BIOL 1201 Chapter : Chapter 2
Chapter 2
The Chemical Context of Life
Matter Consists of chemical elements in pure form and in combinations called compounds
• Organisms are composed of matter, which is defined as anything that takes up space and
has mass. Matter is also made up of element.
• Element is a substance that cannot be broken down to other substances by chemical
reactions.
• A compound is a substance containing of two or more elements combined in a fixed ratio.
• Oxygen, Carbon, Hydrogen, and Nitrogen make up 96% of living matter. Ca, P, K, S, and
a few other elements account for most of the remaining 4% of an organism’s mass.
• Trace elements are required by an organism in only minute quantities. B, Cr, Co, Cu, F, I,
Fe, Mn, Mo, Se, Si, Sn, V, and Zn
• Iodine needed for thyroid gland in vertebrates.
An element’s properties depend on the structure of its atoms
• An atom is the smallest unit of matter that still retains the properties of an element.
• Atomic number – the number of protons
• All atoms of an element have the same number of protons but may differ in number of
neutrons
• Isotopes are two atoms of an element that differ in number of neutrons
• 12C 98.9%, 13C 1.1%, 14C <0.1%
• Radioactive isotopes decay spontaneously, giving off particles and energy
• Radioactive isotopes are also useful as tracers to follow atoms through metabolism.
• Some applications of radioactive isotopes in biological research are
o Dating fossils
o Tracing atoms through metabolic processes
o Diagnosing medical disorders
• Changing the number of protons in an element changes it into a different element
• Energy is defined as the capacity to cause change- for instance, by doing work.
• Potential Energy is the energy that matter possesses because of its location or structure.
• An electron cannot exist between energy levels but its potential energy is determined by
its energy level.
• Electrons are found in electron shells, each with a characteristic average distance and
energy level.
• Valence electrons are those in the outermost shell, or valence shell
• The chemical behavior of an atom is mostly determined by the valence electrons
• Elements with a full valence shell are chemically inert
• The three dimensional space where an electron is found 90% of the time is called an
orbital.
The formation and function of molecules depend on chemical bonding between atoms
• A covalent bond is that sharing of a pair of valence electrons by two atoms.
• Two or more atoms held together by covalent bonds constitute a molecule.
• Atoms in a molecule attract electrons to varying degrees
• Electronegativity is an atom’s attraction for the electrons in a covalent bond
• A more electronegative atom more strongly pulls shared electrons toward itself
