Protons, neutrons, electrons and isotopes

Three particles

An atom is almost entirely empty space. Its nucleus is about 10⁻¹⁵ m across and the whole atom about 10⁻¹⁰ m, so the atom is a hundred thousand times wider than its nucleus, and almost all of its mass is in that nucleus. At the scale of a football pitch, a hydrogen nucleus the size of a grain of sand on the centre spot has its electron about thirty metres away, most of the way to the penalty area, with nothing in between.

The nucleus holds two kinds of particle. Protons carry a positive charge and neutrons carry none; together they are called nucleons. Outside the nucleus are the electrons, each with a negative charge equal in size to the proton's, and a mass so small that it is usually left out when the mass of an atom is worked out.

The masses are relative, so they have no unit. A proton's actual mass is 1.673 × 10⁻²⁷ kg and a neutron's is 1.675 × 10⁻²⁷ kg, close enough to call both 1. An electron is 1840 times lighter than a proton. A sodium atom has 11 electrons, and together they add about 0.006 to a total of 23.

An atom is neutral because it holds equal numbers of protons and electrons, so the charges cancel. Which element an atom belongs to is decided by one number only, its count of protons. Every atom with 6 protons is carbon, whatever else is true of it.

Two consequences follow from that structure. The size of an atom is the size of its electron cloud, so when atoms are said to get larger down a group or smaller across a period, it is the electrons that have moved. And the mass of an atom is in its nucleus, so the mass and the chemistry of an atom come from different places: the mass from the protons and neutrons, the chemistry from the electrons. Isotopes are the clearest case of that split.

Nucleon

A proton or a neutron: either of the two particles found in the nucleus.

Proton

A particle in the nucleus with a relative mass of 1 and a relative charge of +1.

Neutron

A particle in the nucleus with a relative mass of 1 and no charge.

Electron

A particle outside the nucleus with a relative mass of 1/1840 and a relative charge of −1.

Atomic number, mass number and the symbol

Two numbers describe a nucleus. The atomic number, Z, is the number of protons. It is also called the proton number, and it sets the element's place in the periodic table. The mass number, A, is the total number of protons and neutrons. The number of neutrons is therefore A − Z.

Describe the drawing

The nuclide symbol for chlorine-35 drawn large, with the mass number 35 top left labelled as protons plus neutrons and the atomic number 17 bottom left labelled as protons. Beside it aluminium-27 with atomic number 13 and a 3+ charge top right, labelled as three fewer electrons than protons. Underneath, a table of protons, neutrons and electrons: chlorine-35 has 17, 18 and 17; chlorine-37 has 17, 20 and 17; the aluminium ion has 13, 14 and 10. Neutrons equal A minus Z.

In print the atomic number is often left out, because the symbol already fixes it: ³⁵Cl means chlorine-35, and chlorine is always element 17. The mass number is a count of particles, so it is always a whole number. It is not the relative atomic mass printed on the periodic table, 35.5 for chlorine, which is an average over the isotopes found in nature.

Atomic number

The number of protons in the nucleus of an atom, also called the proton number, Z.

Mass number

The total number of protons and neutrons in the nucleus of an atom, A.

Isotopes

Chlorine gas from a cylinder is a mixture. About three in every four of its atoms have 18 neutrons and a mass number of 35; the rest have 20 neutrons and a mass number of 37. All of them have 17 protons, so all of them are chlorine. Atoms like these are isotopes: atoms of the same element, with the same number of protons and different numbers of neutrons.

Hydrogen has three. Ordinary hydrogen, ¹H, has a nucleus that is a single proton. Deuterium, ²H, has a proton and a neutron. Tritium, ³H, has a proton and two neutrons, and it is radioactive. Being an isotope says nothing about being radioactive: chlorine-35 and chlorine-37 are both stable, and so is every chlorine atom in a grain of table salt.

Isotopes of an element react identically, and the reason is the electrons. A chemical reaction is a rearrangement of electrons: they are transferred or shared between atoms, and the nucleus takes no part. Two isotopes have the same number of protons, so their atoms have the same number of electrons in the same arrangement. Chlorine-37 gains an electron to form a chloride ion exactly as chlorine-35 does, and forms the same compounds in the same proportions.

2D₂(g) + O₂(g) → 2D₂O(l)Deuterium burns in oxygen to give heavy water, by the same reaction and in the same ratio as ordinary hydrogen.

Physical properties that depend on mass do differ. Heavy water, D₂O, has a density of 1.11 g cm⁻³ and freezes at 3.8 °C, and an ice cube of it sinks in ordinary water. A gas made of a lighter isotope diffuses slightly faster. The differences are largest for hydrogen, where deuterium has twice the mass of ¹H, and are hard to detect for chlorine, where the two isotopes differ in mass by under 6%.

Isotopes

Atoms of the same element with the same number of protons but different numbers of neutrons.

Ions

An ion is an atom or group of atoms that has gained or lost electrons and so carries a charge. Losing electrons gives a positive ion, because protons now outnumber electrons; gaining electrons gives a negative ion. The nucleus is unchanged, so an ion has the same atomic number and mass number as the atom it came from.

Mg(g) → Mg²⁺(g) + 2e⁻A positive ion forms: the electrons removed are written as products, so the charge balances, 0 on the left and +2 − 2 on the right.

Cl(g) + e⁻ → Cl⁻(g)A negative ion forms by gaining an electron.

Metals in Groups 1, 2 and 3 form ions with charges of 1+, 2+ and 3+. Non-metals in Groups 6 and 7 form ions with charges of 2− and 1−. In each case the ion has the same number of electrons as the nearest noble gas, so Na⁺, Mg²⁺, Al³⁺, F⁻ and O²⁻ all have ten electrons, like neon. Species with the same number of electrons are isoelectronic. Why those numbers are favoured depends on how the electrons are arranged in shells and subshells.

Ion

An atom or group of atoms that carries an overall charge because it has lost or gained electrons.

Isoelectronic

Having the same number of electrons, as Na⁺, Ne and F⁻ do.

How the model of the atom changed

The nucleus-and-electrons atom is the latest of several models, and each one replaced the model before it because an experiment gave a result the old model could not explain. John Dalton's atoms of 1803 were solid spheres that could not be divided, a different kind for each element. That model explained why elements combine in fixed ratios by mass, and it stood until somebody found a piece of an atom.

In 1897 J. J. Thomson showed that cathode rays are streams of negatively charged particles far lighter than a hydrogen atom, and that the same particles came out whatever metal the cathode was made of. Atoms contained electrons. Since atoms are neutral, Thomson proposed that the electrons were dotted through a sphere of spread-out positive charge: the plum pudding model.

In 1909 Hans Geiger and Ernest Marsden, working for Ernest Rutherford, fired alpha particles at gold foil about a thousand atoms thick. In a plum pudding atom the positive charge is spread thin, so every alpha particle should pass through with at most a slight deflection. Most did go straight through. About one in eight thousand was deflected by more than 90°, and a few came almost straight back.

Describe the drawing

Apparatus for alpha scattering: a source in a lead block fires a narrow beam of alpha particles at a thin gold foil, surrounded by a circular zinc sulfide screen where flashes show where each particle lands. A magnified inset of the foil shows three widely spaced gold nuclei marked plus. Four alpha paths pass between them in straight lines, one passing close to a nucleus curves smoothly away through about thirty degrees, and one heading almost straight at a nucleus curves round and back through about one hundred and fifty degrees.

Rutherford's explanation, published in 1911, was the nuclear model. The positive charge and nearly all the mass are concentrated in a tiny nucleus, and an alpha particle is turned sharply only if it passes very close to one. Most of the atom is empty, so most alpha particles met nothing. The electrons were placed around the nucleus, but the model said nothing about how they were arranged.

Niels Bohr supplied an arrangement in 1913. Heated hydrogen gives out light of only a few exact wavelengths, a line spectrum rather than a continuous rainbow. Bohr proposed that the electron can occupy only certain orbits of fixed energy, and that light is given out when it drops from one to a lower one, the energy difference fixing the wavelength. Fixed energy levels are the shells of GCSE chemistry.

Two further changes produced the model used at A level. From 1926 quantum mechanics replaced Bohr's circular orbits with orbitals, regions of space where an electron is likely to be found, and these explain the s, p and d subshells of electron configuration. In 1932 James Chadwick found the neutron, which accounted for the mass a nucleus has beyond its protons: a helium nucleus has a charge of +2 but a mass of 4.

Describe the drawing

Five atom models in date order, joined by arrows labelled with the evidence that changed them. Dalton's 1803 solid sphere; Thomson's 1904 plum pudding, a sphere of spread-out positive charge with electrons scattered through it; Rutherford's 1911 nuclear atom, a tiny positive nucleus with electrons far out and mostly empty space; Bohr's 1913 atom with electrons on three circular orbits labelled n equals 1, 2 and 3; and the orbital model from 1926, a fuzzy cloud densest near the nucleus. The arrows read electron discovered, alpha scattering, line spectrum of hydrogen, and quantum mechanics.

Each new model kept what the old one got right. Bohr's shells are still the quickest way to count electrons, and his idea that an electron can have only certain energies is the basis of ionisation energies and of the flame colours of metal ions. What changed each time was the part of the model an experiment had shown to be wrong. That is how any scientific model is tested: it makes a prediction, an experiment checks the prediction, and a model that fails is revised.

Other boards: beams of particles in an electric field

Cambridge examines how beams of protons, neutrons and electrons behave in an electric field, in its statement 1.1.5; the other three boards do not, though the same link between charge, mass and motion runs the time-of-flight mass spectrometer. A narrow beam of particles passing between two parallel metal plates, one charged positive and one negative, bends. A beam of protons curves towards the negative plate and a beam of electrons towards the positive plate. A beam of neutrons goes straight on, because a particle with no charge feels no force in an electric field. The direction of the bend gives the sign of the charge.

The size of the bend depends on charge and mass together. A larger charge means a larger force and more deflection; a larger mass means the same force changes the particle's direction less. For particles entering the field at the same speed, the angle of deflection is proportional to charge ÷ mass. An electron's charge is the same size as a proton's and its mass is 1840 times smaller, so an electron beam is bent about 1840 times as much as a proton beam, in the opposite direction.

Thomson's discovery of the electron was this measurement. Cathode rays bent towards a positive plate, so they were negatively charged, and they bent so far that their charge ÷ mass had to be far larger than that of a hydrogen ion: the particles were much lighter than any atom.