The Chemical Context of Life: Study Notes for General Biology

The Chemical Context of Life

Vocabulary

This section introduces essential terms for understanding the chemical basis of life. Mastery of these terms is foundational for further study in biology.

• Element: A substance that cannot be broken down into other substances by chemical means.

• Atom: The smallest unit of an element that retains the properties of that element.

• Neutron: A subatomic particle with no electrical charge, found in the nucleus of an atom.

• Proton: A positively charged subatomic particle located in the atomic nucleus.

• Electron: A negatively charged subatomic particle found in electron shells surrounding the nucleus.

• Atomic nucleus: The central core of an atom, composed of protons and neutrons.

• Atomic number: The number of protons in the nucleus of an atom, unique to each element.

• Mass number: The sum of protons and neutrons in an atom's nucleus.

• Isotope: Atoms of the same element with different numbers of neutrons.

• Radioisotope: An isotope with an unstable nucleus that emits radiation during decay.

• Energy: The capacity to cause change or do work.

• Electron shell: The region around the nucleus where electrons are likely to be found.

• Valence electrons: Electrons in the outermost shell, important in chemical bonding.

• Valence shell: The outermost electron shell of an atom.

• Covalent bond: A chemical bond formed by the sharing of electron pairs between atoms.

• Ionic bond: A chemical bond formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions.

• Hydrogen bond: A weak bond between a hydrogen atom and an electronegative atom (often oxygen or nitrogen).

• Molecule: Two or more atoms held together by covalent bonds.

• Molecular formula: A notation indicating the number and type of atoms in a molecule (e.g., H2O).

• Structural formula: A representation showing the arrangement of atoms and bonds in a molecule.

• Single bond: A covalent bond involving one pair of shared electrons.

• Double bond: A covalent bond involving two pairs of shared electrons.

• Triple bond: A covalent bond involving three pairs of shared electrons.

• Electronegativity: The tendency of an atom to attract electrons in a covalent bond.

• Polarity: The distribution of electrical charge over the atoms joined by the bond.

• Polar covalent bond: A covalent bond in which electrons are shared unequally, resulting in partial charges.

• Nonpolar covalent bond: A covalent bond in which electrons are shared equally.

• Ion: An atom or molecule with a net electric charge due to the loss or gain of electrons.

• Cation: A positively charged ion.

• Anion: A negatively charged ion.

• Salt: An ionic compound formed from the neutralization reaction of an acid and a base.

• Van der Waals interactions: Weak attractions between molecules or parts of molecules that result from transient local partial charges.

• Chemical reaction: A process that leads to the transformation of one set of chemical substances to another.

• Reactants: Substances that start a chemical reaction.

• Products: Substances formed as a result of a chemical reaction.

Objectives

After studying this chapter, students should be able to:

• Define matter and distinguish it from energy.

• Define element, atom, and atomic mass.

• Use the periodic table to find information about elements, including atomic symbol, atomic number, and atomic mass.

• List the four main elements (carbon, hydrogen, oxygen, nitrogen) that make up over 96% of living matter.

• Distinguish between atomic number (number of protons) and mass number (protons + neutrons).

• Distinguish between isotopes and radioisotopes.

• Describe the structure of an atom, including the nucleus (protons and neutrons) and electron shells.

• Explain the energy levels (shells) of electrons and the arrangement of electrons in shells and orbitals.

• Define valence shell and valence electrons, and state the minimum and maximum number of electrons in each shell.

• Relate the number of valence electrons to the chemical properties of an atom.

• Use the Bohr model and Lewis dot structure to diagram atoms.

• Distinguish between molecules and compounds.

• Distinguish between molecular formula and structural formula.

• Define covalent bond and distinguish between single, double, and triple covalent bonds.

• Define electronegativity and use it to determine if a covalent bond is polar or non-polar.

• Distinguish between polar molecules, non-polar molecules, and ions.

• Define ionic bond and give examples of ionic compounds (e.g., NaCl).

• Define hydrogen bond and give examples (e.g., between water molecules).

• Describe and calculate the molecular weight of a molecule.

  • Formula: $\text{Molecular weight} = \sum (\text{number of atoms of each element} \times \text{atomic mass of element})$

• Define chemical reaction and distinguish between reactants and products.

• Explain what is meant by a reversible reaction and chemical equilibrium.

Key Concepts and Examples

Elements and Atoms

• Element: Pure substance consisting of only one type of atom (e.g., Oxygen).

• Atom: Smallest unit of an element, composed of protons, neutrons, and electrons.

• Example: A gold bar is made entirely of gold atoms (element: Au).

Atomic Structure

• Protons: Positively charged, found in nucleus.

• Neutrons: No charge, found in nucleus.

• Electrons: Negatively charged, orbit nucleus in shells.

• Atomic number (Z): Number of protons.

• Mass number (A): Number of protons + neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons.

• Radioisotopes: Unstable isotopes that emit radiation.

• Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Electron Shells and Valence

• Electrons occupy energy levels (shells) around the nucleus.

• The valence shell is the outermost shell; valence electrons determine chemical reactivity.

• Example: Oxygen has 6 valence electrons; needs 2 more to fill its shell.

Chemical Bonds

• Covalent bond: Atoms share electron pairs.

• Single bond: One pair shared ($\ce{H-H}$).

• Double bond: Two pairs shared ($\ce{O=O}$).

• Triple bond: Three pairs shared ($\ce{N#N}$).

• Ionic bond: Transfer of electrons creates ions (e.g., Na+ and Cl- form NaCl).

• Hydrogen bond: Weak attraction between a hydrogen atom and an electronegative atom (e.g., between water molecules).

• Van der Waals interactions: Weak, transient attractions between molecules.

Polarity and Electronegativity

• Electronegativity: Atom's ability to attract electrons in a bond.

• Polar covalent bond: Unequal sharing of electrons (e.g., H2O).

• Nonpolar covalent bond: Equal sharing of electrons (e.g., O2).

• Example: Water is a polar molecule; oxygen is more electronegative than hydrogen.

Molecules and Compounds

• Molecule: Two or more atoms held together by covalent bonds.

• Compound: Substance with two or more different elements combined in a fixed ratio (e.g., NaCl).

• Molecular formula: Shows types and numbers of atoms (e.g., C6H12O6).

• Structural formula: Shows arrangement of atoms and bonds.

Chemical Reactions and Equilibrium

• Chemical reaction: Process that transforms reactants into products.

• Reactants: Starting substances.

• Products: Substances formed.

• Reversible reaction: Can proceed in both directions.

• Chemical equilibrium: Forward and reverse reactions occur at the same rate; concentrations of reactants and products remain constant.

• Example: $\ce{H2O <=> H+ + OH-}$

Table: Comparison of Bond Types

Bond Type	Strength	Example	Nature
Covalent	Strong	H2O, O2	Shared electrons
Ionic	Strong (in dry conditions)	NaCl	Transferred electrons
Hydrogen	Weak	Between water molecules	Attraction between partial charges
Van der Waals	Very weak	Gecko feet adhesion	Transient, induced charges

Example Calculation: Molecular Weight

• For water (H2O):

• Hydrogen atomic mass ≈ 1, Oxygen atomic mass ≈ 16

• $\text{Molecular weight} = (2 \times 1) + (1 \times 16) = 18$

Additional info: Some explanations and examples have been expanded for clarity and completeness.