6.1–6.2, 6.5–6.9The structure of an atom
Atoms are very small, having a radius of about 1 × 10⁻¹⁰ metres.
The basic structure of an atom is a positively charged nucleus composed of both protons and neutrons, surrounded by negatively charged electrons.
The radius of a nucleus is less than 1/10 000 of the radius of an atom. Most of the mass of an atom is concentrated in the nucleus.
Put together, those two facts say the atom is almost entirely empty space, with nearly all its mass in a speck at the centre ten thousand times smaller than the atom itself.
The electrons are arranged at different distances from the nucleus — different energy levels. The electron arrangements may change:
• with the absorption of electromagnetic radiation, moving an electron further from the nucleus, to a higher energy level • by the emission of electromagnetic radiation, moving an electron closer to the nucleus, to a lower energy level.
The two go together: absorbing energy moves an electron out, and emitting energy lets it fall back in. Keeping the pairing straight — absorb and move out, emit and move in — is what these questions test.
You should be able to recognise expressions given in standard form, which is why the sizes above are quoted that way.
Common mistake
An electron does not gain or lose energy by "speeding up" between levels. It moves to a different energy level, and the energy is carried away or delivered by electromagnetic radiation.
6.3–6.4Mass number, atomic number and isotopes
In an atom the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge.
All atoms of a particular element have the same number of protons. The number of protons in an atom of an element is called its atomic number.
The total number of protons and neutrons in an atom is called its mass number.
Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
Atoms turn into positive ions if they lose one or more outer electrons. Note the direction: losing a negative electron leaves the atom positive.
You should be able to relate differences between isotopes to differences in conventional representations of their identities, charges and masses.
The method is always the same. The bottom number is the atomic number, which gives the protons and identifies the element. The top number is the mass number, so neutrons = mass number − atomic number. Electrons equal protons unless a charge is shown, in which case a plus sign means that many electrons have been lost.
Worked example
An atom is written ²³⁵₉₂U. Give the numbers of protons, neutrons and electrons. Then state what changes if it becomes ²³⁸₉₂U.
- The bottom number is the atomic number: 92 protons.
- Neutrons = mass number − atomic number = 235 − 92 = 143.
- A neutral atom has as many electrons as protons: 92 electrons.
- ²³⁸₉₂U has the same atomic number, so it is the same element with the same protons and electrons.
- Its mass number is larger: 238 − 92 = 146 neutrons.
Answer: 92 protons, 143 neutrons, 92 electrons. ²³⁸₉₂U is an isotope of the same element with 146 neutrons — three more, and no other difference.
- Atomic number
- the number of protons in an atom of an element; all atoms of a particular element have the same atomic number
- Mass number
- the total number of protons and neutrons in an atom
- Isotope
- atoms of the same element with the same number of protons but different numbers of neutrons
6.17The development of the model of the atom
New experimental evidence may lead to a scientific model being changed or replaced. This section is really about that idea, told through one example — so the marks are for what each piece of evidence forced, not for the dates. It is common content with chemistry.
Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
Two things are asked for directly.
Why the new evidence from the scattering experiment led to a change in the atomic model: the results led to the conclusion that the mass of an atom was concentrated at the centre — the nucleus — and that the nucleus was charged. A plum pudding of positive charge spread throughout the atom could not have produced that result, so the nuclear model replaced it.
The difference between the plum pudding model and the nuclear model: in the plum pudding the positive charge is spread throughout the atom with electrons embedded in it; in the nuclear model the charge and nearly all the mass are concentrated in a tiny central nucleus, with the electrons outside it.
Details of the experimental work supporting the Bohr model are not required, and neither are the details of Chadwick's experimental work.
Common mistake
Niels Bohr did not replace the nuclear model — he adapted it, by suggesting that electrons orbit at specific distances. His calculations agreeing with experimental observations are what made the nuclear model stick.
Before you practise
- An atom is a tiny positive nucleus of protons and neutrons, surrounded by electrons in shells.
- Relative charges: proton +1, electron −1, neutron 0. The positron has the same mass as the electron and a charge of +1.
- The atomic number is the number of protons and identifies the element; the mass number is protons plus neutrons.
- Isotopes have the same number of protons and different numbers of neutrons.
- An atom has equal numbers of protons and electrons, so no overall charge; losing outer electrons makes a positive ion.
- Electrons change orbit when they absorb or emit electromagnetic radiation.
- Plum pudding, then the alpha scattering experiment, then the nuclear model, then Bohr's orbits, then the neutron — each change followed new evidence.