Atoms, elements and compounds, Year 10: the atom and its particles

The plum pudding model

HintIt is named after a British Christmas dessert dotted with fruit.

The whyThe model was put forward after the electron was discovered, to explain how an atom could contain negative particles and still be neutral overall. Before that, atoms were pictured as tiny solid balls that could not be split. It was replaced by the nuclear model after the alpha particle scattering experiment.

A tiny nucleus that is charged and holds most of the atom's mass

HintSomething small, heavy and able to repel a positive particle.

The whyMost alpha particles passing straight through shows that most of the atom is empty space. The few that were turned back must have met something with a great deal of mass packed into a very small space, and charged so that it pushed them away. That is the nucleus.

Its mass and positive charge are spread out, so nothing in it could turn an alpha particle back

HintThink how thinly the 'pudding' is smeared across the whole atom.

The whyIf the plum pudding model were right, every alpha particle should pass through with hardly any change of direction. A few bouncing back was new evidence that the model could not account for, so it was replaced by the nuclear model, in which the mass and the positive charge are concentrated at the centre.

Bohr

HintA Danish physicist; his first name was Niels.

The whyIn the first nuclear model the electrons were simply somewhere around the nucleus. This scientist's version put them in definite orbits at set distances, and his calculations agreed with what experiments showed. Those orbits are the energy levels, or shells, used to write electronic structures such as 2,8,1.

electron; neutron

HintThe earliest find was the light negative particle; the latest was the particle with no charge.

The whyEach discovery forced a new model. The first led to the plum pudding model and the scattering experiment led to the nuclear model. Later work showed that the positive charge of a nucleus comes in whole-number lots, each called a proton, and James Chadwick's experiments then gave the evidence for the last particle in the nucleus.

0

HintIts name is a clue: it sounds like the word for being neither one thing nor the other.

The whyThe relative charges are: proton +1, neutron 0, electron −1. Because the neutron has no charge, changing the number of neutrons changes an atom's mass but not its charge.

It has equal numbers of protons and electrons

HintEach positive particle in the nucleus is cancelled by one negative particle outside it.

The whyA proton is +1 and an electron is −1, so one of each cancels exactly. Sodium, for example, has 11 protons and 11 electrons: +11 and −11 make zero.

The atomic number

HintOn the periodic table it is the figure that rises by one from each element to the next.

The whyEvery atom of an element has the same number of protons, and no two elements share the same number. Carbon is always 6 and oxygen always 8. It is also called the proton number.

No — the number of protons fixes the element

HintAsk which particle count never varies among atoms of one kind.

The whySeventeen protons is always chlorine and eighteen is always argon. Atoms of one element can differ in their neutrons, and can gain or lose electrons, but a different number of protons means a different element.

protons; neutrons

HintOne of the two is positive and the other has no charge at all.

The whyThe nucleus is positive overall because of the positive particles in it. The negative electrons move around the nucleus in energy levels, which are also called shells.

1 × 10⁻¹⁰ m

HintA nanometre is a billionth of a metre; then move the decimal point one more place.

The why1 nm is 1 × 10⁻⁹ m, so 0.1 nm is 0.1 × 10⁻⁹ m, which is 1 × 10⁻¹⁰ m in standard form.

Less than 1/10 000 of it

HintThink of a small pea at the centre of a football stadium.

The whyAn atom is about 1 × 10⁻¹⁰ m in radius and a nucleus about 1 × 10⁻¹⁴ m, which is ten thousand times smaller. So almost the whole volume of an atom is empty space.

1

HintDivide first, then change metres into the smaller unit.

The why100 m ÷ 10 000 = 0.01 m, and 0.01 m × 100 = 1 cm. So if an atom were made 200 m wide, about the length of two football pitches, its nucleus would be only 2 cm across, the size of a large marble.

nucleus

HintIt is where the two heavy kinds of particle are found.

The whyProtons and neutrons each have a relative mass of 1, while the mass of an electron is very small. Since the protons and neutrons are all at the centre, that is where nearly all the mass is, even though it takes up a tiny fraction of the atom's volume.

A nanometre

HintIts prefix is the one used in the name for technology built on the scale of atoms.

The whyThe prefix nano- means 10⁻⁹, so 1 nm = 0.000 000 001 m. It is a convenient unit for atoms and small molecules: an atom is about 0.1 nm in radius.