The Nuclear Model of the Atom

The structure of the atom: a tiny positive nucleus with electrons in orbit

Every atom is built from three particles: protons, neutrons and electrons. The protons and neutrons are packed together at the centre to form the nucleus, and the electrons move in orbits around it. The nucleus carries a positive charge because of its protons, and each electron carries a negative charge, so the atom as a whole is neutral: the number of electrons equals the number of protons.

The size of an atom is set by the orbits of the electrons, not by the nucleus. The radius of the nucleus is only about one ten-thousandth of the radius of the whole atom, so an atom is almost entirely empty space. If an atom were enlarged to the size of a football stadium, the nucleus would be a small marble on the centre spot.

The mass is the other way round. A proton and a neutron each have a relative mass of 1, while an electron has a relative mass of only about 1/1840, small enough to be treated as negligible. Almost all of the mass of the atom is therefore concentrated in the very small nucleus.

Ions: how atoms lose and gain electrons

An ion is a charged particle formed when an atom gains or loses electrons. Only electrons are gained or lost. The nucleus is never changed when an ion forms, because the protons are held tightly inside it, so the number of protons stays the same and it is the number of electrons that decides the charge.

When an atom loses one or more electrons it is left with more protons than electrons, so it becomes a positive ion. A sodium atom has proton number 11, so it has 11 protons and 11 electrons; if it loses one electron it becomes a Na+ ion with 11 protons and 10 electrons, a charge of 1+. A magnesium atom has proton number 12, so if it loses two electrons it becomes a Mg2+ ion with 12 protons and 10 electrons, a charge of 2+.

When an atom gains one or more electrons it has more electrons than protons, so it becomes a negative ion. A chlorine atom has proton number 17, so it has 17 protons and 17 electrons; if it gains one electron it becomes a Cl- ion with 17 protons and 18 electrons, a charge of 1-. An oxygen atom has proton number 8, so if it gains two electrons it becomes an O2- ion with 8 protons and 10 electrons, a charge of 2-.

The pattern follows the periodic table: metals lose electrons to form positive ions and non-metals gain electrons to form negative ions, which is exactly what happens when a metal and a non-metal react to form an ionic compound.

• Losing electrons gives a positive ion; gaining electrons gives a negative ion.
• The number of protons never changes when an ion forms - only electrons move.
• Na+ has 11 protons and 10 electrons; Cl- has 17 protons and 18 electrons.
• An atom is neutral, so the number of electrons equals the proton number.

Inside the nucleus: protons, neutrons, proton number and nucleon number

The nucleus of every atom apart from hydrogen-1 contains both protons and neutrons, and these two particles together are called nucleons. The number of protons decides which element the atom is, while the number of neutrons changes the mass without changing the chemistry.

The proton number (also called the atomic number) is the number of protons in the nucleus, and it has the symbol Z. Every atom of the same element has the same proton number: every carbon atom has Z = 6 and every uranium atom has Z = 92. A neutral atom has the same number of electrons as protons, so the proton number also tells you the number of electrons.

The nucleon number (also called the mass number) is the total number of protons and neutrons in the nucleus, and it has the symbol A. To find the number of neutrons you subtract the smaller number from the larger one: number of neutrons = A - Z.

Worked example. A nucleus of uranium-235 has A = 235 and Z = 92, so the number of neutrons is 235 - 92 = 143. The same nucleus has 92 protons and, if the atom is neutral, 92 electrons. The proton number also fixes the relative charge on the nucleus, because each proton contributes +1 and each neutron contributes 0, so this nucleus has a relative charge of +92. The nucleon number fixes the relative mass, because a proton and a neutron each have relative mass 1 and the electrons are almost weightless, so the relative mass of the nucleus is 235.

Nuclide notation and isotopes

A nuclide is a particular nucleus described by its proton number and its nucleon number. It is written using nuclide notation: the nucleon number A goes at the top left and the proton number Z at the bottom left, with the symbol of the element beside them. So the nuclide of the commonest isotope of uranium is written 235/92 U, which is read as uranium with nucleon number 235 and proton number 92.

You can read everything you need straight off the notation. The bottom number is the number of protons, the top number is the total number of nucleons, and the difference is the number of neutrons. For 235/92 U there are 92 protons, 235 - 92 = 143 neutrons and 92 electrons in the neutral atom.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They therefore have the same proton number and different nucleon numbers. Because chemistry depends on the electrons, and isotopes have the same number of electrons, isotopes of an element have the same chemical properties; their masses differ, so physical properties that depend on mass differ slightly.

An element may have more than one isotope, and most elements do. Chlorine has two common isotopes, 35/17 Cl and 37/17 Cl: both have 17 protons, but 35/17 Cl has 35 - 17 = 18 neutrons and 37/17 Cl has 37 - 17 = 20 neutrons. Hydrogen has three isotopes, hydrogen-1, hydrogen-2 and hydrogen-3, with 0, 1 and 2 neutrons, and all three have 1 proton. Carbon has carbon-12 with 6 neutrons and carbon-14 with 8 neutrons.

• Isotopes have the same number of protons and different numbers of neutrons.
• Chlorine-35 has 18 neutrons and chlorine-37 has 20 neutrons; both have 17 protons.
• Isotopes of an element have the same chemical properties because they have the same number of electrons.
• If the proton number changes, it is a different element, not an isotope.

The alpha particle scattering experiment: evidence for the nuclear model

Before the nuclear model was accepted, the picture of the atom was a sphere of positive charge with electrons buried inside it. This was tested by firing alpha particles at a very thin sheet of metal, usually gold foil, held in a vacuum. An alpha particle is a helium nucleus: 2 protons and 2 neutrons, with a relative charge of +2 and a relative mass of 4. The particles came from a radioactive source, and a detector was moved round the foil to count how many were deflected through each angle.

The results were startling. Most of the alpha particles passed straight through the foil without being deflected at all. Some were deflected through small angles, and about one in 8000 was deflected through more than 90 degrees, coming almost straight back towards the source.

Each observation is evidence for one part of the nuclear model. Because most particles pass straight through, the atom must be mostly empty space, and the positive charge cannot be spread out through the whole atom. Because a few are deflected through very large angles, the positive charge must be concentrated in a very small nucleus, and because a positive alpha particle is pushed away by that nucleus, the nucleus must itself be positively charged. Because only the nucleus can turn an alpha particle back the way it came, the nucleus must contain most of the mass of the atom.

The closer an alpha particle passes to the nucleus, the stronger the electrostatic repulsion and the greater the deflection, which is why the largest deflections are so rare. If the positive charge had been spread evenly through the atom, every alpha particle would have been deflected a little and none would have bounced back.

• Most alpha particles pass straight through, so the atom is mostly empty space.
• A few are deflected through large angles, so the nucleus is very small and dense.
• The nucleus repels the positive alpha particles, so the nucleus is positively charged.
• Only the nucleus can bounce an alpha particle back, so the nucleus contains most of the mass of the atom.

Fission and fusion: splitting and joining nuclei

Nuclear fission is the splitting of a large nucleus into two smaller nuclei, and nuclear fusion is the joining of two small nuclei to make one larger nucleus. Both release energy, and in both cases the energy comes from a change in mass.

Fission happens when a heavy nucleus such as uranium-235 absorbs a neutron and becomes unstable, then splits into two smaller nuclei and releases two or three more neutrons. A typical fission is written 235/92 U + 1/0 n -> 144/56 Ba + 89/36 Kr + 3 1/0 n. Check the balance: the nucleon numbers give 235 + 1 = 236 on the left and 144 + 89 + 3 = 236 on the right, and the proton numbers give 92 + 0 = 92 on the left and 56 + 36 + 0 = 92 on the right. The neutrons released can be absorbed by other uranium nuclei, so a chain reaction builds up. In a reactor the reaction is controlled by control rods, which absorb neutrons, and the neutrons are slowed down by a moderator so that they are more likely to be absorbed.

Fusion happens when two light nuclei, usually hydrogen nuclei, are forced close enough together to join. In the core of the Sun, hydrogen nuclei fuse to form helium and the energy released keeps the Sun shining. A typical fusion reaction is 2/1 H + 3/1 H -> 4/2 He + 1/0 n, and again the nucleon numbers balance, because 2 + 3 = 4 + 1. Fusion needs an extremely high temperature and pressure, because both nuclei are positive and repel each other; only at millions of degrees do they move fast enough to get close enough to join, which is why fusion is so difficult to use as a power source on Earth.

In both processes the total mass of the products is slightly less than the total mass of the nuclei you started with. The missing mass has been converted into energy, which is carried away by the products, so a very small change in mass gives a very large release of energy. Fusion releases more energy per kilogram of fuel than fission, and its fuel can be obtained from water, but no fusion power station yet produces more energy than it uses.

• Fission: a heavy nucleus splits into two smaller nuclei, started by absorbing a neutron.
• Fusion: two light nuclei join to form one heavier nucleus, and it needs a very high temperature and pressure.
• Both release energy because the total mass of the products is less than the mass of the starting nuclei.
• The missing mass becomes energy - never say that the mass is destroyed.