Atomic Theory and Chemical Bonds: Foundations for Biology

Atomic Theory and the Chemical Context of Life

Introduction to Atomic Theory

Understanding atomic theory is essential for grasping the chemical basis of biological processes. Atoms are the fundamental units of matter, and their interactions form the molecules that make up living organisms.

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

• Subatomic particles: Atoms are composed of protons (positive charge), neutrons (neutral), and electrons (negative charge).

• Periodic Table: Organizes elements by atomic number and properties.

Biological Organization: Cellular Level

Biological systems are organized hierarchically, starting from atoms and molecules up to cells and organisms.

• Molecules: Groups of atoms bonded together, forming the chemical basis of life.

• Cells: The basic unit of life, composed of various molecules and organelles.

Biology as a Multidisciplinary Science

Biology integrates principles from chemistry and physics to explain the structure and function of living organisms.

• Biological processes depend on chemical reactions and physical laws.

• Examples include energy transfer, molecular interactions, and cellular organization.

The Chemical Context of Life

Matter and Elements

Matter is anything that takes up space and has mass. Elements are pure substances consisting of only one type of atom.

• States of Matter: Solid, liquid, gas.

• Elements: There are 92 naturally occurring elements; elements 93-118 are man-made.

• Periodic Table: Displays all known elements and their properties.

Chemistry Progression

Atoms combine to form molecules, which interact to create chemical bonds and biological macromolecules.

• Atoms → Molecules → Chemical Bonds → Biology & Life

Atoms and Elements

Atoms consist of a nucleus (protons and neutrons) and an electron cloud. Each element is defined by its atomic number (number of protons).

• Atomic Number (Z): Number of protons in the nucleus.

• Mass Number (A): Number of protons plus neutrons.

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

Isotopes and Their Uses

Isotopes have important applications in biology and medicine, such as radiometric dating and medical imaging.

• Radioactive Isotopes: Unstable isotopes that decay over time, emitting radiation.

• Uses: Tracing metabolic pathways, dating fossils, medical diagnostics.

Electron Arrangement and Chemical Properties

The arrangement of electrons in shells determines an atom's chemical behavior. Atoms are most stable when their outer electron shell is full.

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

• Electron Shells: Energy levels where electrons reside; the first shell holds 2 electrons, the second holds 8, etc.

Chemical Bonds

Ionic Bonds

Ionic bonds form when electrons are transferred from one atom to another, resulting in oppositely charged ions that attract each other.

• Example: Sodium chloride (NaCl) forms when sodium donates an electron to chlorine.

Bond Type	Electron Movement	Example
Ionic	Transfer	NaCl
Covalent	Sharing	H2, O2

Covalent Bonds

Covalent bonds form when atoms share pairs of electrons. These bonds can be single, double, or triple, depending on the number of shared electron pairs.

• Single Bond: One pair of shared electrons (e.g., H-H).

• Double Bond: Two pairs of shared electrons (e.g., O=O).

• Triple Bond: Three pairs of shared electrons (e.g., N≡N).

Polarity and Electronegativity

Polarity arises when atoms in a covalent bond have different electronegativities, causing unequal sharing of electrons.

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

• Polar Covalent Bond: Electrons are shared unequally (e.g., H2O).

• Nonpolar Covalent Bond: Electrons are shared equally (e.g., O2).

Bond Type	Electronegativity Difference	Polarity
Nonpolar Covalent	0-0.4	Nonpolar
Polar Covalent	0.5-1.7	Polar
Ionic	>1.7	Ionic

Important Biological Elements

Six elements are most important for life: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Phosphorus (P), and Sulfur (S).

• These elements form the backbone of biological molecules.

Vocabulary and Study Guide

Key Terms

• Matter-

• Element-

• Atom-

• Proton-

• Neutron-

• Electron-

• Atomic number-

• Atomic weight/mass-

• Isotope-

• Ionic/Covalent bond-

• Valence shell-

• Electronegativity-

Study Guide

• 1. Name the 3 states of matter discussed in class. Answer: Solid, Liquid, Gas

  2. Describe biological organization starting from an atom up to a cell. Answer: Atoms → Molecules → Cells

  3. Take any tile off the periodic table and draw the corresponding atom. Place the correct number of subatomic particles, be careful with the electron shells. Answer: (Example: Oxygen)

  • Atomic number = 8 → 8 protons

  • Mass number ≈ 16 → 8 neutrons

  • 8 electrons

    • First shell: 2 electrons

    • Second shell: 6 electrons

  4. Identify the four major elements found in living organisms Answer: Carbon (C) Hydrogen (H) Oxygen (O) Nitrogen (N)

  5. What is the difference between essential elements and trace elements? Answer: Essential elements are required in large amounts. Trace elements are needed in very small amounts.

  6. Distinguish between the following pairs of terms: neutron and proton, atomic number, atomic mass, atomic weight. Answer: Neutron vs Proton

  • Proton = positive charge

  • Neutron = no charge

  Atomic number vs atomic mass/weight

  • Atomic number = number of protons

  • Atomic mass/weight = protons + neutrons

  7. All isotopes are not radioactive. Why not? Answer: Only unstable isotopes are radioactive. Stable isotopes do not decay.

  8. What role do electrons play in chemical bonds? Answer: Electrons—especially valence electrons—are shared or transferred to form chemical bonds.

  9. Distinguish between ionic and covalent chemical bonds. How do you predict what type of bond any two elements will form with each other. Answer:

• Ionic bond: electrons are transferred

• Covalent bond: electrons are shared If difference is less than 0.5 equal electron sharing (nonpolar) If difference is = or greater than 0.5 unequal electron sharing (polar)

  10. What does it mean when a molecule has “polarity”. Answer: Polarity means electrons are shared unequally because atoms have different electronegativities.

  11. Review the differences between a polar covalent bond and a nonpolar covalent bond. Use the following words: electronegativity, unequally and equally, sharing/shared/share. Answer:

• Polar covalent bond: electrons are shared unequally due to different electronegativity

• Nonpolar covalent bond: electrons are shared equally because electronegativity is simila

  12. Identify which common elements have strong electronegativity versus those that are weak. Answer:

• Strong electronegativity: Oxygen, Nitrogen

• Weak electronegativity: Hydrogen, Carbon