0625 · 5.1The nuclear model of the atom

Atomic structure and ions

0:00A solid piece of metal is made from atoms that are mostly empty space. Almost all their mass is concentrated in tiny centres called nuclei. That surprising structure explains the results of an important experiment. In this video, we will connect atomic structure, ions and isotopes, then explain what changes when nuclei split or join. We need one electrical idea before looking inside an atom. Like charges repel, while opposite charges attract.

0:25Positive and negative describe electrical charge, not whether something is useful or harmful. These forces help explain both the attraction between electrons and nuclei, and the deflection of particles fired towards an atom. An atom has a small, positively charged nucleus, with negatively charged electrons in orbit around it. The nucleus contains protons and, in most atoms, neutrons. Electrons occupy the region surrounding the nucleus. Our diagram shows the nuclear model used at this level, but the nucleus is drawn much larger than its true relative size.

0:57A proton has relative charge plus one, a neutron has relative charge zero, and an electron has relative charge minus one. Relative charge compares each particle with the charge on a proton. A neutron is electrically neutral, but it still has mass. Its lack of charge does not mean it contributes nothing to the nucleus. Protons and neutrons each have a relative mass of approximately one. An electron has a much smaller mass than either of them.

1:21This explains why the nucleus contains almost all the atom’s mass, even though it occupies very little space. A neutral atom has equal numbers of protons and electrons, so their charges cancel. An ion is an atom, or a group of atoms, with an overall electrical charge. Here we will consider ions formed from single atoms. An atom forms a positive ion by losing electrons. Removing negative charge leaves more protons than electrons, so the remaining particle is positively charged overall.

1:49An atom forms a negative ion by gaining electrons. It then has more electrons than protons, giving it an overall negative charge. In both processes, the nucleus stays unchanged. Losing an electron does not mean gaining a proton, and becoming an ion does not turn the atom into a different element.

Alpha scattering evidence

2:06The alpha scattering experiment provides evidence for this nuclear model. This explanation is Supplement content for Extended students. Alpha particles are positively charged, each containing two protons and two neutrons. In the experiment, a narrow beam was directed towards a very thin sheet of metal, such as gold. A detecting screen showed where the alpha particles went after encountering the foil. Most passed straight through with little or no deflection.

2:29Some changed direction, and a very small number were deflected through large angles, including backwards. The important reasoning connects each observation to a feature of the atom. Most alpha particles passed straight through because most of each atom is empty space. Only a small fraction approached a nucleus closely enough to experience a large deflection. This shows that the nucleus occupies a very small fraction of the atom’s volume. A large nucleus would obstruct or strongly deflect far more particles.

2:57Large deflections also show that the nucleus contains a concentrated mass. Alpha particles are far more massive than electrons, so electrons are too light to turn them back. A tiny, massive centre can produce these large changes in direction. The evidence therefore supports a nucleus containing most of the atom’s mass. The nucleus is positively charged, so it repels the positive alpha particles. An alpha particle passing near a nucleus follows a curved path because of this repulsion.

3:24A very close approach can produce a large deflection. The particle does not need to touch the nucleus like a ball striking a wall.

Proton number and nuclides

3:30Proton number, also called atomic number, is the number of protons in the nucleus of an atom. It identifies the element, because no two elements share a proton number. Every sodium nucleus contains eleven protons. Gaining or losing electrons alters the charge, but a different number of protons means a different element. Nucleon number, also called mass number, is the total number of protons and neutrons in the nucleus of an atom.

3:55A nucleon is either a proton or a neutron. Therefore, the number of neutrons equals nucleon number minus proton number. We subtract the protons because the nucleon number already includes them. Nuclide notation puts the element symbol beside two numbers. The nucleon number is written at the upper left, with the proton number below it. Our example represents sodium with nucleon number twenty-three and proton number eleven. These numbers describe the nucleus, so they do not directly tell us whether electrons have been lost.

4:24Let us calculate the particles in a sodium ion with nucleon number twenty-three and charge plus one. Its proton number is eleven, so it contains eleven protons. Its neutron number is 23 − 11, which equals twelve. A neutral sodium atom would have eleven electrons, but this ion has lost one, leaving ten. We can check that answer using the charges. Eleven protons contribute relative charge plus eleven, while ten electrons contribute relative charge minus ten.

4:50Adding those contributions gives an overall relative charge of plus one. The twelve neutrons do not affect this result because each has zero electrical charge. For a second example, consider a chloride ion with nucleon number thirty-five, proton number seventeen and charge minus one. The proton number gives seventeen protons. 35 − 17 gives eighteen neutrons. The negative charge means one extra electron has been gained, so 17 + 1 gives eighteen electrons.

Isotopes and neutron numbers

5:18The relationship between nuclear charge and proton number is Supplement content. A nucleus has relative charge equal to its proton number, with a positive sign, because only its protons contribute charge. Its relative mass is approximately equal to its nucleon number. That follows because each proton and neutron contributes approximately one unit of relative mass. Isotopes are atoms of the same element, with equal proton numbers but different neutron numbers.

5:44An element may have more than one isotope. Carbon twelve and carbon fourteen both have six protons, so they are the same element. Their nucleon numbers differ, showing that their nuclei contain unequal numbers of neutrons. Carbon twelve has 12 − 6, giving six neutrons. Carbon fourteen has 14 − 6, giving eight neutrons. Isotopes therefore differ in nuclear composition, whereas an ion has gained or lost electrons. An atom can belong to a particular isotope and also become an ion, because these descriptions concern different particles.

Fission, fusion and recap

6:17Fission and fusion are also Supplement content. Fission is the splitting of a large nucleus into smaller nuclei. In one example, a uranium nucleus absorbs a neutron and then splits into two smaller nuclei, releasing additional neutrons. Energy is released, mostly carried away by the products as they move apart rapidly. The equation shows uranium two hundred and thirty-five plus a neutron producing barium one hundred and forty-one, krypton ninety-two and three neutrons.

6:44This is one possible fission outcome, not the only one. A neutron has nucleon number one and proton number zero. Those values allow us to check the equation systematically. First, check the total nucleon number on each side. Before fission, 235 + 1 = 236. Afterwards, 141 + 92 + 3 also equals two hundred and thirty-six. For proton number, ninety-two before equals 56 + 36 afterwards, with the neutrons contributing zero.

7:13Fusion is the joining of small nuclei to form a larger nucleus. In this example, two isotopes of hydrogen, with nucleon numbers two and three, join to form helium four and a neutron. The nucleon numbers balance because 2 + 3 = 4 + 1. The proton numbers balance because 1 + 1 = 2 + 0. Both this fusion reaction and the earlier fission example release energy. The products have a slightly smaller total mass than the particles that reacted.

7:40That decrease in mass corresponds to the energy released. Balanced nucleon numbers do not mean the masses are exactly equal, because that number counts particles rather than measuring mass. Several mistakes become easier to avoid once these distinctions are clear. Electrons orbit outside the nucleus; they are not included in nucleon number. A positive ion has lost electrons rather than gained protons. Isotopes have different neutron numbers, while each element is identified by its own proton number.

8:08An exam question may ask you to describe how alpha scattering supports the nuclear model. Use an observation followed by the conclusion it supports. Most alpha particles pass straight through, showing that the atom is mostly empty space. Very few undergo large deflections, showing that the nucleus occupies a tiny volume. Large deflections show that most of the atom’s mass is concentrated in the nucleus. Positive alpha particles are repelled by the nucleus, showing that it is positively charged.

8:35These are separate, creditworthy points. Simply describing the paths without explaining their significance would leave the connection to atomic structure incomplete. For nuclide questions, read the proton number before calculating anything else. Subtract it from the nucleon number to find the neutron number. If an ionic charge is given, adjust the electron number while leaving the nucleus unchanged. For nuclear equations, confirm that nucleon numbers and proton numbers each balance.

9:01The nuclear model connects several ideas through one small centre. The nucleus contains protons and neutrons, carries positive charge and holds almost all the atom’s mass. Electrons orbit around it and determine whether the atom is neutral or ionised. Alpha scattering provides evidence for this structure, rather than merely giving us a picture to memorise. Proton number identifies the element, while nucleon number counts protons and neutrons together. Isotopes share a proton number but have different neutron numbers.

9:30Fission splits a large nucleus, while fusion joins small nuclei. In the examples studied here, a small decrease in total mass corresponds to released energy.