Protons neutrons electrons — mass and charge — Chemistry

Before this

The atom — protons, neutrons, electrons · coming soon
Electron shells · coming soon

This leads to

Electron configuration
How ionic bonds form
Atomic number, mass number and isotopes (Physics) · coming soon

How can we know what is inside something we cannot see?

Everything you can touch is made of atoms: this screen, the air, your hand. A full stop on a printed page is about half a millimetre wide. That is roughly two million atoms side by side.

No one has ever looked inside an atom with their eyes. No microscope shows a proton. And yet chemists can tell you exactly what is inside an atom of sodium: how many particles, of which kinds, where they sit and what they weigh. How?

The answer starts with one of the most surprising experiments in the history of science. It shows that an atom has an inside, and that the inside is very strange. Once you understand that inside, you hold the key to most of chemistry. Which atoms bond, which ions form, why the periodic table has its shape: all of it follows from three particles and two numbers.

A surprise in 1909

In the early 1900s, most scientists pictured the atom as a ball of positive charge, spread out evenly, with tiny negative electrons dotted through it. This was called the plum pudding model. The electrons were the plums. The positive charge was the pudding.

In 1909, Hans Geiger and Ernest Marsden, working in Ernest Rutherford's laboratory in Manchester, tested this picture. They fired alpha particles at very thin metal foil. Alpha particles are small, fast and positively charged. Gold was a favourite target, because gold can be beaten into foil less than a thousandth of a millimetre thick. Before you read on, decide what you would expect.

Predict, then reveal — firing at gold foil

If the plum pudding model were right, what would happen to the alpha particles? Choose one, then press "Show me".

The question: fast, positive alpha particles are fired at very thin gold foil. What happens?

What Geiger and Marsden saw.

Every particle passes straight through. This is what the plum pudding model predicted, because spread-out charge is too weak to turn a fast particle. Most particles did pass through, but not all.
Most pass straight through, but a very few bounce back. This is what happened, and it is why the plum pudding model had to go.
Most bounce back. This would need the foil to be packed solid. It is not: most particles met nothing at all.
They all stop inside the foil. The foil is so thin that the particles do not lose all their energy inside it.

Almost every alpha particle went straight through the foil. But a very small number were turned through large angles, and a few came almost straight back. In one early measurement, about 1 in 8000 bounced back. Rutherford later called it the most incredible event of his life.

In 1911, Rutherford explained the result. Spread-out charge could never turn a fast particle around. So the positive charge, and almost all the mass, must be packed into a tiny region at the centre of the atom: the nucleus. Most alpha particles never came near a nucleus, so they passed straight through. The rare particle that came very close to one was pushed back hard.

How tiny is the nucleus? Its diameter is roughly 100 000 times smaller than the diameter of the atom. If the nucleus were the size of a pea, about 1 cm across, the whole atom would be about 1 km across. Everything in between is the region where the electrons are found. The atom is almost entirely empty space.

Three particles

We now know that an atom is built from three kinds of particle. Two of them, the protons and neutrons, are in the nucleus. The third, the electrons, are outside it. The neutron was the last of the three to be discovered, by James Chadwick in 1932. Its lack of charge made it very hard to detect.

Particle	Where it is	Relative charge	Relative mass
proton	nucleus	+1	1
neutron	nucleus	0	1
electron	shells around the nucleus	−1	$\frac{1}{1836}$

Why "relative"? The real masses are extremely small. A proton has a mass of about 0.000 000 000 000 000 000 000 001 67 g. Numbers like that are hard to work with, so chemists compare each particle with the proton instead. On that scale, the neutron is 1. In fact it is very slightly heavier, by about 0.1%, but 1 is close enough for chemistry. The electron is about $\frac{1}{1836}$ of a proton. That is so small that we usually treat it as zero.

The charges work the same way. The proton's charge is +1 on this scale, and the electron's is exactly −1: the same size, opposite sign. The neutron has no charge, which is where its name comes from.

Two facts follow straight from this table.

First, almost all of an atom's mass is in the nucleus. So if you count the protons and neutrons, you know the mass of the atom.

Second, an atom has no overall charge. It contains equal numbers of protons and electrons, so every +1 is cancelled by a −1.

A sodium atom. The nucleus holds 11 protons and 12 neutrons. The 11 electrons are arranged in three shells: 2, 8, 1.

Two numbers describe every atom

Chemists use two numbers to describe the nucleus.

The atomic number, $Z$ , is the number of protons. This number decides which element the atom is. Every atom with 6 protons is carbon. Every atom with 11 protons is sodium. Change the number of protons, and you have a different element. The periodic table is arranged in order of atomic number.

The mass number, $A$ , is the number of protons plus the number of neutrons. Together these are called nucleons, the particles in the nucleus. Since electrons have almost no mass, the mass number tells you the mass of the atom on the relative scale.

Counting the particles

protons $= Z$ · neutrons $= A - Z$ · electrons in a neutral atom $= Z$

Both numbers are written next to the element's symbol in nuclide notation. The mass number goes at the top left and the atomic number at the bottom left:

_{11}^{23} Na_{6}^{12} C_{17}^{35} Cl

So $_{6}^{12} C$ has 6 protons, $12 - 6 = 6$ neutrons and 6 electrons. It is often called carbon-12. Notice that the bigger number is always on top: a nucleus cannot have fewer nucleons than protons.

Stop and think

A particle has 17 protons, 18 neutrons and 18 electrons. Argon has 18 electrons in its neutral atom. Is this particle argon?

Show the answer

No. Only the protons decide the element. It has 17 protons, so it is chlorine, whatever its electrons and neutrons are doing. It has one more electron than protons, so its overall charge is $17 - 18 = - 1$ . It is a chloride ion, Cl⁻, with mass number $17 + 18 = 35$ . The number of electrons tells you the charge, not the element.

Electrons in shells

The electrons are not scattered at random around the nucleus. They are arranged in shells, also called energy levels. The shell closest to the nucleus has the lowest energy, so electrons fill it first. They then fill outwards, one shell at a time.

For the first 20 elements, from hydrogen to calcium, you can use these limits:

The first shell holds up to 2 electrons.
The second shell holds up to 8 electrons.
The third shell takes 8 electrons before the fourth shell starts.

The arrangement is written as a list: sodium, with 11 electrons, is 2, 8, 1. Calcium, with 20, is 2, 8, 8, 2. The third shell can actually hold more than 8, as you will see in the next lecture. But for the first 20 elements, the 2, 8, 8 pattern gives the right answer every time.

Now build some atoms yourself. Change one number at a time and watch what changes with it.

Explore — build an atom

Add or remove one particle at a time. Watch which results change: the element, the mass number or the charge.

Protons

Neutrons

Electrons

This builder only counts. Some combinations are unstable or do not exist in nature. It goes up to 20 protons and 20 electrons, where the 2, 8, 8 rule holds.

What changes when you change each particle:

Protons	Neutrons	Electrons	Result
6	6	6	$_{6}^{12} C$ : a carbon atom, neutral, 2, 4
6	8	6	$_{6}^{14} C$ : still carbon (an isotope), mass number 14
7	7	7	$_{7}^{14} N$ : one more proton makes nitrogen
11	12	10	$_{11}^{23} Na^{+}$ : a sodium ion, charge +1, 2, 8
17	18	18	$_{17}^{35} Cl^{-}$ : a chloride ion, charge −1, 2, 8, 8

Protons change the element. Neutrons change the mass number. Electrons change the charge.

Changing the counts: ions and isotopes

The builder shows a pattern. Each particle controls one thing. The script of chemistry follows from it.

Change the electrons, and you get an ion. An atom that loses electrons has more protons than electrons, so it becomes positive. A sodium atom (11 protons, 11 electrons) that loses one electron becomes Na⁺: 11 protons and 10 electrons, charge +1. An atom that gains electrons becomes negative. A chlorine atom that gains one electron becomes Cl⁻. Write the number before the sign: Mg²⁺, not Mg⁺². In every case, the charge is protons minus electrons.

Change the neutrons, and you get an isotope. Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12, carbon-13 and carbon-14 all have 6 protons and 6 electrons. They have 6, 7 and 8 neutrons. Because chemical reactions involve electrons, and their electrons are the same, isotopes react in the same way. Their masses differ, and some are radioactive. Carbon-14 is the one used to date ancient wood and bone.

Change the protons, and you get a different element. No chemical reaction can do this, because chemical reactions only move electrons. Turning one element into another needs a nuclear reaction.

Worked examples

Worked Example 1 — Sodium

Give the numbers of protons, neutrons and electrons in $_{11}^{23} Na$ , and its electron arrangement.

Protons $= Z = 11$ . Neutrons $= A - Z = 23 - 11 = 12$ . Electrons: it is a neutral atom, so 11.

Arrangement: 2 fill the first shell, 8 fill the second, and 1 is left for the third. So 2, 8, 1. Check: $2 + 8 + 1 = 11$ .

Worked Example 2 — Chlorine, and the classic slip

Do the same for $_{17}^{35} Cl$ .

Protons = 17, the bottom number. Neutrons $= 35 - 17 = 18$ . Electrons = 17. Arrangement: 2, 8, 7.

The slip to avoid: taking the protons from the top number. A student who writes "35 protons" gets every other answer wrong as well, because every count in the problem depends on the proton number. Say it to yourself each time: the bottom number counts the protons.

Worked Example 3 — A mystery element

An atom has 20 protons, 20 neutrons and 20 electrons. Identify it, and write it in nuclide notation.

20 protons means $Z = 20$ . On the periodic table, element 20 is calcium, Ca. The mass number is $20 + 20 = 40$ . So it is $_{20}^{40} Ca$ , with arrangement 2, 8, 8, 2.

Worked Example 4 — An ion

An aluminium atom, $_{13}^{27} Al$ , loses three electrons. Describe the particle it becomes.

Protons stay at 13, so it is still aluminium. Neutrons stay at $27 - 13 = 14$ . Electrons: $13 - 3 = 10$ .

Charge $= 13 - 10 = + 3$ . It is the ion Al³⁺, with arrangement 2, 8. Its outer shell is now full, which is a clue to why aluminium forms this ion. You will meet that idea properly when you study bonding.

Worked Example 5 — Two isotopes of chlorine

Compare $_{17}^{35} Cl$ and $_{17}^{37} Cl$ .

Both have 17 protons and 17 electrons, so both are chlorine, with arrangement 2, 8, 7. Chlorine-35 has 18 neutrons. Chlorine-37 has $37 - 17 = 20$ neutrons.

They react in the same way, because their electrons are the same. Only their masses differ.

Three classic mistakes

Trap 1 — Taking the protons from the mass number

In $_{17}^{35} Cl$ , the protons are 17, not 35. The top number counts protons and neutrons together. If you are unsure which number is which, remember that the top number is always the bigger one.

Trap 2 — Getting the sign of an ion backwards

Losing electrons makes a particle positive, not negative. Electrons carry the negative charge, so taking them away leaves the protons' positive charge unbalanced. Always work it out: charge = protons − electrons.

Trap 3 — Reading 35.5 as a mass number

Many periodic tables show 35.5 for chlorine. That is not a mass number: no atom has half a neutron. It is the relative atomic mass, $A_{r}$ , an average over all the chlorine atoms in a natural sample. About three-quarters are chlorine-35 and one quarter chlorine-37, and the average comes out at 35.5.

Try it yourself

Work each one on paper first. Open a hint only when you are stuck.

Give the numbers of protons, neutrons and electrons in $_{15}^{31} P$ , and its electron arrangement.

Hint 1
The bottom number is the proton count. Neutrons are top minus bottom.

Hint 2
Fill the shells in order: 2, then up to 8, then the rest.

Full answer
Protons 15, neutrons $31 - 15 = 16$ , electrons 15 (neutral atom). Arrangement: 2, 8, 5.
A potassium atom, $_{19}^{39} K$ , loses one electron. Write the symbol of the particle formed, and give its protons, neutrons, electrons and electron arrangement.

Hint 1
Which counts change when an electron is lost, and which stay the same?

Hint 2
Charge = protons − electrons. Then fill 18 electrons into shells.

Full answer
Protons stay at 19 and neutrons at $39 - 19 = 20$ . Electrons fall from 19 to 18. Charge $= 19 - 18 = + 1$ , so the particle is K⁺. Arrangement: 2, 8, 8.
A particle has 16 protons, 16 neutrons and 18 electrons. Identify it fully: element, mass number, charge, symbol and electron arrangement.

Hint 1
Only one of the three counts decides the element. Element 16 on the periodic table is sulfur.

Hint 2
Compare the protons with the electrons to find the charge.

Full answer
16 protons means sulfur, S. Mass number $= 16 + 16 = 32$ . Charge $= 16 - 18 = - 2$ . It is the sulfide ion, S²⁻, written $_{16}^{32} S^{2 -}$ . Arrangement of its 18 electrons: 2, 8, 8.
Boron has two natural isotopes, boron-10 and boron-11. The periodic table gives boron a relative atomic mass of 10.8. Which isotope is more common? Estimate the percentage of each.

Hint 1
10.8 is an average of 10 and 11. Which of the two is it closer to?

Hint 2
Let the fraction of boron-10 be $x$ . Then the fraction of boron-11 is $1 - x$ , and $10 x + 11 (1 - x) = 10.8$ .

Full answer
10.8 is much closer to 11, so boron-11 is more common. Solve $10 x + 11 (1 - x) = 10.8$ : $11 - x = 10.8$ , so $x = 0.2$ . About 20% of boron atoms are boron-10 and 80% are boron-11. Check: $0.2 \times 10 + 0.8 \times 11 = 2 + 8.8 = 10.8$ .

The bigger idea

Almost all the mass of an atom sits in its nucleus. Almost all of its chemistry happens in its electrons. When two atoms meet, it is their electrons that meet first, so electrons decide how atoms bond and react. The heavy nucleus mostly stays out of chemistry, except in one way: its protons decide which element the atom is.

For centuries, alchemists tried to turn lead into gold with chemical reactions. Now you can see why they never could. Lead has 82 protons and gold has 79. Chemistry only moves electrons. Changing an element means changing its nucleus.

Key concepts in this lesson

An atom has a tiny, dense, positive nucleus of protons and neutrons, surrounded by electrons, and is almost entirely empty space.
Protons have relative charge +1 and relative mass 1, neutrons have charge 0 and mass 1, and electrons have charge −1 and mass about $\frac{1}{1836}$ .
The atomic number $Z$ is the number of protons and decides the element, and the mass number $A$ is protons plus neutrons.
Neutrons $= A - Z$ , and a neutral atom has as many electrons as protons.
Changing the electrons makes an ion with charge protons − electrons, and changing the neutrons makes an isotope of the same element.
For the first 20 elements, electrons fill shells from the inside out as 2, 8, 8, 2.