AQA Chemistry 5.1.1.6 · Combined and Triple
Relative atomic mass, atomic number and isotopes
Two numbers label an atom. Isotopes share one of them. Relative atomic mass is the weighted mean of their mass numbers, and you can calculate it from percentage abundances.
Triple Science · Higher tier
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Four friends, one quiz.
Three friends score 10 each. One scores 20.
If you had to guess, is the group’s average closer to 10 or to 20?
Two numbers, two places
The nuclear symbol and the periodic-table box.
A nuclear symbol labels one atom: mass number on top (protons + neutrons), atomic number below (protons). The periodic table shows something different on top: the relative atomic mass, which belongs to the element as a whole.
Sodium-23 has atomic number 11. How many neutrons?
Pick one.
Counting the particles in atoms and ions is taught in Subatomic particles.
Isotopes
Same protons, different neutrons.
Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes of that element. Same atomic number, so the same element; different mass number. Isotopes have the same number of electrons, and chemical reactions involve the electrons, so isotopes of an element have the same chemical properties.
Which is different between chlorine-35 and chlorine-37?
Pick one.
Equation · relative atomic mass
The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element. It is a weighted sum, not a triangle: one line, with the whole top bracketed.
Ar = [ Σ (% abundance × mass number) ] ÷ 100
Multiply each isotope’s abundance by its mass number, add them all, then divide the whole total by 100. Without the outer bracket, only the last product is divided.
Slide the mix. Chlorine-35 and chlorine-37: choose how much is chlorine-37 and watch Ar move.
With 75% chlorine-35 and 25% chlorine-37: Ar = [(75 × 35) + (25 × 37)] ÷ 100 = 35.5. Real chlorine is about 75% chlorine-35 and 25% chlorine-37, so Ar = 35.5. Many Ar values are whole numbers (C 12, O 16, Na 23); chlorine’s is often the first one pupils meet that is not.
Think again · the bracket
“Ar = (20 × 10) + (80 × 11) ÷ 100 = 10.8”
Your call
ChooseA pupil writes Ar = (20 × 10) + (80 × 11) ÷ 100 for boron and gets 10.8. The answer is right. What is wrong with the line?
Sort
Isotopes of one element
Same or different?
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Calculate
- Ar from percentage abundances; neutrons from the two numbers.
- Define
- Atomic number, mass number, isotope, relative atomic mass.
- Explain
- Why Ar is often not a whole number.
Key fact
Isotopes: same atomic number, different mass number. Ar = [Σ(% × mass number)] ÷ 100. Bracket the whole top before dividing.
Key note · AQA 5.1.1.6 · Combined and Triple
Relative atomic mass, atomic number and isotopes
- Atomic number = number of protons. Mass number = protons + neutrons. Neutrons = mass number − atomic number. The nuclear symbol shows mass number (top) and atomic number (bottom); the periodic table shows relative atomic mass and atomic number.
- Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes. Same chemical properties, different mass.
- The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
- Ar = [Σ(% abundance × mass number)] ÷ 100, with the whole top bracketed. Ratios become percentages first; if one abundance is given, the other is 100 minus it.
- Chlorine: [(75 × 35) + (25 × 37)] ÷ 100 = 35.5. Ar is often, not always, a non-whole number.
Photograph this card. Tomorrow, cover it and say each line first.
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Ask MrBadmus
Getting 208.8 instead of 10.8?
Abundances in the practice questions are rounded for arithmetic.