Every card in Atoms, elements and compounds, Year 10: isotopes, relative atomic mass and electrons
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- Atoms of the same element that have different numbers of neutrons
Isotopes
HintCarbon-12 and carbon-14 are a well-known pair.
WhyThese atoms have the same number of protons, so they are the same element and react in the same way; only their mass numbers differ. Chlorine-35 and chlorine-37 both have 17 protons, with 18 and 20 neutrons.
- An atom has atomic number 17 and mass number 37. How many neutrons does it have? (number only)
20
HintThe mass number counts two kinds of particle; take away the kind you already know.
WhyMass number = protons + neutrons, so neutrons = mass number − atomic number = 37 − 17 = 20. This atom is chlorine-37.
- An oxide ion has a charge of 2−. Its atomic number is 8 and its mass number is 16. How many electrons does it have? (number only)
10
HintStart from the neutral atom, then think which way a negative charge moves the count.
WhyA neutral oxygen atom has 8 protons and so 8 electrons. A 2− charge means two extra electrons: 8 + 2 = 10. The numbers of protons (8) and neutrons (16 − 8 = 8) are unchanged.
- The total number of protons and neutrons in an atom
The mass number
HintIt is the larger of the two figures written beside an element's symbol.
WhyProtons and neutrons each have a relative mass of 1 and the mass of an electron is very small, so counting protons and neutrons gives the relative mass of the atom. Subtracting the atomic number from it gives the number of neutrons.
- An atom of sodium can be shown as the symbol Na with a small 23 at its top left and a small 11 at its bottom left. What does the top number, 23, tell you?
The mass number: its protons and neutrons add up to 23
HintIt is the bigger figure, so it must be counting more than one kind of particle.
WhyIn this notation the mass number goes at the top and the atomic number at the bottom. So this sodium atom has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
- The average mass of the atoms of an element, taking account of how abundant each isotope is
Relative atomic mass
HintOn the periodic table it is the figure that is often not whole, such as 35.5 for chlorine.
WhyMost elements are a mixture of isotopes, so their atoms do not all have the same mass. This quantity is a weighted average: the more abundant an isotope is, the more it counts towards the value.
- Boron is 20% boron-10 and 80% boron-11. Calculate its relative atomic mass. (number only, to 1 decimal place)
10.8
HintMultiply each mass number by its percentage, add, then divide by the total percentage.
Why(10 × 20) + (11 × 80) = 200 + 880 = 1080, and 1080 ÷ 100 = 10.8. The answer lies nearer 11 than 10 because boron-11 is the more abundant isotope.
- The relative atomic mass of chlorine is 35.5, but no atom can contain half a proton or half a neutron. How can the value not be a whole number?
It is an average of isotopes that differ in mass
HintThink what happens when you find the mean of 35 and 37, when there are more 35s.
WhyAbout three quarters of chlorine atoms are chlorine-35 and one quarter are chlorine-37. The weighted average is (35 × 75 + 37 × 25) ÷ 100 = 35.5. No single chlorine atom has that mass.
- Copper has two isotopes, copper-63 and copper-65, and its relative atomic mass is 63.5. Which isotope is the more abundant?
Copper-63
HintSee which of the two mass numbers the average sits closer to.
WhyA weighted average is pulled towards the isotope there is more of. If the two were equally common the value would be 64; 63.5 is below that, so the lighter isotope must be the more abundant.
- To calculate a relative atomic mass, multiply each isotope's mass number by its percentage abundance, add the results together, and divide by ____.
100
HintIt is what all the percentages must add up to.
WhyDividing by the total percentage turns the sum into an average for one atom. For an element that is half mass 79 and half mass 81: (79 × 50 + 81 × 50) ÷ 100 = 80.
- Chlorine has atomic number 17. Write its electronic structure. (numbers separated by commas, no spaces, as in 2,1)
2,8,7
HintFill the innermost shell, then the next, and put whatever is left in the third.
WhyA chlorine atom has 17 electrons. The first shell takes 2, the second takes 8, and the remaining 17 − 10 = 7 go into the third shell.
- An atom has the electronic structure 2,8,3. Which element is it?
Aluminium
HintAdd the three numbers to get the number of protons, then count along the periodic table.
Why2 + 8 + 3 = 13 electrons, and a neutral atom has the same number of protons, so the atomic number is 13. Element 13 is aluminium.
- In the electronic structures of the first twenty elements, the first shell is full with ____ electrons and the second shell is full with ____.
2; 8
HintLook at how the structure of sodium, which ends in 1, begins.
WhySodium is 2,8,1: two electrons fill the first shell, eight fill the second, and the eleventh starts the third. At this level the third shell also takes eight before a fourth begins, as in calcium, 2,8,8,2.
- Electrons go into the ____ energy level that still has room, which is the innermost shell that is not yet full.
lowest
HintThe energy levels nearest the nucleus are the ones meant here.
WhyThe shell nearest the nucleus fills first, and a new shell is started only when the one inside it is full. 'Energy level' and 'shell' mean the same thing here, and either word is accepted in an answer.
- Calcium has 20 electrons. How many of its shells contain electrons? (number only)
4
HintWrite out the structure, filling each shell in turn, and count the numbers you wrote.
WhyCalcium is 2,8,8,2: the first three shells take 2 + 8 + 8 = 18 electrons, and the last two start a fourth shell.
1★ GCSE-CHEM-ATO-0016
2★ GCSE-CHEM-ATO-0017
3★ GCSE-CHEM-ATO-0018
4★ GCSE-CHEM-ATO-0019
5★ GCSE-CHEM-ATO-0020
6★ GCSE-CHEM-ATO-0021
7★ GCSE-CHEM-ATO-0022
8★ GCSE-CHEM-ATO-0023
9★ GCSE-CHEM-ATO-0024
10★ GCSE-CHEM-ATO-0025
11★ GCSE-CHEM-ATO-0026
12★ GCSE-CHEM-ATO-0027
13★ GCSE-CHEM-ATO-0028
14★ GCSE-CHEM-ATO-0029
15★ GCSE-CHEM-ATO-0030
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