Five numbers, one element

Look up a single element and you are handed five different numbers, several of which are close together, and at least two of which students routinely swap. The atomic number. The mass number. The relative atomic mass. The number of neutrons. The number of electrons. ⚠️ ALMOST EVERY MISTAKE IN THIS TOPIC IS DOING THE RIGHT ARITHMETIC ON THE WRONG PAIR OF THEM. The subtraction is correct; the two things subtracted were not the two the question needed. So the useful skill is not memorising formulas. It is knowing what each number COUNTS, and what it does not. Two of them are counts of particles in a single atom. One counts only protons. One counts protons and neutrons together, which is why the difference between them gives you the neutrons and why that subtraction is a reason rather than a rule. ⚠️ ONE OF THEM IS NOT A COUNT AT ALL. RELATIVE ATOMIC MASS IS AN AVERAGE ACROSS A WHOLE SAMPLE, which is exactly why it is usually not a whole number when a mass number always is. Those are two different kinds of quantity, and treating an average as though it were a count is where a great many marks go. And one number quietly runs the whole of the rest of the topic. ⚠️ THE PROTON COUNT FIXES THE ELECTRON COUNT IN A NEUTRAL ATOM, WHICH FIXES THE ELECTRONIC STRUCTURE, WHICH FIXES WHERE THE ELEMENT SITS IN THE TABLE AND HOW IT BEHAVES. That is also why the modern table is ordered by atomic number. It is ordered by a count of the thing that causes chemistry, rather than by a count of everything that happens to be in the nucleus. ⚠️ AND IT MATTERS MORE ON THIS SPECIFICATION THAN ON ITS SIBLINGS. MATHEMATICAL SKILLS ARE AT LEAST 20% OF THIS QUALIFICATION, DOUBLE WHAT THE WJEC BIOLOGY SPECIFICATION REQUIRES. Step 7 explains what follows from that.

What each of the five actually counts

Five quantities, defined by what they count. No worked example here; matching them to real cases comes next.

Match each quantity to what it counts

The quantity that decides which element this is, and cannot change without changing the element
The quantity that counts everything in the nucleus at once
The quantity that is usually not a whole number, because it describes a whole sample rather than one atom
The quantity you never read directly and always work out from two others
The quantity that matches the protons in a neutral atom but not in an ion

THE ATOMIC NUMBER, a count of protons only
THE MASS NUMBER, a count of protons and neutrons together
THE RELATIVE ATOMIC MASS, a weighted average across the isotopes
THE NEUTRON COUNT, obtained by subtraction rather than read off
THE ELECTRON COUNT, fixed by charge balance rather than by the nucleus

The arithmetic is right. What went wrong

A student is asked for the number of neutrons in an atom of chlorine. They take the relative atomic mass of 35.5, subtract the atomic number of 17, and answer 18.5. Their subtraction is correct. What is the error?

They used the relative atomic mass, which is an average across isotopes, where the calculation needs the mass number of one specific atom
They should have rounded 35.5 to 36 before subtracting
They should have added rather than subtracted
Nothing is wrong, since 18.5 is a valid answer

Three kinds of quantity, not five facts

The five numbers fall into three kinds, and knowing which kind you are holding tells you what you are allowed to do with it.

True about these quantities

Select the TWO statements that are true.

Relative atomic mass is usually not a whole number because it is an average across isotopes rather than a count of particles
The atomic number decides which element an atom is, so changing it would make it a different element
Mass number and relative atomic mass are two names for the same quantity
The number of electrons always equals the number of protons

What the maths weighting means for how you write

One fact about your specification should change how you revise, and it is easy to miss. ⚠️ MATHEMATICAL SKILLS ARE AT LEAST 20% OF THIS QUALIFICATION. THAT IS DOUBLE WHAT THE WJEC BIOLOGY SPECIFICATION REQUIRES, and it is a deliberate difference rather than an accident. One mark in five is waiting on your ability to handle a number correctly. So a chemistry answer is not judged only on whether the final figure is right. ⚠️ SET THE WORKING OUT SO EACH LINE SAYS WHAT IT IS. Not a column of bare digits, but a line that names the quantity: mass number minus atomic number, then the subtraction, then the answer. If the final number is wrong but the method is visible and correct, there are usually marks in the working. If only an answer appears and it is wrong, there is nothing to credit. ⚠️ AND STATE WHICH QUANTITY YOU HAVE USED, ESPECIALLY WHERE TWO ARE SIMILAR. Writing "mass number = 35" before you subtract shows the marker you chose deliberately rather than grabbed the nearest figure. Then the extended-writing question, the QER, where the marks include how clearly you communicate and not only the chemistry. ⚠️ IN CHEMISTRY THAT NEARLY ALWAYS MEANS SAYING WHAT YOUR NUMBERS MEAN. A calculation dropped into a paragraph with no sentence around it is not an explanation. Say what you worked out, say what it tells you, and say why that answers the question. And check that your answer is the right KIND of thing before you write it down. ⚠️ A COUNT OF PARTICLES THAT COMES OUT FRACTIONAL IS TELLING YOU SOMETHING WENT WRONG, and noticing that in the exam is worth more than any formula you could memorise.

One element, all five numbers

Take chlorine, because it is the element where the trap is sharpest. Its atomic number is 17. So every chlorine atom has 17 protons, and in a neutral atom 17 electrons as well. Two of the five numbers settled already, from one fact. Now the mass number, and here chlorine has two common forms. One kind of chlorine atom has a mass number of 35; the other has a mass number of 37. Both are chlorine, because both have 17 protons. Work the neutrons for each. For the first, 35 counted protons and neutrons together, so removing the 17 protons leaves 18 neutrons. For the second, 37 minus 17 leaves 20 neutrons. ⚠️ NOTICE WHAT DID NOT CHANGE. BOTH HAVE 17 ELECTRONS ARRANGED IDENTICALLY, SO BOTH BEHAVE IDENTICALLY IN EVERY CHEMICAL REACTION. The extra neutrons sit in the nucleus and take no part. And now the fifth number, which is where students lose marks. The relative atomic mass of chlorine is about 35.5. ⚠️ THERE IS NO CHLORINE ATOM WITH A MASS NUMBER OF 35.5. NOT ONE. The figure is an average across a real sample, and because the lighter kind is roughly three times as common as the heavier one, the average sits closer to 35 than to 37. ⚠️ SO 35.5 IS THE ONE NUMBER OF THE FIVE THAT DESCRIBES NO INDIVIDUAL ATOM AT ALL, AND IT IS THE ONE MOST OFTEN DROPPED INTO A COUNTING CALCULATION. One last thing this case shows. Chlorine's identity, its electron arrangement, its group and its chemical behaviour all followed from the 17. The mass numbers changed and none of that moved. That is the reason the modern table is ordered by the count of protons: it is the number that actually decides how an element behaves.

Subtract to find the neutrons

An atom of iron has an atomic number of 26 and a mass number of 56. How many neutrons are in its nucleus?

The five numbers run

Five questions on which quantity is which. Three lives.

Spot the true atomic number facts

Tap the TWO statements that are true.

Relative atomic mass describes an average across a sample, so no single atom need have that mass
The modern table is ordered by the count of protons, which is the quantity that decides chemical behaviour
Mass number and relative atomic mass are two names for the same quantity
Two isotopes of one element react differently because their masses differ

Complete the atomic structure facts

The number of protons in a nucleus is the _____. The number of protons and neutrons counted together is the _____. An average mass across all the isotopes of an element, weighted by how common each one is, is the _____. In a neutral atom the number of _____ is equal to the number of protons, because the charges balance.

atomic number mass number relative atomic mass electrons neutrons isotope ion period

Write the working so it earns marks

Assemble a line of working for the neutron count of an atom with atomic number 26 and mass number 56, set out the way the step-7 advice describes.

Three calls about numbers

Three cases. Each answer has to carry the reasoning, not just the verdict.

A student needs the neutron count for copper and uses the relative atomic mass of 63.5, getting a fractional answer. They ask whether to round it. What should you tell them?
Two samples contain atoms with the same atomic number but different mass numbers. A student predicts the heavier sample will react more slowly. Are they right?
In a calculation question a student writes only the final answer, which turns out to be wrong. Given that mathematical skills are at least a fifth of this qualification, what was the cost?

Explain which number is which

A classmate keeps using the number printed largest on the periodic table in every calculation, and cannot see why they sometimes get fractional answers for counts of particles. Write them an explanation.

Explain what the atomic number and the mass number each count, and how the neutron count follows from them
Explain why relative atomic mass is usually not a whole number, and what it describes that the others do not
Explain why two atoms with the same atomic number but different mass numbers behave identically in reactions
Explain why the modern periodic table is ordered by atomic number
Finish by explaining why setting out working that names each quantity matters on a specification where mathematical skills are at least a fifth of the marks

Atomic Structure and the Periodic Table

Get the method right under pressure

What you'll cover