Atoms, Molecules, and Life: Study Notes

Atoms, Molecules, and Life

Section 2.1: What are Atoms?

Atoms are the basic building blocks of matter and the foundation of all chemical substances. Understanding their structure and behavior is essential for studying biological molecules and life processes.

• Proton: Positively charged particle found in the nucleus.

• Neutron: Uncharged particle found in the nucleus.

• Electron: Negatively charged particle found in orbitals around the nucleus.

The number of protons in an atom determines its atomic number. The chemical behavior of an atom is largely determined by the arrangement of its electrons.

Elements and Isotopes

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

• Isotope: Atoms of the same element with different numbers of neutrons. Some isotopes are stable, while others are radioactive and decay over time, releasing energy.

• Example: Carbon-12 and Carbon-14 are isotopes of carbon. Carbon-14 is radioactive and used in radiometric dating.

Electron Shells

• Electrons are arranged in specific energy levels called electron shells.

• The outermost shell (valence shell) determines an atom's chemical reactivity.

• Atoms are most stable when their outermost shell is full (octet rule).

Energy Capture and Release

• Atoms can absorb or release energy as electrons move between shells.

• This energy transfer is fundamental to chemical reactions and biological processes such as photosynthesis and cellular respiration.

Section 2.2: How do Atoms Interact to Form Molecules?

Atoms combine to form molecules through chemical bonds, which involve the sharing or transfer of electrons. The type of bond formed affects the properties of the resulting molecule.

Chemical Bonds

• Ionic Bonds: Formed when electrons are transferred from one atom to another, creating oppositely charged ions that attract each other.

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

• Hydrogen Bonds: Weak attractions between a hydrogen atom (already covalently bonded to another atom) and an electronegative atom (like oxygen or nitrogen).

Free Radicals

• Free radicals are molecules with unpaired electrons, making them highly reactive and potentially damaging to cells.

Types of Chemical Bonds

Bond Type	Description	Example
Ionic	Transfer of electrons, attraction between ions	NaCl (table salt)
Covalent	Sharing of electrons	H2O (water)
Hydrogen	Weak attraction between polar molecules	Between water molecules

Section 2.3: Why is Water So Important to Life?

Water is essential for life due to its unique chemical and physical properties, which support biological processes and maintain homeostasis.

Cohesion and Adhesion

• Cohesion: Water molecules stick to each other due to hydrogen bonding, resulting in surface tension.

• Adhesion: Water molecules stick to other substances, aiding in processes like capillary action in plants.

Solvent Properties

• Water is an excellent solvent, dissolving many ionic and polar substances, which facilitates chemical reactions in cells.

pH Scale

• The pH scale measures the acidity or basicity of a solution based on the concentration of hydrogen ions ($[H^+]$).

• Acidic solutions: $[H^+] > [OH^-]$, pH < 7

• Basic solutions: $[OH^-] > [H^+]$, pH > 7

• Neutral solution: $[H^+] = [OH^-]$, pH = 7

Buffers

• Buffers are substances that help maintain a relatively constant pH in a solution by accepting or releasing hydrogen ions as needed.

• Example: Blood contains buffers to maintain a pH around 7.4.

Temperature Moderation

• Water has a high specific heat, meaning it can absorb or release large amounts of heat with little temperature change. This property helps organisms maintain stable internal temperatures.

• Evaporative cooling (e.g., sweating) helps regulate body temperature.

Density of Ice

• Ice is less dense than liquid water because its molecules form a rigid lattice structure, causing it to float.

• This property insulates aquatic environments, protecting life during cold periods.