Inside the Atom: Protons, Neutrons, Electrons and Isotopes

1Bullets that bounced back

By 1900 scientists knew that atoms contain tiny negative particles, electrons, discovered by J. J. Thomson in 1897. Because atoms are neutral overall, there had to be positive charge too. Thomson pictured the atom as a ball of spread-out positive charge with electrons dotted through it, like raisins in a bun.

In 1909 Hans Geiger and Ernest Marsden, working for Ernest Rutherford, tested this picture. They fired alpha particles, small, fast, positively charged particles given out by a radioactive substance, at a sheet of gold only a few hundred atoms thick. If Thomson were right, every alpha particle should pass through with hardly any change of direction. Figure 2.1 shows what they actually saw.

Figure 2.1: The gold-foil experiment. Each circle is a gold atom; the dot at its centre is the nucleus, drawn far too big.

2What the results mean

Rutherford explained the results in 1911:

• Most alpha particles went straight through, so most of an atom is empty space.
• A few were pushed sideways, and about 1 in 8,000 bounced back. They must have come very close to, or hit, something small, heavy and positive that repelled them. Rutherford called it the nucleus.

The nucleus is tiny. If an atom were as wide as Amahoro Stadium in Kigali, its nucleus would be about the size of a bead on the centre spot, yet it would hold almost all of the atom's mass. The electrons move around the nucleus through the rest of the "stadium". Later experiments found the two kinds of particle in the nucleus: the proton (Rutherford, 1919) and the neutron (James Chadwick, 1932).

Figure 2.2: The structure of a lithium atom. Protons and neutrons make up the nucleus; electrons occupy shells around it.

The masses and charges are given relative to the proton, because the real values are extremely small: a proton has a mass of only $1.67\times {10}^{-27}\mathrm{k}\mathrm{g}1.67\; \backslash times\; 10^\{-27\}\backslash ,\backslash mathrm\{kg\}$.

Table 2.1: The three particles in an atom

3Writing an atom in symbols

Chemists write the mass number at the top left of the symbol and the atomic number at the bottom left:

$${}_{\mathrm{Z}}^{\mathrm{A}}\mathrm{X}\text{for example}{}_{11}^{23}\mathrm{N}\mathrm{a}\backslash mathrm\{^\{A\}\_\{Z\}X\}\; \backslash qquad\; \backslash text\{for\; example\}\; \backslash qquad\; \backslash mathrm\{^\{23\}\_\{11\}Na\}$$

From these two numbers you can find every particle in the atom:

$$\text{protons}=Z\text{neutrons}=A-Z\text{electrons (neutral atom)}=Z\backslash text\{protons\}\; =\; Z\; \backslash qquad\; \backslash text\{neutrons\}\; =\; A\; -\; Z\; \backslash qquad\; \backslash text\{electrons\; (neutral\; atom)\}\; =\; Z$$

4Isotopes: same element, different mass

All carbon atoms have 6 protons, but they do not all have the same number of neutrons. Most have 6 neutrons (carbon-12), about 1 in 100 has 7 (carbon-13), and a very small number have 8 (carbon-14). These different forms of one element are called isotopes. The name comes from Greek for "same place": isotopes sit in the same place in the periodic table.

Isotopes of an element have the same number of electrons, so they react in the same way. They differ only in mass, and in some cases in whether the nucleus is stable. Carbon-14 is slightly radioactive, and scientists measure how much is left in old wood, charcoal or bone to find its age. In S2 you will use isotopes to explain why chlorine's relative atomic mass is 35.5 and not a whole number.

Figure 2.3: The isotopes of hydrogen. Only the number of neutrons changes.

Table 2.2: Some isotopes

Try it yourself

Open the Build an Atom simulation. Drag protons, neutrons and electrons into the atom and watch the element name, the charge and the mass number change. Build ${}_3^7\mathrm{L}\mathrm{i}\backslash mathrm\{^\{7\}\_\{3\}Li\}$, then add one neutron: what changes, and what stays the same?