Atoms and Elements

Essential Elements for Life

Living organisms require certain elements to survive, grow, and reproduce. These elements are fundamental to biological molecules and processes.

• Essential elements: Elements needed by an organism to live a healthy life and reproduce.

• Of 92 natural elements, about 20 percent are essential elements.

• Four main elements make up most of the mass of living organisms: Oxygen (O), Carbon (C), Nitrogen (N), and Hydrogen (H).

• Some elements can be toxic to organisms, even in small amounts.

Structure of Atoms

Atoms are the smallest units of matter that retain the properties of an element. They are composed of subatomic particles:

• Protons: Positively charged particles found in the nucleus.

• Neutrons: Electrically neutral particles found in the nucleus.

• Electrons: Negatively charged particles that orbit the nucleus.

Atomic Number, Mass Number, and Isotopes

The identity and properties of an atom are determined by its atomic number and mass number.

• Atomic number: Number of protons in the nucleus of an atom.

• Mass number: Total number of protons and neutrons in the nucleus.

• Isotopes: Atoms of the same element that differ in the number of neutrons.

• Radioactive isotopes are unstable and decay over time, while stable isotopes do not decay.

Applications of Isotopes

• Dating fossils: Using radioactive decay to estimate the age of fossils.

• Tracing atoms: Tracking the movement of substances in biological systems.

• Medical diagnosis: Using radioactive isotopes in imaging and treatment.

Potential Energy in Atoms

Potential energy is the energy that matter possesses due to its location or structure. In atoms, electrons have potential energy based on their distance from the nucleus.

• Electrons are found in electron shells around the nucleus.

• The valence shell is the outermost electron shell of an atom.

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

Chemical Bonds and Interactions

Types of Chemical Bonds

Atoms with incomplete valence shells can share or transfer electrons to achieve stability, forming chemical bonds.

• Ionic bonds: Formed when electrons are transferred from one atom to another, resulting in oppositely charged ions.

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

• Hydrogen bonds: Weak attractions between a hydrogen atom covalently bonded to one electronegative atom and another electronegative atom.

• Van der Waals interactions: Weak attractions due to transient local partial charges.

Ions

• Cation: Positively charged ion (loses electrons).

• Anion: Negatively charged ion (gains electrons).

Summary Table: Types of Chemical Bonds

Water: Properties and Importance

Properties of Water That Make Life Possible

Water's unique properties are essential for life on Earth. These properties arise from its molecular structure and hydrogen bonding.

• Cohesive behavior: Water molecules stick together due to hydrogen bonding.

• Expansion upon freezing: Ice is less dense than liquid water, so it floats.

• Versatility as a solvent: Water can dissolve many substances, making it a universal solvent.

• Moderation of temperature: Water absorbs and releases heat slowly, stabilizing temperatures in organisms and environments.

Acids, Bases, and pH

Water can dissociate into hydrogen ions (H+) and hydroxide ions (OH-), affecting the pH of solutions.

• Acids: Increase the concentration of H+ ions in a solution.

• Bases: Increase the concentration of OH- ions in a solution.

• pH scale: Measures the concentration of H+ ions; ranges from 0 (most acidic) to 14 (most basic).

• Acidic solutions: pH less than 7.

• Basic solutions: pH greater than 7.

• Neutral solutions: pH of 7.

Example: Ocean Temperature Regulation

• Oceans have more stable temperatures than land due to water's high specific heat, which allows it to absorb and release heat slowly.

Key Equation: pH Calculation

The pH of a solution is calculated as:

Additional info: The notes have been expanded to include definitions, examples, and a summary table for bond types, as well as a key equation for pH calculation.