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Chapter 2: The Chemical Context of Life – Study Notes

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Chapter 2: The Chemical Context of Life

Introduction

Understanding the chemical basis of life is fundamental to biology. All living organisms are composed of matter, which is organized into elements and compounds. The properties and interactions of these substances underpin all biological processes.

Elements, Compounds, and Matter

What is Matter?

  • Matter is anything that takes up space and has mass.

  • All organisms are composed of matter.

Elements and Compounds

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

  • Compound: A substance consisting of two or more elements combined in a fixed ratio.

  • Compounds have emergent properties that are different from those of their constituent elements.

Example: Sodium (Na) is a soft, reactive metal, and chlorine (Cl) is a poisonous gas. When combined, they form sodium chloride (NaCl), or table salt, which is safe to eat.

Elements of Life

  • About 20–25% of the 92 natural elements are essential for life. These are called essential elements.

  • Major elements: Carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) make up approximately 96% of living matter.

  • Other important elements: Calcium (Ca), phosphorus (P), potassium (K), and sulfur (S) account for most of the remaining 4%.

  • Trace elements are required by organisms in very small amounts but are still essential for health (e.g., iron, iodine, zinc).

Atomic Structure and Properties

Atoms and Subatomic Particles

  • An atom is the smallest unit of matter that retains the properties of an element.

  • Atoms are composed of subatomic particles:

    • Protons (positive charge)

    • Neutrons (no charge)

    • Electrons (negative charge)

Atomic Number and Mass

  • The atomic number is the number of protons in an atom's nucleus and defines the element.

  • The mass number is the sum of protons and neutrons in the nucleus.

  • The atomic mass is the atom's total mass, which can be approximated by the mass number.

Isotopes

  • Isotopes are atoms of the same element that have different numbers of neutrons.

  • Radioactive isotopes decay spontaneously, releasing particles and energy. They are used as diagnostic tools in medicine (e.g., radioactive tracers).

Electron Configuration and Chemical Properties

Energy Levels of Electrons

  • Energy is the capacity to cause change.

  • Potential energy is the energy matter possesses due to its location or structure.

  • Electrons have different amounts of potential energy depending on their distance from the nucleus, organized into electron shells.

Electron Distribution and Chemical Behavior

  • The chemical behavior of an atom is determined by the distribution of electrons in its electron shells, especially the outermost shell (valence shell).

  • Valence electrons are those in the outermost shell and are most involved in chemical bonding.

  • Atoms with a full valence shell are chemically inert (unreactive).

Chemical Bonds and Molecules

Covalent Bonds

  • A covalent bond is the sharing of a pair of valence electrons by two atoms.

  • A single covalent bond involves one pair of shared electrons; a double covalent bond involves two pairs.

  • The valence of an atom is its bonding capacity, usually equal to the number of unpaired electrons in its valence shell.

  • Molecules are two or more atoms held together by covalent bonds.

Electronegativity and Bond Polarity

  • Electronegativity is an atom's attraction for the electrons in a covalent bond.

  • Nonpolar covalent bond: Electrons are shared equally between atoms.

  • Polar covalent bond: Electrons are shared unequally, resulting in partial charges on the atoms.

Example: In water (H2O), oxygen is more electronegative than hydrogen, so the shared electrons spend more time near oxygen, making it partially negative and hydrogen partially positive.

Ionic Bonds

  • Sometimes, atoms strip electrons from their bonding partners, forming ions (charged atoms or molecules).

  • A cation is a positively charged ion; an anion is a negatively charged ion.

  • Ionic bonds are attractions between oppositely charged ions.

  • Ionic compounds (salts) are often found as crystals (e.g., NaCl).

Weak Chemical Interactions

  • Many large biological molecules are held in their functional forms by weak bonds.

  • Hydrogen bonds form when a hydrogen atom covalently bonded to one electronegative atom is attracted to another electronegative atom (commonly oxygen or nitrogen).

  • Van der Waals interactions are weak attractions that occur when electrons are distributed asymmetrically in molecules, creating temporary partial charges.

Chemical Reactions

  • Chemical reactions involve the making and breaking of chemical bonds.

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

  • Photosynthesis is a key chemical reaction in biology:

  • This reaction converts carbon dioxide and water into glucose and oxygen using sunlight.

Table: Major 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

-

<0.01%

Summary: Key Concepts to Master

  • Identify the basic unit of matter and the subatomic particles that make them up.

  • Identify the essential elements in the human body.

  • Distinguish between and discuss the biological importance of polar covalent bonds and hydrogen bonds.

  • Understand how to identify valence electrons.

  • Determine the difference between covalent and ionic bonds.

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