Atoms, elements and compounds, Year 10: isotopes, relative atomic mass and electrons

Isotopes

HintCarbon-12 and carbon-14 are a well-known pair.

The 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.

20

HintThe mass number counts two kinds of particle; take away the kind you already know.

The whyMass number = protons + neutrons, so neutrons = mass number − atomic number = 37 − 17 = 20. This atom is chlorine-37.

10

HintStart from the neutral atom, then think which way a negative charge moves the count.

The 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 mass number

HintIt is the larger of the two figures written beside an element's symbol.

The 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.

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.

The 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.

Relative atomic mass

HintOn the periodic table it is the figure that is often not whole, such as 35.5 for chlorine.

The 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.

10.8

HintMultiply each mass number by its percentage, add, then divide by the total percentage.

The 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.

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.

The 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-63

HintSee which of the two mass numbers the average sits closer to.

The 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.

100

HintIt is what all the percentages must add up to.

The 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.

2,8,7

HintFill the innermost shell, then the next, and put whatever is left in the third.

The 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.

Aluminium

HintAdd the three numbers to get the number of protons, then count along the periodic table.

The 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.

2; 8

HintLook at how the structure of sodium, which ends in 1, begins.

The 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.

lowest

HintThe energy levels nearest the nucleus are the ones meant here.

The 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.

4

HintWrite out the structure, filling each shell in turn, and count the numbers you wrote.

The whyCalcium is 2,8,8,2: the first three shells take 2 + 8 + 8 = 18 electrons, and the last two start a fourth shell.