Objectives
Describe the structure of an atom.
Explain how isotopes can be used to study biological processes.
Explain the role of an element's electrons in determining its chemical reactivity.
Key Terms
atom nucleus
proton atomic number
electron isotope
neutron radioactive isotope
Different elements have different properties. Some are solid metals at room temperature. Some are invisible gases. Some elements readily react with other elements, while others hardly react at all. These properties affect the roles that different elements play in biological processes. This section describes how an element's properties are related to its structure.
Atoms
Each element consists of a single kind of atom that is different from the atoms of all other elements. An atom, which gets its name from the Greek word atomos meaning "indivisible," is the smallest possible particle of an element. In other words, a carbon atom is the smallest possible "piece" of the element carbon. And that's a very small piece—it would take more than three million carbon atoms to stretch across the period printed at the end of this sentence.
Atoms of all elements are made up of even smaller components called subatomic particles. A proton is a subatomic particle with a single unit of positive electrical charge (+). An electron is a subatomic particle with a single unit of negative electrical charge (-). A third type of subatomic particle, the neutron, is electrically neutral, meaning it has no electrical charge. An element's physical and chemical properties depend on the number and arrangement of its subatomic particles. For example, the shiny luster of copper metal and the boxy crystals of sulfur are based on the structure and interactions of the atoms that make up those elements.
An atom's protons and neutrons are tightly packed together, forming a central core called the nucleus. Electrons, which have much less mass than neutrons and protons, continually move about the outside of the nucleus at great speed. The attraction between the negatively charged electrons and the positively charged protons keeps the electrons close to the nucleus.
charge as spread out, like a cloud, in all the places the electron might be. In a real atom,
the electron cloud is much larger than the nucleus. To give you an idea of the difference,
consider that if the electron cloud of an atom were big enough to fill a baseball stadium,
the nucleus would be only the size of a housefly on the field!
Figure 4-4
This model of a helium atom indicates the number of each kind of subatomic particle it contains. Though no visual model can accurately show an atom's structure, models can help you in understanding certain aspects of an element's chemical behavior.
Isotopes
Some elements have alternate forms called isotopes. Isotopes of an element have the same number of protons in their atoms but different numbers of neutrons. Figure 4-5 shows the numbers of subatomic particles in atoms of the three isotopes of carbon. Carbon-12 (usually written 12C), which has atoms containing 6 neutrons, makes up about 99 percent of all naturally occurring carbon. Most of the other 1 percent is carbon-13 (13C), which has atoms with 7
neutrons. A third isotope, carbon-14 (14C), has atoms with 8 neutrons and is very rare. Notice that atoms of all three carbon isotopes still have 6 protons—otherwise, they would not be carbon. Both 12C and 13C are stable isotopes, meaning their nuclei do not change with time. The
isotope 14C, on the other hand, is unstable, or radioactive. A radioactive isotope
is one in which
the nucleus decays (breaks down) over time, giving off radiation in the form of matter
and energy.
Figure 4-5
Atoms of three isotopes of carbon differ only in their numbers of neutrons. The isotopes are named for the total number of particles in their nuclei (protons plus neutrons). Carbon-13, for example, has 6 protons and 7 neutrons, for a total of 13.
Electrons and Reactivity
How does an atom's structure determine how it reacts with other atoms? The key is the atom's electrons. Electrons differ in the amount of energy they have and how tightly they are held by the protons in the nucleus. Based on these properties, chemists describe an atom's electrons as belonging to certain energy levels. Usually it is the electrons in the highest energy level of an atom that determine how that atom reacts.
The first, or lowest, energy level (nearest the nucleus) can hold 2 electrons, while the
second energy level can hold 8 electrons. For example, a hydrogen atom has 1 electron.
Since electrons fill the lowest energy level first, hydrogen's electron occupies its first
energy level (Figure 4-7).
Figure 4-7
An atom's lowest (first) energy level can hold up to 2 electrons. The second level can hold up to 8. Notice that the second energy levels of carbon, nitrogen, and oxygen atoms are unfilled with 4, 5, and 6 electrons, respectively. (Remember that atomic models are limited in what they can represent. Energy levels are not actual physical locations.)
Helium (modeled in Figure 4-4) has 2 electrons, filling its lowest energy level. Carbon has 2 electrons in its lowest energy level, and 4 more electrons in its second level. Note that both hydrogen and carbon have a partly-filled energy level, as do nitrogen and oxygen. That condition makes these atoms chemically reactive—they tend to react with other atoms, filling their highest occupied energy levels. In contrast, a helium atom, which has no partly-filled energy levels, is inert—it does not tend to react. In the next section, you'll read more about the ways electrons are involved in the reactions among atoms.
Concept Check 4.2
1. Describe three kinds of subatomic particles and tell how they are arranged in an atom.
3. Describe the significance of the number of electrons in an atom's highest energy level.
4. Explain the significance of an element's atomic number.