Atoms, Elements, and Chemical Bonds: Foundations for Biology

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Introduction to Matter, Elements, and Compounds

Defining Matter

Matter is anything that takes up space and has mass. It exists in a vast assortment of forms, including solids, liquids, and gases, and is the physical substance that makes up all living and nonliving things.

Matter includes rocks, metals, organisms, and all substances in the universe.

Elements

An element is a pure substance that cannot be broken down into other substances by chemical means. Each element is defined by a unique type of atom and is represented by a name and a symbol (e.g., Carbon - C, Iron - Fe).

Elements are naturally occurring and are the building blocks of matter.
There are 92 naturally occurring elements, each with distinct properties.

Compounds

A compound is a substance formed when two or more elements combine in a fixed ratio. Compounds have characteristics different from those of their constituent elements.

Example: Table salt (NaCl) is formed from sodium (a metal) and chlorine (a poisonous gas), but together they form an edible compound.
Example: Water (H2O) is formed from hydrogen and oxygen in a 2:1 ratio.

Elements Essential for Life

Major Elements in Biology

Only a small subset of elements are essential for life. The most important elements are:

Sulfur (S)
Phosphorus (P)
Oxygen (O)
Nitrogen (N)
Carbon (C)
Hydrogen (H)

These are often remembered by the acronym SPONCH and are called the elements of life.

Trace Elements

Trace elements are required by organisms in only minute quantities but are essential for proper biological function.

Example: Iodine (I) is necessary for the thyroid gland; deficiency can lead to health problems. The recommended daily intake is about 0.15 mg/day.

Both deficiency and excess of trace elements can be harmful, as shown in the following table:

Element	Biological Function	Effect of Deficiency	Effect of Excess
Iodine (I)	Thyroid hormone production	Goiter, developmental issues	Toxicity symptoms
Zinc (Zn)	Enzyme function	Impaired immune function	Nausea, vomiting
Iron (Fe)	Oxygen transport (hemoglobin)	Anemia	Organ damage
Additional info: ...	Other trace elements include copper, manganese, selenium, etc.

Atomic Structure and Its Biological Importance

Atoms: The Building Blocks of Matter

An atom is the smallest unit of matter that retains the properties of an element. Atoms are composed of subatomic particles:

Protons: Positively charged particles found in the nucleus.
Neutrons: Electrically neutral particles found in the nucleus.
Electrons: Negatively charged particles that orbit the nucleus in electron shells.

Atomic Number and Mass Number

Atomic Number: The number of protons in an atom, which defines the element.
Mass Number: The sum of protons and neutrons in the nucleus.
Isotopes: Atoms of the same element with different numbers of neutrons.

For example, Carbon-12 has 6 protons and 6 neutrons, while Carbon-14 has 6 protons and 8 neutrons.

Atomic Mass

The atomic mass is the weighted average mass of an atom based on the abundance of its isotopes. It is measured in atomic mass units (amu) or Daltons.

Example: The atomic mass of carbon is approximately 12.01 due to the presence of both Carbon-12 and Carbon-13 isotopes.

Radioactive Isotopes

Some isotopes are unstable and undergo radioactive decay, emitting particles and energy. These radioactive isotopes have important applications in biology:

Radiometric Dating: Determining the age of fossils by measuring isotope ratios (e.g., Carbon-14 dating).
Radioactive Tracers: Tracking biological processes using isotopes (e.g., PET scans for cancer detection).

The half-life of a radioactive isotope is the time required for half of the isotope to decay.

Electron Arrangement and Chemical Properties

Electron Shells and Orbitals

Electrons are arranged in shells around the nucleus, each with a characteristic energy level. The first shell holds up to 2 electrons, the second up to 8, and so on.

Valence electrons are those in the outermost shell and determine chemical reactivity.
Atoms are most stable when their valence shell is full (octet rule).

Energy Levels

Electrons closer to the nucleus have lower potential energy, while those farther away have higher potential energy. Electrons can absorb or release energy to move between shells.

Chemical Bonds and Interactions

Covalent Bonds

A covalent bond is formed when two atoms share one or more pairs of valence electrons. Covalent bonds can be:

Nonpolar covalent bonds: Electrons are shared equally (e.g., H2, O2).
Polar covalent bonds: Electrons are shared unequally due to differences in electronegativity (e.g., H2O).

Electronegativity is the ability of an atom to attract electrons in a covalent bond.

Ionic Bonds

An ionic bond is formed when one atom donates an electron to another, resulting in oppositely charged ions that attract each other (e.g., Na+ and Cl- form NaCl).

Ionic compounds often form crystalline solids and dissolve easily in water.

Hydrogen Bonds and Van der Waals Interactions

Hydrogen bonds: Weak bonds that form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (e.g., between water molecules).
Van der Waals interactions: Weak attractions between molecules due to transient local charges, important in large biological molecules.

Chemical Reactions

Nature of Chemical Reactions

Chemical reactions involve the making and breaking of chemical bonds. Atoms are not created or destroyed, only rearranged.

Reactants: Starting substances in a chemical reaction.
Products: Substances formed as a result of the reaction.

Example of a chemical reaction:

Combustion of propane:

Balancing chemical equations ensures the conservation of mass.

Summary Table: Elements Essential for Life

Element	Symbol	Biological Role
Oxygen	O	Component of water and organic molecules; cellular respiration
Carbon	C	Backbone of organic molecules
Hydrogen	H	Component of water and organic molecules
Nitrogen	N	Component of proteins and nucleic acids
Phosphorus	P	Component of nucleic acids and ATP
Sulfur	S	Component of some amino acids and vitamins
Additional info: Trace elements (e.g., Fe, Zn, I) are required in small amounts for enzyme function and hormone production.