Campbell and Reece 8 Edition
The Chemical Context of Life
I. Atoms and Molecules
Matter - anything that takes up space and has mass. Matter is composed of elements.
Element - a substance that can not be broken down to other substances by chemical reactions. In
other words, an element is composed of a single type of Atom (i.e., the smallest unit of matter
having the physical and chemical properties of an element).
• 70 - 95% of all cells is water (2 O)
• 96% of living matter is made up of C, H, O, and N.
• other major elements include P, S, K, Ca, Mg, Na, Cl
Trace elements - required in minute quantities (< 0.01%) but are nonetheless essential for life
e.g., Fe, Zn, I, Mn, V…
Compounds - Elements are organized into compounds ▯ two or more elements combined in a
1. Atomic structure
• Atoms are mostly space - they have a dense nucleus and a large amount of empty space
occupied by scattered electrons
• Atoms are composed of three stable subatomic particles.
Charge Mass (g)
protons (p) +1 1.7 x 10-2▯ ~1 Da (dalton)*
neutrons (n) 0 1.7 x 10 ▯ ~1 Da
electrons (e ) -1 0.0005 x mass of a proton or neutron
Note: the mass of e is ignored in computing total mass of an atom
* Dalton is equivalent to an atomic mass unit (amu)
(mass number = sum of protons and neutrons)
atomic mass 16
O Element symbol
atomic number 8
(number of protons)
(If the atom has a neutral charge then atomic number = the number of electrons) 2
• All atoms of an element have the same number of protons, however, the mass of some atoms
of an element can vary (i.e., some atomic forms have more neutrons than others). These
different forms are called isotopes
e.g., C - atomic mass of 12, 13, 14
P - atomic mass of 31, 32, 33
• Isotopes are considered stable if their nuclei do not have a tendency to lose particles.
• Radioactive isotopes are unstable and their nuclei decay or give off particles and energy.
Radioactive isotopes are used in medicine, research, energy, bombs
II. Electrons - Shells, Energy Levels and Chemical Properties
• The distance of an electron from the nucleus is determined by its energy level
Energy = capacity to cause change - ability to do work
Recall that electrons are negatively charged and are attracted to the nucleus (i.e.,
positively charged protons).
• Therefore, the farther an electron is away from the nucleus, the more potential energy it has
(potential energy = stored energy due to position or location).
• An electron’s average distance from the nucleus is represented symbolically by electron
shells (Recall: an electron’s energy level is correlated with its average distance from the
• Electrons are arranged in orbitals within a shell -An orbital is the 3-D space where an
electron is likely to be found 90% of the time. There are a number of orbitals in each shell.
Each orbital may hold a maximum of two electrons.
Shell Number of electrons
• An electron can change energy level (and shell) by absorbing or losing the energy equivalent
to the difference in potential energy (stored energy due to position or location). Processes
such as photosynthesis and cellular respiration use this movement of electrons to store or
2 Campbell and Reece 8 Edition
Why are we talking about these concepts?
Electron configuration is responsible for the chemical behavior of an atom. In particular, the
number of electrons in the outer most shell is the most important factor in determining the
chemical properties of an atom. Electrons found within the outer shell, or valence shell, are
called valence electrons
• Aperiodic table arranges elements in tiers or periods (i.e., rows of the periodic table)
corresponding to the number of electron shells in their atoms. (Figure 2.9 shows an
abbreviated periodic table)
What changes as you move from left to right in a row of the periodic table?
• 92 elements in nature (upwards of 105) in the periodic table.
• 25 of the natural elements are required for life
• Elements wi