Inside the atom: protons, neutrons and electrons

Science · Matter & Materials · ages 14-15

Name ______________________ Date ____________

Where does nearly all the mass of an atom sit?
- spread evenly through the whole atom
- in the electron shells
- in the empty space between the shells
- in the nucleus
Which number in an element's box on the chart counts the protons?
- the mass number
- the atomic number
- the group number
- the period number
Where are an atom's electrons found?
- packed inside the nucleus
- stuck onto the protons
- in shells around the nucleus
- mixed in among the neutrons
An atom of fluorine has atomic number 9 and mass number 19. How many neutrons does it hold?
Answer: ______________
An atom of aluminium has atomic number 13 and mass number 27. How many electrons does this neutral atom hold?
Answer: ______________
An atom holds 17 protons and 18 neutrons. What is its mass number?
- 1
- 35
- 18
- 17
Calcium has atomic number 20 and mass number 40. Sam copies the atomic number across as the neutron count and writes '20 neutrons'. He lands on the right figure. Why was that luck rather than method?
- the atomic number counts protons, and 40 happens to be double 20
- he should have added the two numbers, since a mass number is always a sum
- the atomic number is never a whole number, so it cannot be copied across like that
- nothing was lucky about it, because neutrons always match protons exactly
Most of an atom's volume is taken up by the nucleus.
Circle one: True False
An atom of tin has a mass number of 119 and holds 69 neutrons. How many electrons does this neutral atom hold?
Answer: ______________
Rafi says an atom must come out positive, because a proton weighs far more than an electron. Where does his reasoning go wrong?
- charge and mass are separate, and the two charges are the same size
- protons really are lighter than electrons, so the balance tips the other way
- the neutrons sit between them and cancel the extra positive charge
- atoms really are slightly positive, and that is what makes static work

Answer key

For grown-ups. Fold this page away before handing over the rest.

Inside the atom: protons, neutrons and electrons W1-mt_WqDXAr2IkA-s1

in the nucleus · Protons and neutrons each count as 1, and they are all in the nucleus. The electrons outside weigh almost nothing by comparison.
the atomic number · The atomic number is the proton count, and it is printed in the element's box. The bigger number printed beside it is a different quantity, and it waits for another day. The mass number comes with the atom you are handed, as in sodium-23.
in shells around the nucleus · Electrons move around the nucleus in shells. Only protons and neutrons are inside the nucleus itself.
10 · Take the 9 protons off the mass number: 19 - 9 = 10 neutrons.
13 · A neutral atom has as many electrons as protons. The atomic number gives 13 protons, so there are 13 electrons. The mass number is not needed.
35 · The mass number is protons plus neutrons: 17 + 18 = 35.
the atomic number counts protons, and 40 happens to be double 20 · The atomic number is a proton count. The neutron count is 40 - 20 = 20, and it only lands on the same figure because 40 is exactly twice 20.
False · The nucleus holds nearly all the mass but almost none of the room. The volume belongs to the electrons and the empty space they move through.
50 · Protons = 119 - 69 = 50. A neutral atom has as many electrons as protons, so 50 electrons.
charge and mass are separate, and the two charges are the same size · A proton is about two thousand times heavier than an electron, and that is true. It has nothing to do with charge: +1 and -1 cancel however heavy the carriers are.