The particles inside an atom
Dalton thought atoms could not be divided. Experiments around 1900 showed otherwise: atoms contain smaller subatomic particles.
Particle | Discovered by | Charge | Approximate mass | Where it is |
|---|---|---|---|---|
Electron (e⁻) | J. J. Thomson | −1 | about 1/2000 u | outside the nucleus |
Proton (p⁺) | E. Goldstein (canal rays) | +1 | 1 u | in the nucleus |
Neutron (n) | J. Chadwick (1932) | 0 | 1 u | in the nucleus |
Thomson's model
Thomson pictured the atom as a sphere of positive charge with electrons embedded in it, like seeds in a watermelon (or plums in a pudding). The total positive and negative charges are equal, so the atom is neutral.
This model could not explain the results of Rutherford's experiment.
Rutherford's gold foil experiment
Rutherford fired fast-moving alpha particles (positively charged, about 4 u in mass) at a very thin gold foil.
Observation | Conclusion |
|---|---|
Most alpha particles passed straight through | most of the atom is empty space |
Some were deflected by small angles | the positive charge occupies a very small space |
About 1 in 12 000 bounced right back | all the positive charge and most of the mass are in a tiny, dense nucleus |
Rutherford's model: a small, positively charged nucleus at the centre, with electrons revolving around it in circular paths.
Its problem: a charged particle moving in a circle should lose energy and spiral into the nucleus. Atoms would then be unstable, but we know atoms are stable.
Bohr's model
Niels Bohr proposed that:
electrons revolve only in certain discrete orbits, called energy levels or shells,
while in these orbits, electrons do not radiate energy.
The shells are labelled K, L, M, N… (or n = 1, 2, 3, 4…), starting nearest the nucleus.
Filling the shells
The maximum number of electrons a shell can hold is 2n²:
Shell | n | Maximum electrons |
|---|---|---|
K | 1 | 2 |
L | 2 | 8 |
M | 3 | 18 |
For the first 18 elements, the outermost shell holds at most 8 electrons, and shells are filled step by step, inner ones first.
Examples: sodium (11 electrons) is 2, 8, 1. Chlorine (17) is 2, 8, 7. Argon (18) is 2, 8, 8.
Valency
The electrons in the outermost shell are valence electrons. Atoms with a full outer shell (8, or 2 for helium) are stable and unreactive. These are the noble gases.
Other atoms react to reach a stable outer shell, by losing, gaining or sharing electrons. The number of electrons involved is the valency.
If an atom has 1 to 4 valence electrons, its valency usually equals that number. Sodium (2, 8, 1) has valency 1.
If an atom has 5 to 7, its valency is 8 minus that number. Chlorine (2, 8, 7) has valency 8 − 7 = 1, and oxygen (2, 6) has valency 2.
Atomic number and mass number
Atomic number (Z) = the number of protons. It defines the element. Every carbon atom has Z = 6.
Mass number (A) = the number of protons + the number of neutrons, because nearly all the mass is in the nucleus.
Number of neutrons = A − Z
Example. An atom of sodium has Z = 11 and A = 23. It has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
Isotopes and isobars
Isotopes are atoms of the same element with the same atomic number but different mass numbers, because they have different numbers of neutrons. Hydrogen has three: protium (¹H), deuterium (²H) and tritium (³H). Chlorine occurs as ³⁵Cl and ³⁷Cl in the ratio of about 3 : 1, which is why its average atomic mass is 35.5 u.
Isotopes have the same chemical properties but different physical properties. Uses: uranium-235 as fuel in nuclear reactors, cobalt-60 in cancer treatment, iodine-131 in treating goitre.
Isobars are atoms of different elements with the same mass number: for example, calcium-40 (Z = 20) and argon-40 (Z = 18).