GCSE · Chemistry · Edexcel · Spec 1CH0

Isotopes

Chlorine's relative atomic mass is 35.5. But a nucleus is built from whole protons and whole neutrons. So where does the half come from?

Chemistry · Isotopes

Which of these atoms are neon?

Is each atom neon, or a different element?

Still to sort

Neon (0)

The same element as Atom A

Where the line is: An atom is neon if it has 10 protons, however many neutrons it has.

A different element (0)

Not neon at all

Where the line is: Change the number of protons and you change the element. Changing the number of neutrons never does.

6 of 6 still to sort.

Each card is one atom, described by what's inside it. Atom A is a neon atom. Decide which of the others are neon too, then read why.

Watch out: Heavier does not mean a different element. Atom C has two more neutrons than Atom A, and both are neon.

Chemistry · Writing isotopes

Two ways to write an isotope
C126mass number (A)protons + neutronsatomic number (Z)protonschemical symbol (X)which element it is

View: Nuclear notation. Showing 1 layer: Nuclear notation

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Explore

Switch view ↑, then tap a part

Carbon-12 in nuclear notation: mass number 12 on top, atomic number 6 underneath, symbol C.

Switch between the two ways of writing, then tap each part to see what it tells you.

Chemistry · Counting neutrons

Your turn: fill in the missing steps

A chlorine atom has atomic number 17 and 18 neutrons. What is its mass number? Then, copper has atomic number 29: how many neutrons are in copper-63, and in copper-65?

  1. Start from what the mass number counts: every particle in the nucleus. So mass number = atomic number + number of neutrons.
  2. missing step
Which line is step 2?

Chemistry · Do isotopes react differently?

Does a heavier isotope react differently?

Lithium-6 and lithium-7 are two isotopes of lithium: lithium-7 has one more neutron. A piece of each is dropped into water.

Which is closest to what you think happens?
How sure are you?

Chemistry · Relative atomic mass

Where does copper's 63.5 fit?

Every copper atom is either copper-63 or copper-65. The periodic table gives copper a relative atomic mass of 63.5. Place all three on the line.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Two atoms can weigh different amounts and still be exactly the same element. Find out what really decides which element an atom is, and why chlorine's relative atomic mass is 35.5 instead of a whole number.

What you need to know

  • Every atom of an element has the same number of protons. That number is the atomic number, and it decides which element the atom is.
  • The mass number is the total number of protons and neutrons in the nucleus.
  • Isotopes are atoms of the same element with the same number of protons but a different number of neutrons, so they have the same atomic number and different mass numbers.
  • Carbon-12 has 6 protons, 6 neutrons and 6 electrons; carbon-13 has 6 protons, 7 neutrons and 6 electrons. Nitrogen-14 and nitrogen-15 both have 7 protons, and nitrogen-15 has one more neutron.
  • Number of neutrons = mass number − atomic number. A chlorine atom with atomic number 17 and 18 neutrons has mass number 35. Copper-63 and copper-65 both have 29 protons, with 34 and 36 neutrons.
  • Nuclear notation writes the mass number (A) above and the atomic number (Z) below, beside the symbol. The shorthand is the name or symbol, a hyphen and the mass number: carbon-14, C-14, Ne-21, Cu-63.
  • Isotopes of an element react in the same way, because they have the same number of electrons and electrons determine chemical properties.
  • Elements usually exist as a mixture of isotopes. The relative atomic mass on the periodic table is the mean mass of the isotopes, taking account of how much of each there is (its abundance), relative to carbon-12. That's why chlorine's is 35.5 and copper's is 63.5.

The big picture

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. The atomic number (protons) fixes the element; the mass number counts protons plus neutrons, so neutrons = mass number − atomic number. Isotopes are written in nuclear notation or as shorthand such as C-14. With the same electrons, isotopes react in the same way. Elements usually exist as a mixture of isotopes, so the relative atomic mass is a mean, which is why chlorine's is 35.5.

Key points

1Protons decide the element; neutrons only change the mass.
2Isotopes: same atomic number, different mass number.
3Neutrons = mass number − atomic number.
4Same electrons means same chemical properties, so isotopes react in the same way.
5A relative atomic mass is a mean over the mixture of isotopes, not the mass number of the most common isotope, and not the mass number of any single atom.

Worked example

Problem

One atom is written in nuclear notation as N with 15 on top and 7 underneath. Another atom is nitrogen-14. Work out the number of protons, neutrons and electrons in each, and decide whether they are isotopes.

⚠ Watch out

Thinking an atom with a different mass must be a different element. Only a change in the number of protons makes a different element. And don't read C-14 as '14 protons': the number in the shorthand is always the mass number.

🧠

Memory hook

Protons are the passport, neutrons are the luggage. An atom can carry more or less luggage and still have the same passport: same element, just a different mass.

✓

Check yourself

Without looking back: say what makes two atoms isotopes, find the neutrons in neon-22 (atomic number 10), and explain why chlorine's relative atomic mass isn't a whole number.

Flashcards

(13)
What is an isotope?
An atom of the same element with the same number of protons but a different number of neutrons.
What does the atomic number tell you?
The number of protons in the nucleus. It decides which element the atom is.
What does the mass number tell you?
The total number of protons and neutrons in the nucleus.
How do you work out the number of neutrons?
Number of neutrons = mass number − atomic number.
In nuclear notation, which number goes on top and which underneath?
Mass number (A) on top; atomic number (Z) underneath, the smaller number.
In the shorthand Ne-21, what does 21 stand for?
The mass number (protons + neutrons), not the atomic number.
How is a chemical symbol written?
A capital letter, followed by a lower-case letter if there is one: Ne, Cu.
Carbon-12 and carbon-13: what is different inside the atom?
Carbon-13 has one extra neutron (7 instead of 6). Both have 6 protons and 6 electrons.
How many neutrons do neon-20, neon-21 and neon-22 have?
10, 11 and 12. All three have 10 protons.
Why do isotopes of an element react in the same way?
They have the same number of electrons, and electrons determine chemical properties.
Give an example of isotopes reacting in the same way.
Lithium-6 and lithium-7 react in the same way with water; hydrogen-1, -2 and -3 all burn explosively with oxygen.
Why is chlorine's relative atomic mass 35.5 and not a whole number?
Chlorine exists as a mixture of isotopes, and the relative atomic mass is the mean mass of that mixture.
Is the relative atomic mass the mass number of the most common isotope?
No. It's the mean mass of all the isotopes, taking account of how much of each there is, relative to carbon-12.

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