Chapter 2: The Chemical Basis for Life

What is an Element?

An element is a pure substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons.

• Examples: Carbon (C), Oxygen (O), Nitrogen (N), Hydrogen (H)

What is a Compound?

A compound is a substance formed when two or more elements are chemically combined in fixed ratios.

• Example: Water (H2O) is a compound of hydrogen and oxygen.

Are Trace Elements Essential?

Trace elements are required by organisms in minute quantities but are vital for proper physiological functioning.

• Iron (Fe): Deficiency causes anemia.

• Iodine (I): Deficiency causes goiter and mental retardation.

• Fluorine (F): Deficiency causes tooth decay.

• Zinc (Zn): Deficiency causes acne, diarrhea, and wasting of body tissue.

The four major elements of life are:

• Carbon (C)

• Oxygen (O)

• Nitrogen (N)

• Hydrogen (H)

How do organisms obtain these elements?

• Animals: Through consumption, eating, drinking, and breathing.

• Plants: Mainly through the soil.

Atoms: The Building Blocks of Matter

All matter is composed of atoms, which consist of subatomic particles:

• Protons (p+): Positively charged, found in the nucleus.

• Neutrons (n): No charge, found in the nucleus.

• Electrons (e-): Negatively charged, orbit the nucleus.

• Atomic number: Number of protons in the nucleus (defines the element).

• Atomic mass: Sum of protons and neutrons.

• Number of electrons: In a neutral atom, equals the number of protons.

Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons.

• Radioactive isotopes: Unstable isotopes that decay spontaneously, emitting radiation.

• Applications:

  • Phosphorus-32: Labels DNA and RNA.

  • Carbon-14: Used in dating fossils and biological molecules.

  • Fluorine-18: Used in PET scans for imaging glucose metabolism.

Electron Shells and Chemical Properties

The arrangement of electrons in electron shells determines an atom's chemical properties.

• Electrons occupy orbitals within shells; each shell has a specific number of orbitals.

• Valence electrons (in the outermost shell) are most important for chemical bonding.

Chemical Bonds

Types of Chemical Bonds

Chemical bonds are forces that hold atoms together in molecules and compounds. The reactivity of atoms is based on filling their outermost shell (valence shell).

• Covalent Bonds: Formed when two atoms share one or more pairs of electrons.

• Single Bond: One pair of shared electrons.

• Double Bond: Two pairs of shared electrons (e.g., O2 molecule).

• Polar Covalent Bonds: Electrons are shared unequally, leading to partial charges (e.g., H2O).

• Ionic Bonds: Formed when one atom transfers electrons to another, creating oppositely charged ions that attract each other (e.g., NaCl).

• Hydrogen Bonds: Weak attractions between a hydrogen atom in one molecule and an electronegative atom (like oxygen or nitrogen) in another.

• Van der Waals Interactions: Weak, transient attractions between molecules due to temporary charge fluctuations.

Determining the Number of Bonds

The number of covalent bonds an atom can form is usually determined by the number of unpaired electrons in its valence shell.

• Example: Oxygen has 6 valence electrons and typically forms 2 bonds (as in O2).

Electronegativity and Polarity

Electronegativity is an atom's ability to attract shared electrons in a covalent bond. Differences in electronegativity between atoms lead to polar covalent bonds.

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

Chemical Reactions

Chemical reactions involve the making or breaking of chemical bonds, resulting in the formation of new substances. Atoms are not created or destroyed, only rearranged.

• Example: Formation of water from hydrogen and oxygen:

• Reactions can release or absorb energy.

• Reactants are the starting substances; products are the resulting substances.

Summary Table: Types of Chemical Bonds

Additional info:

• Electronegativity values can be found in the periodic table and are important for predicting bond polarity.

• Hydrogen bonds are crucial for the structure of DNA and proteins.

• Radioactive isotopes are widely used in biological research and medicine for tracing and imaging.