Chapter 2: The Chemical Context of Life – Study Notes

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Chapter 2: The Chemical Context of Life

Concept 2.1: Matter Consists of Chemical Elements in Pure Form and in Combinations Called Compounds

All living organisms are composed of matter, which is anything that occupies space and has mass. Matter is made up of elements, and elements can combine to form compounds with unique properties.

Matter: Anything that takes up space and has mass.
Element: A substance that cannot be broken down to other substances by chemical reactions.
Compound: A substance consisting of two or more elements in a fixed ratio. Compounds have characteristics different from those of their constituent elements.

Example: Sodium (Na) and chlorine (Cl) are elements; when combined, they form sodium chloride (NaCl), or table salt, which has properties distinct from either element.

Table: Elements in the Human Body

Element	Symbol	Percentage of Body Mass (including water)
Oxygen	O	65.0%
Carbon	C	18.5%
Hydrogen	H	9.5%
Nitrogen	N	3.3%
Calcium	Ca	1.5%
Phosphorus	P	1.0%
Potassium	K	0.4%
Sulfur	S	0.3%
Sodium	Na	0.2%
Chlorine	Cl	0.2%
Magnesium	Mg	0.1%

Trace elements are required in minute amounts (less than 0.01% of mass).

Concept 2.2: An Element's Properties Depend on the Structure of Its Atoms

Each element consists of unique atoms, which are the smallest units of matter that retain the properties of the element.

Atom: The smallest unit of matter that retains the properties of an element.
Subatomic particles: Atoms are composed of neutrons (no charge), protons (positive charge), and electrons (negative charge).
Neutrons and protons form the atomic nucleus; electrons form a "cloud" around the nucleus.
Neutron and proton mass are nearly identical and measured in daltons.

Atomic Number and Atomic Mass

Atomic number (Z): Number of protons in the nucleus.
Mass number (A): Sum of protons and neutrons in the nucleus.
Atomic mass: Total mass of the atom, approximately equal to the mass number.

Example: Nitrogen has 7 protons (atomic number 7) and a mass number of 14 (7 protons + 7 neutrons).

Isotopes

Atoms of the same element with different numbers of neutrons are called isotopes.
Radioactive isotopes decay spontaneously, emitting particles and energy.

Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon, differing in neutron number.

Electron Distribution and Chemical Properties

The chemical behavior of an atom is determined by the distribution of electrons in electron shells.
The periodic table shows the electron distribution for each element.
Valence electrons are those in the outermost shell (valence shell) and determine chemical reactivity.
Elements with a full valence shell are chemically inert.

Table: Subatomic Particle Counts in Selected Elements

Element	P (Protons)	N (Neutrons)	E (Electrons)	A.# (Atomic Number)	A.M. (Atomic Mass)
Hydrogen	1	0	1	1	1
Helium	2	2	2	2	4
Carbon	6	6	6	6	12
Carbon-14	6	8	6	6	14
Nitrogen	7	7	7	7	14
Calcium 2+	20	20	18	20	40

Concept 2.3: The Formation and Function of Molecules Depend on Chemical Bonding Between Atoms

Atoms with incomplete valence shells can share or transfer valence electrons, resulting in chemical bonds that hold atoms together in molecules and compounds.

Chemical bond: An attraction between two atoms resulting from sharing or transferring valence electrons.

Covalent Bonds

Covalent bond: The sharing of a pair of valence electrons by two atoms.
Shared electrons count as part of each atom’s valence shell.
Molecule: Two or more atoms held together by covalent bonds.
Single bond: Sharing of one pair of valence electrons.
Double bond: Sharing of two pairs of valence electrons.
Structural formula: Notation representing atoms and bonds (e.g., H—H).
Molecular formula: Abbreviated notation (e.g., H2).

Example: Water (H2O) is a molecule formed by covalent bonds between hydrogen and oxygen.

Electronegativity and Bond Polarity

Electronegativity: An atom’s attraction for electrons in a covalent bond.
In a nonpolar covalent bond, electrons are shared equally.
In a polar covalent bond, one atom is more electronegative, causing unequal sharing and partial charges.

Example: In H2O, oxygen is more electronegative than hydrogen, resulting in a polar molecule with partial charges (δ- and δ+).

Ionic Bonds

Atoms may transfer electrons, resulting in charged atoms called ions.
Cation: Positively charged ion.
Anion: Negatively charged ion.
Ionic bond: Attraction between an anion and a cation.
Ionic compounds (salts): Compounds formed by ionic bonds, often found as crystals (e.g., NaCl).

Hydrogen Bonds

Hydrogen bond: Forms when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom.

Example: Hydrogen bonds occur between water molecules and between water and ammonia molecules.

Additional info: These notes cover the foundational chemical principles necessary for understanding biological molecules and processes, as outlined in Chapter 2 of a typical General Biology textbook.