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Protons, neutrons and electrons, and which of the three counts is the element.
Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.
By the end of this lesson you will be able to read any nuclide symbol as three counts — protons, neutrons and electrons — and say which of the three decides what element it is. You will be able to work the neutron count out as a subtraction, adjust the electron count for a charge, and say what an ion has in common with the atom it came from, which is everything except its electrons.
You know that everything is made of atoms, that an element is a substance with only one kind of atom in it, and that a compound holds more than one kind joined together. What this lesson adds is what one kind of atom means, exactly: a number, and only one of the three numbers you are about to meet.
| Term | What it means |
|---|---|
| Proton | A positively charged particle in the nucleus. |
| Neutron | An uncharged particle in the nucleus, of almost the same mass as a proton. |
| Electron | A very light, negatively charged particle outside the nucleus. |
| Nucleus | The tiny, dense center of an atom, holding the protons and neutrons. |
| Atomic number | The number of protons; it decides the element. |
| Mass number | The number of protons and neutrons together. |
| Isotope | One of two or more atoms of an element with different neutron counts. |
| Ion | An atom that has lost or gained electrons and so carries a charge. |
Every atom is three numbers. They are not equally important, and knowing which is which is most of this unit.
A symbol is written with two of the three numbers beside it:
$$\mathrm{^{23}_{11}Na}$$
The bottom-left number, $11$, is the atomic number: the protons. The top-left number, $23$, is the mass number: protons and neutrons together. The neutrons are not written down, because they are the subtraction $23 - 11 = 12$.
So $\mathrm{^{23}_{11}Na}$ is eleven protons, twelve neutrons and — since nothing says otherwise — eleven electrons.
Another way: picture
Think of a football stadium with a marble on the fifty-yard line. The marble is the nucleus: every proton and every neutron is in it, and so is almost all the mass. The electrons are somewhere in the stands, and they are what any other atom meets first. That is why the electrons do the chemistry and the nucleus does not: nothing in an ordinary reaction ever gets anywhere near the marble.
Another way: steps
To read any nuclide symbol:
The numbers in this table are the ones every later unit assumes, and two of them are worth a second look.
| Particle | Where it is | Relative mass | Relative charge |
|---|---|---|---|
| proton | in the nucleus | 1 | $+1$ |
| neutron | in the nucleus | 1 | $0$ |
| electron | outside the nucleus | about $\tfrac{1}{1840}$ | $-1$ |
The first is the electron's mass. It is so small that the mass of an atom is, to any precision this course uses, the mass of its nucleus alone — which is why the mass number counts only protons and neutrons and why an ion weighs the same as the atom it came from.
The second is the neutron's charge, which is nothing. That is what lets a nucleus hold more neutrons or fewer without the atom's charge changing, and it is the reason isotopes exist at all.
Find the atomic number. It is the bottom-left number of the symbol, or the number given for the element. That is the proton count, and it names the element.
Find the neutrons. Subtract the atomic number from the mass number. The subtraction always goes this way round.
Find the electrons. Start from the proton count. If there is no charge, the electrons equal the protons. A positive charge takes that many electrons away; a negative charge adds that many.
Write all three with their labels. Protons, neutrons, electrons — three numbers, each with its name, so they cannot be swapped.
Check the work. Do protons plus neutrons give the mass number back? Does protons less electrons give the charge on the symbol, with its sign? Is the proton count the same as the atomic number of the element you were told it is? A neutron count can be zero, but it can never be negative; if yours is, the two numbers were read the wrong way round.
Reading the element from the atomic number is allowed because the proton count is what defines an element. It is not a property sodium happens to have; it is what the word sodium means.
Subtracting to find the neutrons is allowed because the nucleus holds only protons and neutrons, and the mass number counts both. Whatever is in the nucleus and is not a proton must be a neutron.
Setting the electrons equal to the protons in a neutral atom is allowed because a proton and an electron carry equal and opposite charges, and neutral means the charges cancel exactly.
Adjusting only the electrons for an ion is allowed because ordinary chemistry never reaches the nucleus. An ion forms when electrons are lost or gained in the outer part of the atom; the protons and neutrons are untouched, so the element and the mass stay the same.
An atom is about a ten-billionth of a meter across, and the nucleus at its center is about a hundred thousand times smaller again. Almost all of the atom is space through which the electrons move. Yet almost all of its mass is in that tiny nucleus: a proton or a neutron is about 1,840 times as heavy as an electron.
That difference of scale is why the two numbers on a symbol mean such different things. The mass number is a count of the heavy particles packed into the nucleus, so it sets the mass. The atomic number counts the positive charges in the nucleus, and those charges decide how many electrons the atom holds and how tightly. Every property a chemist measures in a test tube — what an element reacts with, the formulas of its compounds, whether it is a metal — follows from the electrons, and so from the atomic number.
It also explains why ordinary chemistry cannot change one element into another. The energies of chemical reactions are enough to move electrons between atoms, but far too small to add or remove a proton from a nucleus. Changing the proton count takes the very much larger energies of nuclear reactions, which are the subject of the last unit of this course.
It would be tidier if the bigger number were the identity, and it is not. The reason is that chemistry happens between electrons, the electron count follows the proton count, and the neutron count is invisible to all of it.
Carbon-12 and carbon-14 differ by two neutrons. They have the same six protons, therefore the same six electrons in a neutral atom, therefore the same reactions: both burn to carbon dioxide, both form four bonds, both sit in living tissue. The only difference a chemist can see is the mass — and, for carbon-14, that its nucleus is unstable, which is a nuclear property and not a chemical one. That is exactly why carbon dating works: the two behave identically going into a plant, so the proportion that goes in is the proportion the atmosphere had.
Nuclear medicine departments across the United States rely on exactly the distinction this lesson draws. Iodine-131 has 53 protons, like every iodine atom, and 78 neutrons. Because its proton count is 53, the body treats it as iodine and the thyroid gland takes it up just as it takes up ordinary iodine-127, which has 74 neutrons. But the extra neutrons make iodine-131's nucleus unstable, and the radiation it gives off as it decays destroys overactive thyroid tissue from the inside.
The same idea runs the most common imaging scan in American hospitals. Technetium-99m has 43 protons and 56 neutrons. It is attached to a carrier molecule chosen for where it travels in the body — bone, heart muscle — and a camera tracks the radiation it gives off. The chemistry decides where the atom goes, and that chemistry depends on the 43 protons and their electrons; the neutron count decides that the nucleus will give off radiation the camera can see.
A hospital pharmacist ordering a dose works with exactly these counts. The mass number is in the name, the atomic number is the element, and the neutrons are the difference: for fluorine-18, used in PET scans, $18 - 9 = 9$.
Archaeologists date wood, bone and cloth by measuring the share of carbon-14 left in them. Carbon-14 and carbon-12 both have six protons, so living things take them up alike; after death, the carbon-14 decays and the ratio falls at a known rate.
The mass number is read as the element. It is the eye-catching number and it is the wrong one. Two atoms with mass number $14$ can be two different elements — $\mathrm{^{14}_{6}C}$ and $\mathrm{^{14}_{7}N}$ are carbon and nitrogen — because their proton counts differ.
The neutrons are read straight off the symbol. They are never written. The top-left number includes them, so the neutron count is always a subtraction.
An ion is treated as a different element. A sodium ion has ten electrons, which is what neon has, and it is still sodium: the nucleus has eleven protons and nothing about forming an ion touched it.
A positive charge is read as electrons gained. It is the reverse. A $+2$ ion has two electrons fewer than its protons, because losing negative charge leaves the atom positive.
A missing neutron count is read as an error. $\mathrm{^{1}_{1}H}$ has one proton and no neutrons at all. It is the commonest atom in the universe, and its nucleus is a single proton.
Read the atomic number.
$\mathrm{^{27}_{13}Al}: Z = 13$
The bottom-left number.
Name the element.
$13 \text{ protons: aluminum}$
The atomic number is the element.
Read the mass number.
$A = 27$
The top-left number counts the whole nucleus.
Find the neutrons.
$27 - 13 = 14$
The neutron count is never written; it is the difference.
Find the electrons.
$13 \text{ electrons}$
No charge is shown, so the atom is neutral.
Read the atomic number of magnesium.
$Z = 12$
Start from the proton count, always.
Count the neutral atom's electrons.
$12 \text{ electrons}$
Neutral means as many electrons as protons.
Read the ion's charge.
$\mathrm{Mg^{2+}}: +2$
A charge is a statement about electrons only.
Find the ion's electrons.
$12 - 2 = 10$
A 2+ charge means two electrons lost.
Check the charge.
$12 - 10 = +2$
Protons less electrons gives the charge back.
Compare the masses.
$\text{the same, to the precision used here}$
The two electrons that left had almost no mass.
Read the atomic number of chlorine.
$Z = 17$
Every chlorine particle has 17 protons.
Find the neutrons in chlorine-35.
$35 - 17 = 18$
Mass number less atomic number.
Find the neutrons in chlorine-37.
$37 - 17 = 20$
Only the neutron count differs between isotopes.
Count a neutral atom's electrons.
$17 \text{ in each isotope}$
The same electrons, so the same chemistry.
Read the chloride ion's charge.
$\mathrm{Cl^{-}}: -1$
A negative charge is an electron gained.
Find the chloride ion's electrons.
$17 + 1 = 18$
One more electron than protons.
Check the charge.
$17 - 18 = -1$
Protons less electrons gives the charge back.
Read the protons.
$8 \text{ protons: oxygen}$
Always start at the atomic number.
Find the neutrons.
$16 - 8 = 8$
Mass number less atomic number.
Find the electrons.
An atom of hydrogen-1 is written $\mathrm{^{1}_{1}H}$. The small number at the bottom left is the atomic number and the one at the top left is the mass number. Fill in how many of each particle one neutral atom of it contains.
| number in one neutral atom | |
|---|---|
| protons | |
| neutrons | |
| electrons |
Complete the worked solution: an ion has atomic number $20$, mass number $33$ and a charge of $2+$. Find its neutrons, its electrons, and the total number of particles it contains.
Find the neutrons.
$(\text{mass number}) - (\text{atomic number}) =$ a
The mass number counts protons and neutrons together.
Find the electrons.
$(\text{atomic number}) - \text{two} =$ b
A charge of 2+ means two electrons fewer than the protons.
Find the particles in total.
$(\text{protons}) + (\text{neutrons}) + (\text{electrons}) =$ c
Every particle in the ion, counted once.
Complete the sentence about carbon-14.
One neutral atom of $\mathrm{^{14}_{6}C}$ has a proton count of a, a neutron count of b and an electron count of c.
How many neutrons are there in one atom of chlorine-35, written $\mathrm{^{35}_{17}Cl}$?
Answer:
A hospital in Rochester, Minnesota uses iodine-131 for treating an overactive thyroid. The number in the name is the mass number, and the element's atomic number is $53$. How many neutrons are in one atom of it?
The answer: a.
A supplier sends a laboratory two bottles. Both are labeled bromine, $\mathrm{Br}$, and both are pure — but one has been enriched in the lighter of the element's two isotopes, mass number $79$, and the other in the heavier, mass number $81$. Both isotopes have atomic number $35$. Fill in what one atom from each bottle contains.
| in an atom from the lighter bottle | in an atom from the heavier bottle | |
|---|---|---|
| protons | ||
| neutrons | ||
| electrons |
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Three ions: the iron(III) ion written $\mathrm{Fe^{3+}}$, the potassium ion written $\mathrm{K^+}$, and the fluoride ion written $\mathrm{F^-}$. For each one, give the number of protons, the number of electrons and the overall charge.
| protons | electrons | overall charge | |
|---|---|---|---|
| the iron(III) ion | |||
| the potassium ion | |||
| the fluoride ion |
You can turn a symbol into three numbers and say which one is the element. Say out loud how many protons, neutrons and electrons there are in one chloride ion, and why the answer for protons would be the same for any chlorine particle at all. Next: what a mass spectrum shows, and why chlorine's relative atomic mass is not a whole number.
16. Your turn: what does one ion of $\mathrm{^{16}_{8}O^{2-}}$ contain?, step 3
$8 + 2 = 10$
A 2− charge means two electrons more than the protons.