Place protons, neutrons and electrons in the atom with their relative charges and masses. Read the atomic number and mass number off the periodic table and use them to count every particle in an atom.
The wall chart above the salvage bench is the periodic table you already read. Rows are periods, columns are groups. Each box carries a small whole number, the atomic number, and it counts the protons in one atom of that element. The chart gives you that one for free. The other number this stop needs is the mass number, and it does not come off the chart. It belongs to one particular atom, is written after the name as in sodium-23, and counts the protons and the neutrons together. Both numbers are counts of particles, not weights in grams.
An atom is built from three kinds of particle. Protons and neutrons sit in the nucleus at the centre. Electrons move around it in shells. A proton carries a relative charge of +1 and a relative mass of 1. A neutron carries no charge and has a relative mass of 1 as well. An electron carries a relative charge of -1, but its mass is about one two-thousandth of a proton's. Relative here means measured against a proton, which is why a proton scores 1 for charge and 1 for mass.
Take sodium. The chart gives its atomic number as 11, so one sodium atom holds 11 protons. The label on the bench jar reads sodium-23, so the mass number is 23, and that counts protons and neutrons together. Subtract to find the neutrons: 23 - 11 = 12. The atom is neutral, so the electron count matches the proton count at 11. Write it in the notebook as 11 protons, 12 neutrons, 11 electrons. Then check the total: 11 + 12 = 23, which is the mass number you started from. That check catches most slips.
A proton's charge and an electron's charge are the same size and opposite in sign. One proton and one electron together therefore add up to nothing. Sodium's 11 protons give +11, and its 11 electrons give -11. Add those and the total is zero, which is why a sodium atom shows no charge to the outside world. Neutrons change nothing here, since their charge is zero. Mass does not come into it. A proton is far heavier than an electron, and the two charges still cancel exactly.
The nucleus is tiny next to the atom around it. An atom is roughly 100,000 times wider than its nucleus. Blow one nucleus up to the size of a grain of sand and the whole atom would stretch about as far as a football pitch is long. Almost all of an atom is empty space for electrons to move through. Yet nearly all of the mass sits in that speck. Protons and neutrons weigh about 1 each, while 11 electrons together weigh less than one hundredth of a proton. That is why the mass number can ignore them.
The atomic number off the chart is the proton count. The mass number that comes with the atom, minus the atomic number, gives the neutrons. A neutral atom carries as many electrons as protons. Nearly all the mass sits in a nucleus that takes up almost none of the room.
The wall chart is still taped above the bench. A council skip gave the club an old orange street lamp, and inside its tube is sodium.
The chart puts sodium's atomic number at 11, and the tube is labelled sodium-23, so its mass number is 23. How many protons does one sodium atom hold?
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24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.