Protons, neutrons and electrons

The big idea

Every atom is built from just three kinds of particle. The nucleus (the tiny central core of the atom) contains protons and neutrons, which together are called nucleons. Around it are the electrons. The nucleus is about 100 000 times smaller across than the whole atom, yet it holds more than 99.9% of the mass. So an atom is mostly empty space with a heavy, positive dot in the middle.

Explained simply

Picture a football stadium. A marble on the centre spot is the nucleus; the electrons are like midges buzzing around the outer stands. Nearly all the "stuff" is in the marble, but the size of the atom is set by where the electrons are.

Chemists compare the particles using relative values, which means each one is measured against the proton:

Particle	Where it is	Relative charge	Relative mass
Proton	nucleus	+1	1
Neutron	nucleus	0	1
Electron	shells around the nucleus	−1	$\frac{1}{1836}$ (about 0.0005)

Two numbers describe a particular atom:

• Atomic number (proton number), $Z$ — the number of protons. It decides which element the atom is.
• Nucleon number (mass number), $A$ — the number of protons plus neutrons.

So the number of neutrons is $A - Z$. A neutral atom has as many electrons as protons. An ion is an atom or group of atoms with an overall charge: a positive ion has lost electrons, a negative ion has gained them. The protons never change in a chemical reaction.

$\text{electrons in an ion} = Z - (\text{charge on the ion})$

Beams in an electric field. If beams of the three particles, all moving at the same speed, pass between two charged plates, each behaves differently. Protons are attracted towards the negative plate. Electrons are attracted towards the positive plate. Neutrons have no charge, so they carry straight on. The electrons bend far more than the protons, even though the size of their charge is the same, because they are about 1836 times lighter. The rule is:

$\text{angle of deflection} \propto \frac{\text{charge}}{\text{mass}}$

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Beams of protons, neutrons and electrons passing between charged plates: electrons curve sharply towards the positive plate, protons curve slightly towards the negative plate, and neutrons go straight on (not to scale)

Worked example

(a) Give the numbers of protons, neutrons and electrons in $^{56}_{26}\text{Fe}^{3+}$ and $^{32}_{16}\text{S}^{2-}$.

For iron, $Z = 26$ and $A = 56$:

• protons $= 26$
• neutrons $= 56 - 26 = 30$
• electrons $= 26 - 3 = 23$ (a 3+ ion has lost three electrons)

For sulfur, $Z = 16$ and $A = 32$:

• protons $= 16$
• neutrons $= 32 - 16 = 16$
• electrons $= 16 - (-2) = 18$ (a 2− ion has gained two electrons)

(b) A beam of $^{1}\text{H}^{+}$ ions is deflected through an angle of 8.0°. Under the same conditions, predict the deflection of a beam of $^{4}\text{He}^{2+}$ ions.

Work out charge ÷ mass for each, using relative values:

$^{1}\text{H}^{+}: \ \frac{+1}{1} = 1 \qquad ^{4}\text{He}^{2+}: \ \frac{+2}{4} = 0.5$

The helium ion has half the charge-to-mass ratio, so it is deflected half as much:

$\text{angle} = 0.5 \times 8.0^\circ = 4.0^\circ$

Both are positive, so both bend towards the negative plate.

Common mistakes

• Changing the protons to make an ion. Ions form by losing or gaining electrons only. If the proton number changed, it would be a different element.
• Getting the sign backwards for negative ions. $\text{S}^{2-}$ has more electrons than protons: $16 + 2 = 18$.
• Saying electrons bend more "because they are negative". The direction comes from the sign of the charge; the amount of bending comes from charge ÷ mass.
• Writing the electron's relative mass as 0. It is tiny ($\frac{1}{1836}$), not zero. Saying "negligible" is fine.

Where you see this in real life

Old television sets and some electron microscopes steer beams of electrons with electric and magnetic fields. Mass spectrometers, used to identify drugs in blood or check the purity of medicines, separate ions using exactly the charge-to-mass idea above. Neutrons, because they are uncharged, pass easily into materials, which is why neutron beams are used to study the structure of metals and proteins.