The nucleus of hydrogen, the simplest atom, consists of a single particle—a proton. Of the two basic nuclear particles, the proton is the one with the positive charge. Its charge attracts electrons and holds them in their swarming orbits. But the nuclei of atoms may also contain additional particles that are electrically neutral—neutrons. This combination of protons and neutrons determines an atom’s atomic weight, or mass number.
Protons and neutrons can be divided into even smaller parts. But chemistry is not concerned with this—that is the realm of atomic or particle physics. Chemistry focuses on what happens between atoms during chemical changes, while nuclear physics focuses on activity within the atom itself.
Of the two nuclear particles, the proton has a special importance—it serves to distinguish atoms of one element from atoms of another element How? By the number of protons in the nucleus. The number of protons in the atomic nucleus is called the atomic number. Hydrogen, the only element that has just one proton in its nucleus, has an atomic number of 1. Helium, with two protons, has an atomic number of 2, and so on for all the other elements. The atomic number also serves to identify each element in the periodic table, the chart that lists all the known elements, arranged in order of increasing atomic number.
The energy that binds together the nucleus and its swarm of electrons is electrical. A proton is electrically positive. An electron, in motion around the nucleus, is electrically negative. The attraction between these opposite charges keeps the electron of a hydrogen atom spinning around the proton of the hydrogen nucleus. In its electrically neutral state, with an oxidation number of zero, an atom has as many electrons as protons. In its charged state as an ion, oxidized or reduced, it has either more electrons than protons or fewer.
Unlike the proton, neutrons—the particles of the atomic nucleus that have no charge—may vary in number among atoms of the same element. This variation gives rise to what are called isotopes.
The number of protons in the atoms of an element is fixed and uniquely identifies the element, but the number of neutrons can vary. Thus, although the atomic number of an element is constant, the mass number (atomic weight—the sum of protons and neutrons) varies. The mass number identifies the isotopes of an element.
Different isotopes of an element generally behave in the same way during chemical reactions, which is not surprising since the number of valence electrons is the same in all the isotopes. However, the physical properties of the isotopes of an element may vary slightly. The hydrogen atom, for example, normally has just one proton, but a few such atoms may each contain a neutron as well. These rare atoms belong to the hydrogen isotope known as deuterium, or “heavy hydrogen,” the basis of the “heavy water” used in the hydrogen bomb. Deuterium has the same atomic number as hydrogen, 1, but its mass number is 2, whereas hydrogen’s mass number is 1. Tritium, an even heavier hydrogen isotope, contains two neutrons, but does not occur in nature. It is produced in atomic reactors by bombarding the element lithium with neutrons.
Isotopes that are radioactive are calle dioisotopes. Radioactive elements spontaneously lose nuclear particles as they decay to lighter elements. A number of these are used in medicine to detect disease or abnormalities, and some are used in the treatment of cancer. The radioisotope of iodine,1311, for example, is used to detect malfunction of the thyroid gland, as well as to treat thyroid cancer. In basic research on the biochemical process called photosynthesis, scientists used the isotope ,4C as a tracer to determine the pathways taken by carbon molecules during metabolism in plants.
Many familiar elements, such as lead, are made up of mixtures of different isotopes. Eight isotopes of lead exist in nature. Three of them—Z06Pd, 207Pb, and 208Pb—are stable end products of the radioactive decay of uranium and thorium, and four of the other five are radioactive, thus unstable.
When two hydrogen atoms get close enough (A), their atomic orbitals overlap and form a molecular orbital. The electrons, one from each hydrogen atom, are shared equally in the resulting covalent bond in a hydrogen molecule. When hydrogen forms a similar bond with an atom of fluorine (B), an irregular
pear-shaped molecular orbital results. This is because the fluorine atom has a larger share of the bonding electrons.
An electron cloud can be visualized as arranged in shells, like an onion. This concept, the classic model based on the work of Danish physicist Niels Bohr (1885-1962), is useful for describing the aspects of the atom that determine an element’s place in the periodic table.
In the table, all the elements listed in vertical rows have the same number of electrons in their atoms’ outer shell.
A shell is a group of electrons at the same average distance from the nucleus of an atom. At a certain number of electrons—specified by the quantum laws described in the next section—a shell of electrons is complete and can hold no more. The atom is then stable. The atom of the next heavier element in the periodic table has one more electron, and it starts a new shell. The single electron in this outer shell is relatively easy to remove.
Sodium (Na) and potassium (K) are two such elements with just a single electron in their outer shells. Both are metals that react violently with water. Both easily lose that single electron and form ionic bonds with other elements, creating ionic compounds—chlorides such as sodium chloride and potassium chloride. Despite the similarities, there is a basic difference between sodium and potassium: the heavier element, potassium, nas one more complete electron shell between its outer valence electron and its nucleus, and its nucleus has eight more protons than sodium. One result of these differences is that of the two, potassium is the more chemically reactive because its outer electron is held less tightly by the nucleus.
Elements that have one more proton and electron than sodium or potassium, namely magnesium (Mg) and calcium (Ca), form ionic compounds by donating two electrons. Atoms of both have just two valence electrons in their outer shells. In the periodic table, elements such as these, which are grouped vertically in columns, have similar characteristics based on the fact that they have the same number of electrons in their outer shells.
The motion of an electron moving in orbits around the atomic nucleus is wavelike in some respects. Just as a wave of water rises and falls as it moves across the surface of a pond, an electron moves in a wavelike path called an orbital around the nucleus. The path may be circular or take any of a variety of shapes, which complicates the picture of concentric shells that can be used to describe the arrangement of electrons within an atom. Rather than thinking of it strictly as a path, then, an orbital is defined as a volume of space in which an electron moves.
Despite this uncertainty, different orbitals can be distinguished according to the average distance from the nucleus of the electrons occupying them, just as electron shells are distinguished. An electron at its average distance is in its “ground state”—it neither releases nor absorbs energy. When it releases or absorbs energy, it jumps to another orbital, at a new average distance from the nucleus. This, however, is more properly the concern of a field of physics called quantum mechanics.
When hydrogen atoms join to form Hz molecules and share their total of two electrons between them, the two electrons then move along a single pathway called a molecular orbital. The molecular orbital surrounds the two hydrogen nuclei. By providing each nucleus with a share in both electrons, the molecular orbital helps hold the nuclei together, creating a stable combination.
The electrons of each element are characterized by four different quantum numbers, the first two of which have a major impact on chemical reactions. The first number describes the average distance of an electron from the nucleus and thus indicates which shell the electron occupies. (All the electrons at a given distance from the nucleus form a shell.) The second number, denoted by a letter (s, p, d, or f), defines the shape of the orbital— the path the electron follows in its motion around the nucleus.
The distribution of electrons around an atom can be described by a kind of shorthand, using the two quantum numbers, with the number of electrons in a particular orbital shown as a superscript. Quantum rules stipulate that when the first number equals 1, only an s orbital is allowed. When it equals 2, both s and p orbitals are allowed. When it equals 3, s, p, and d orbitals are allowed. When it is 4, s, p, d, and f orbitals are possible. The rules also stipulate the maximum number of electrons that can occupy an orbital, such as 2 for an s orbital, 6 for p, 10 for d, and 14 for f.
Argon (Ar), for example, has 18 electrons: 21 s 22s 62p 23s 63p, which means that the first shell has 2 electrons in an s orbital, the second shell has 2 electrons in an s orbital and 6 electrons in a p orbital, and the third shell also has 2 electrons in an s orbital and 6 in a p orbital.