal path. Sizes and distances are not to scale.
A positive ion forms by losing electrons, not by gaining protons. Changing the proton number would change the element. Ordinary ion formation changes the electrons outside the nucleus.
Find the ion’s charge
A magnesium atom has 12 protons and 12 electrons. It loses two electrons.
Can atoms form negative ions too?
Yes. An atom can gain electrons and become a negative ion. For example, gaining one electron gives an overall relative charge of −1. Our main focus here is how losing outer electrons creates positive ions.
Electrons can change energy levels
In the GCSE model, electrons occupy particular energy levels at set distances from the nucleus. They do not have any possible energy in between these levels.
An electron can absorb electromagnetic radiation and move to a higher energy level, further from the nucleus. When it moves to a lower energy level, closer to the nucleus, it emits electromagnetic radiation.
The energy absorbed or emitted matches the difference between the two levels. An electron moving to a higher level is called excitation. It is still part of the atom; this is different from removing it completely to form an ion.
Absorb energy or emit energy?
Watch the slow repeating sequence: absorb energy → higher level → emit energy → lower level.
* The blue − marker is one electron. The pink wave represents radiation, not another particle in the atom. The repeating jumps and timing are illustrative; this is not a continuous orbit. Each cycle assumes suitable incoming radiation. Circles represent energy levels, not literal tracks; the hydrogen nucleus and distances are not to scale.
Absorb energy or emit energy?
Watch the electron jump to a higher energy level when energy is absorbed, then return to a lower level when energy is emitted.
* The circles represent energy levels, not actual electron paths.
Isotope, ion or excited atom?
Name each change before revealing the explanation.
1 · Same protons, different neutrons
Different isotopes of the same element. Their mass numbers differ, but their atomic numbers are the same.
2 · A neutral atom loses an outer electron
A positive ion forms. The number of electrons decreases. The nucleus and element remain unchanged.
3 · An electron moves higher but stays in the atom
The atom is excited. Energy has been absorbed, but no electron has left, so a neutral atom remains neutral.
Choose one answer for each question. Feedback appears immediately.
A neutral carbon-14 atom loses one electron. A student says, “It has become carbon-13 because there is one less particle.” Explain why she is wrong, and give the numbers of protons, neutrons and electrons in the ion.
Think about which particles contribute to mass number, and which particle has left.
Click here to reveal the answer
Carbon-14 has six protons and eight neutrons. These 14 nucleons remain in its nucleus, so its mass number is still 14.
One electron leaves, reducing the electron count from six to five. The ion contains 6 protons, 8 neutrons and 5 electrons and has relative charge +1. It is still carbon-14. Carbon-13 would have seven neutrons.
CHAPTER 2
Chapter summary
- Use the atomic number to find the number of protons; in a neutral atom, electrons equal protons.
- Number of neutrons = mass number − atomic number.
- Isotopes have the same proton number but different neutron numbers.
- A neutral atom has equal numbers of protons and electrons. Losing electrons forms a positive ion.
- Ion formation does not change the nucleus, atomic number or mass number.
- Electrons absorb electromagnetic radiation to move higher and emit it when moving lower.
- Excitation leaves an electron in the atom; ionisation removes it.