READ2.1 The Nature of AtomsAny substance in the universe that has mass and occupies space is defined as matter. All matter is composed of extremely small particles called atoms.Atomic structure includes a central nucleus and orbiting electronsObjects as small as atoms can be "seen" only indirectly, by using complex technology such as tunneling microscopy. We not know a great deal about the complexities of atomic structure, but the simple view put forth in 1913 by the Danish physicist Niels Bohr provides a good starting point for understanding atomic theory. Bohr proposed that every atom possesses an orbiting cloud of tiny subatomic particles called electrons whizzing around a core, like the planets of a miniature solar system. At the center of each atom isa small, very dense nucleus formed of two other kinds of subatomic particles: protons and neutrons. Atomic number and the elementsWithin the nucleus, the cluster of protons and neutrons is held together by a force that works only over short, subatomic distances. Each proton carries a positive (+) charge, and each neutron has no charge. Each electron carries a negative (-) charge. Typically, an atom has one electron for each proton and it, thus, electrically neutral. Different atoms are defined by the number of protons, a quantity called the atomic number. The chemical behavior of an atoms is due to the number and configuration of electrons, as we will see later in this chapter. Atoms with the same atomic number (that is, the same number of protons) have the same chemical properties and are said to belong to the same element. Formally speaking, an element is any substance thatcannot be broken down to any other substance by ordinary chemical means.Atomic massThe terms mass and weight are often used interchangeably, but they have slightly different meanings. Mass refers to the amount of substance, but weight refers to the force gravity exerts on a substance. An object has the same mass whether it is on the Earth or the Moon, but its weight will be greater on the earth because the Earth's gravitational force is greater on the Earth because the Earth's gravitational force is greater that the Moon's. The atomic mass ofan atom is equal to the sum of the masses of its protons and neutrons. Atoms that occur naturally on Earth contain from 1 to 92 protons and up to 146 neutrons.The mass of atoms and subatomic particles is measured in units called daltons. To give you an idea of just how small these units are, not that it takes 602 million million billion (6.02 x 10²³) daltons to make 1 gram (g). A proton weighs approximately 1 dalton (actually 1.007 daltons), as does a neutron (1.009 daltons). In contrals, electrons weight only 1/1840 of a dalton, so they contribute almost nothing to the overall mass of an atom.ElectronsThe positive charges in the nucleus of an atom are neutralized, or counterbalanced, by negatively charged electrons, which are located in regions called orbitals that lie at varying distances around the nucleus. Atoms with the same number of protons and electrons are electrically neutral; that is, they have no net charge, and are therefore called neutral atoms. Electrons are maintained in their orbitals by their attraction to the positively charged nucleus. Sometimes other forces overcome this attractions, and an atom loses one or more electrons. In other cases, atoms gain additional electrons. Atoms in which the number of electrons does notequal the number of protons are known as ions, and they are charged particles. An atom having more protons that electrons has a net positive charge and is called a cation. For example, an atom of sodium (Na) that has lost on electron becomes a sodium ion (Na+), with a charge of +1. An atom having fewer protons that electrons carries a net negative charge and is called an anion. A chlorine atom (Cl) that has gained one electron becomes a chloride ion (Cl-), with a charge of -1. IsotopesAlthough all atoms of an element have the same number of protons, they may not all have the same number of neutrons. Atoms of a single element that possess different numbers of neutrons are called isotopes of that element.Most elements in nature exist as mixtures of different isotopes. Carbon (C), for example, has three isotopes, all containing six protons. Over 99% of carbon found in nature exists as an isotopes that also contains six neutrons.Because the total mass of this isotope is 12 daltons (6 from protons plus 6 from neutrons), it is referred to as carbon-12. Most of the rest of the naturally occuring carbon is carbon-13, and isotope with seven neutrons. The rarest carbon isotope is carbon-14, with eight neutrons. Unlike the other two isotopes, carbon-14 is unstable: This means that its nucleus tends to break up into elements with lower atomic numbers. This nuclear breakup, which emits a significant amount of energy, is called radioactive decay, and isotopes that decay in this fashion are radioactive isotopes. Some radioactive isotopes are more unstable that others, and therefore they decay more readily. For any given isotope, however, the rate of decayis constant. The decay time is unusually expressed as the half-life, the time it takes for one-half of the atoms in a sample to decay. Carbon-14, for example, often used in the carbon dating of fossils and other materials, has a half-life of 5730 years. A sample of carbon containing 1 g of carbon-14 today would contain 0.5 g of carbon-14 after 5730 years, 0.25 g 11,460 years from now, 0.125 g 17,190 years from now, and so on. By determining the ratios of thedifferent isotopes of carbon and other elements in biological samples and in rocks, scientists are able to accurately determine when these materials formed.Radioactivity has many useful applications in modern biology. Radioactive isotopes are one way to label, or "tag", a specific molecule and then follow its progress, either in a chemical reaction or in living cells and tissue. The downside, however, is that the energetic subatomic particles emitted by radioactive substances have the potential to severelydamage living cells, producing genetic mutations an, at high doses, cell death. Consequently, exposure to radiation is carefully controlled and regulated. Scientists who work with radioactivity follow strict handling protocols and wear radiation-sensitive badges to monitor their exposure over time to help ensure a safe level of