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Nucleus region in space where an electron is present most of the time.
(6 protons + 6 neutrons) The area in space where electrons circle around the nucleus is
known as an energy level or electron shell. The innermost

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e - energy level may contain one or two electrons. As you can see
e -
in FIGURE 1.3 , hydrogen has one electron in the first energy

level, which is also its only energy level. Many elements that
e - e - Electron are important in biology, such as carbon, nitrogen, and oxygen,
contain two energy levels. In these atoms, the second energy
+ + Proton level may hold up to eight electrons. Figure 1.3 shows how
+
+ carbon’s six electrons are placed in the energy levels: two are in
+ + + Neutron the first energy level and four are in the second energy level.
The amount of energy in a level depends on its location.
e - Electrons closer to the nucleus have less energy and are less
e - reactive than those further from the nucleus. Atoms are most
stable when their energy levels are full. When electrons fill the
energy levels closest to the nucleus, rather than ones further
e - away, the element becomes more stable. If an electron gains
energy and moves or “jumps” to a level further away from the
nucleus, the atom is less stable. As we will see later, molecules
Carbon atom
are often formed when atoms share electrons to fill their out-
FIGURE 1.2 A carbon atom ermost energy levels.

Most carbon atoms have six protons, which are positively charged; Since chemicals often react to complete their outermost
six neutrons, which are neutral; and six electrons, which are energy levels, simple diagrams like those in Figure 1.3 give
negatively charged. a sense of how many electrons an element must gain or lose
to have a full outermost energy level. In the case of carbon, it
Typically, an atom has equal numbers of protons and must gain four electrons in the outer shell for a total of eight
electrons. As you can see in Figure 1.2 carbon possesses electrons. Hydrogen may either gain one electron or lose an
,
six positively charged protons and six negatively charged electron to have its outermost shell complete. If an electron is
+

electrons. The charges add up to zero, so this carbon atom is lost, a hydrogen ion (H ) forms because it now has one more

electrically neutral. proton than electrons. We will study hydrogen in greater
Certain chemical processes cause an atom to either detail when we discuss cellular energetics in Unit 3.
gain or lose electrons. An atom that has lost an electron is
positively charged, and one that has gained an electron is neg- Chemical Properties of Elements
atively charged. Electrically charged atoms are called ions. The electrons in an element’s outermost energy level are

The charge of an ion is specified as a superscript to the right known as the valence electrons . The periodic table of

+

of the chemical symbol. For example, H indicates a hydrogen
ion that has lost an electron and is positively charged. Posi-
tively charged ions are called cations and negatively charged
atoms are anions. In this simpli ed diagram, the
electron energy levels are depicted
Electrons as circles and the electrons that
occupy them as dots. The cloud in
The movement of electrons from one molecule to another Hydrogen the center is the nucleus.
is the foundation of energy transfer in many biologi- Carbon
cal reactions. Therefore, understanding electron transfer is FIGURE 1.3 Energy levels for hydrogen and carbon

essential to the study of cellular biochemistry. The hydrogen atom contains one energy level with a single electron,
Electrons move around the nucleus as a cloud of points while the carbon atom has two energy levels with two electrons in the
that is denser where the electron is most likely to be. The first energy level and four electrons in the second. The energy levels
exact path of an electron varies, but it is possible to identify a are depicted as circles and the electrons as orange dots.

MODULE 1 ELEMENTS OF LIFE 31

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