Chapter 2: The Chemical Context of Life

Concept 2.1: Matter and Chemical Elements

Matter is composed of chemical elements, which may exist in pure form or combine to create compounds. Understanding the distinction between elements and compounds is fundamental in biology and chemistry.

• Element: A substance that cannot be broken down into other substances by chemical means. Each element is defined by its number of protons (atomic number).

• Compound: A substance formed when two or more elements are chemically bonded in fixed ratios. Compounds have properties different from their constituent elements.

• Example: Water (H2O) is a compound made from hydrogen and oxygen elements.

Concept 2.2: Atomic Structure and Element Properties

The properties of an element are determined by the structure of its atoms. Atoms consist of subatomic particles and have unique characteristics such as atomic number, mass number, and valence.

• Subatomic Particles:

  • Protons: Positively charged particles found in the nucleus.

  • Neutrons: Neutral particles found in the nucleus.

  • Electrons: Negatively charged particles orbiting the nucleus.

• Atomic Number (): The number of protons in an atom's nucleus. Determines the element's identity.

• Mass Number (): The sum of protons and neutrons in the nucleus.

• Atomic Weight: The average mass of atoms of an element, accounting for isotopes.

• Valence: The number of electrons in the outermost shell, which determines chemical reactivity.

• Distinguishing Subatomic Particles:

  • Neutron and Proton: Both found in the nucleus; neutrons have no charge, protons are positive.

  • Atomic Number and Mass Number: Atomic number is protons only; mass number is protons plus neutrons.

• Isotopes: Atoms of the same element with different numbers of neutrons. Isotopes have the same atomic number but different mass numbers.

• Electron Configuration: The arrangement of electrons in an atom's shells. Influences chemical behavior and bonding.

• Principle of Electron Distribution: Electrons fill shells in order of increasing energy; the outermost shell determines reactivity.

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

• Additional info: The chemical properties of elements are largely determined by the number and arrangement of electrons, especially those in the valence shell.

Concept 2.3: Chemical Bonds and Molecule Formation

Molecules and ionic compounds are formed through chemical bonding between atoms. The type of bond affects the properties and behavior of the resulting compound.

• Types of Chemical Bonds:

  • Nonpolar Covalent Bond: Electrons are shared equally between atoms.

  • Polar Covalent Bond: Electrons are shared unequally, creating partial charges.

  • Ionic Bond: Electrons are transferred from one atom to another, resulting in oppositely charged ions.

• Electron Sharing: The number of electrons shared determines the bond type:

  • Single Covalent Bond: One pair of electrons shared.

  • Double Covalent Bond: Two pairs of electrons shared.

  • Triple Covalent Bond: Three pairs of electrons shared.

• Bond Strength and Chemical Behavior: Covalent bonds are generally stronger than ionic bonds; polarity influences solubility and interactions.

• Hydrogen Bonds: Weak attractions between a hydrogen atom (covalently bonded to a highly electronegative atom) and another electronegative atom. Important in water and biological molecules.

• Comparison Table:

• Example: Water molecules are held together by polar covalent bonds, and interact via hydrogen bonds.

• Additional info: The type of chemical bond influences molecular shape, polarity, and biological function.