Atoms and Chemical Elements

Essential Elements in Living Organisms

Living organisms are primarily composed of a small subset of chemical elements, which are crucial for biological processes.

• Essential elements: 20-25 out of 92 naturally occurring elements are necessary for life.

• Major elements: Approximately 96% of living matter consists of oxygen (O), carbon (C), hydrogen (H), and nitrogen (N).

• Trace elements: Elements required in minute quantities, but still vital for proper biological function.

Atomic Structure

Subatomic Particles

The properties of an element depend on the structure of its atoms, which are composed of subatomic particles.

• Proton: Positively charged particle found in the nucleus.

• Neutron: Neutral particle found in the nucleus.

• Electron: Negatively charged particle orbiting the nucleus.

• Atomic number (Z): Number of protons in the nucleus; determines the element.

• Mass number (A): Sum of protons and neutrons in the nucleus.

Example: Sodium-23 (Na-23) has 11 protons and 12 neutrons. Atomic number = 11, mass number = 23.

Isotopes and Radioactivity

Isotopes are atoms of the same element with different numbers of neutrons.

• Isotope: Same number of protons, different number of neutrons.

• Some isotopes are stable, others are radioactive (radioactive isotopes).

Electron Distribution and Orbitals

Electrons are arranged in shells and orbitals around the nucleus, influencing chemical reactivity.

• Atoms strive for a full outer electron shell (valence shell).

• Electron shells fill from the inside out: 1st shell (2 electrons), 2nd shell (8 electrons), 3rd shell (up to 8 electrons).

• Orbital: Region where an electron is found 90% of the time; each shell contains specific orbitals.

Chemical Bonds and Molecules

Types of Chemical Bonds

Chemical bonds form when atoms interact to achieve stable electron configurations.

• Covalent bond: Atoms share electron pairs to fill their valence shells. Strong bond.

• Single covalent bond: One pair of electrons shared.

Electronegativity and Bond Polarity

The type of covalent bond depends on the difference in electronegativity between atoms.

• Electronegativity: Atom's ability to attract electrons in a bond.

• Nonpolar covalent bond: Electrons shared equally; electronegativity difference < 0.5.

• Polar covalent bond: Electrons shared unequally; electronegativity difference 0.5 to 1.6, resulting in partial charges.

• Ionic bond: Electrons transferred completely; electronegativity difference > 1.7.

Example: In water (H2O), oxygen is more electronegative than hydrogen, creating a polar covalent bond with partial charges ( and ).

Ions and Ionic Bonds

Ions are atoms or molecules with a net electric charge due to loss or gain of electrons.

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• Ionic bond: Attraction between cations and anions.

Weak Chemical Interactions

In addition to covalent and ionic bonds, weak interactions play important roles in biological systems.

• Hydrogen bonds: Attraction between a hydrogen atom covalently bonded to an electronegative atom and another electronegative atom.

• Van der Waals forces: Weak attractions due to transient local charges in molecules.

Molecular Shape and Function

Orbital Hybridization and Molecular Geometry

The shape of a molecule is determined by the positions of its orbitals and the arrangement of atoms.

• Hybrid orbitals: s and p orbitals combine to form new shapes (e.g., tetrahedral geometry in methane).

• Molecular shape affects function and interactions.

Chemical Reactions

Reactants, Products, and Reaction Mechanisms

Chemical reactions involve the making and breaking of chemical bonds, transforming reactants into products.

• Reactant: Starting substance in a chemical reaction.

• Product: Substance formed as a result of a chemical reaction.

Example:

Summary Table: Types of Chemical Bonds

Additional info: These notes expand on the original content by providing definitions, examples, and tables for clarity and completeness, suitable for General Biology students.