Atoms, Elements, and Chemical Bonding: The Chemical Context of Life

Atoms: The Smallest Unit of Matter

Definition and Hierarchy of Matter

Matter is anything that takes up space and has mass, including organisms, rocks, and oceans. All matter consists of at least one chemical element, which is a pure substance made of only one type of atom. The atom is the fundamental unit of an element and, therefore, the smallest unit of matter.

• Matter: Composed of chemical elements.

• Chemical Element: Pure substance consisting of one type of atom.

• Atom: Smallest unit of an element and matter.

Atoms in Biological and Non-Biological Contexts

Atoms make up both living and non-living matter. For example, a diamond is composed entirely of carbon atoms, while a honey bee contains atoms of carbon, oxygen, and hydrogen arranged in complex molecules like glucose.

• Example: Diamond (carbon atoms), Honey Bee (carbon, oxygen, hydrogen atoms in glucose).

Atomic Structure

Subatomic Particles

Atoms are made of three main subatomic particles, each with distinct properties:

Subatomic Particle	Electric Charge	Atomic Mass Unit (AMU)	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	-1	~0	Orbiting Nucleus

Atomic Model

Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in energy shells.

• Protons: Positive charge, in nucleus.

• Neutrons: Neutral charge, in nucleus.

• Electrons: Negative charge, orbit nucleus.

Elements of Life and the Periodic Table

Major and Trace Elements

Of all known elements, only a subset is found in living organisms. About 97% of the mass of most life is composed of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS). Trace elements are required in small amounts for life.

• Major Elements: CHNOPS.

• Trace Elements: Required in minute quantities.

Atomic Properties

Each atom of an element has unique properties:

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

• Mass Number: Sum of protons and neutrons in the nucleus.

• Atomic Mass: Average mass of all isotopes of an element.

Electron Orbitals and Energy Shells

Electron Configuration

Electrons occupy three-dimensional regions called orbitals, visualized as energy shells. Shells closer to the nucleus are lower in energy, while distant shells are higher in energy. Valence electrons are found in the outermost energy shell (valence shell).

• 1st shell: Holds up to 2 electrons.

• 2nd shell: Holds up to 8 electrons.

• Valence Electrons: Determine chemical reactivity.

The Octet Rule

Stability of Atoms

The octet rule states that atoms are more stable (less reactive) when their valence shells are fully occupied. Atoms are most reactive when their outer valence shells are not full.

• 1st shell: Up to 2 electrons.

• 2nd shell: Up to 8 electrons.

• Full valence shell: Atom is stable and unreactive.

Isotopes

Definition and Properties

Isotopes are atoms of the same element that vary in the number of neutrons. All isotopes have the same atomic number (protons) but different mass numbers (protons + neutrons). Atomic mass is the average mass of all isotopes.

• Example: Carbon-12, Carbon-13, Carbon-14.

Radioactive Isotopes

Radioactive isotopes are unstable and break down, emitting energy as rays or particles. The half-life is the time it takes for half of all radioactive atoms in a sample to decay. Radioactive isotopes are used in medicine and radiometric dating of fossils.

• Half-life: Key concept in radiometric dating.

• Applications: Medical imaging, fossil dating.

Chemical Bonding

Introduction to Chemical Bonds

Chemical bonds are attractive forces between atoms, holding them together to form molecules and compounds. A molecule contains two or more chemically bound atoms, while a compound is a molecule composed of two or more different elements. Chemical formulas reveal the types and numbers of atoms in a molecule.

• Molecule: O2 (oxygen gas).

• Compound: H2O (water), C6H12O6 (glucose).

Intramolecular vs. Intermolecular Bonds

Bonds between atoms can be intramolecular (within a molecule) or intermolecular (between molecules). Intramolecular bonds hold atoms together within a molecule, while intermolecular bonds connect different molecules.

• Intramolecular Bonds: Strong, within molecules.

• Intermolecular Bonds: Weaker, between molecules.

Types of Chemical Bonds

Chemical bonds are classified as covalent or noncovalent. Covalent bonds involve the sharing of electrons, while noncovalent bonds involve electrostatic interactions or weak forces.

• Covalent Bonds: Nonpolar (equal sharing), Polar (unequal sharing).

• Noncovalent Bonds: Ionic, hydrogen, van der Waals interactions.

Covalent Bonds

Definition and Types

Covalent bonds are interactions between two atoms resulting from the sharing of electrons. There are two main types: nonpolar covalent and polar covalent bonds. The type depends on the difference in electronegativity between the atoms.

• Electronegativity: Measure of an atom's attraction to electrons (scale 0-4).

Nonpolar Covalent Bonds

Nonpolar covalent bonds involve equal sharing of electrons between atoms with similar electronegativities. Examples include hydrogen gas (H2) and methane (CH4).

• Example: H2, O2, CH4.

Polar Covalent Bonds

Polar covalent bonds involve unequal sharing of electrons between atoms with different electronegativities, resulting in partial charges (δ+ and δ-). Examples include water (H2O) and hydrogen chloride (HCl).

• Example: H2O, HCl, NH3.

Noncovalent Bonds

Types and Properties

Noncovalent bonds are interactions between atoms resulting from full or partial charges, without sharing electrons. Types include ionic bonds, hydrogen bonds, and van der Waals interactions.

• Ionic Bonds: Strong electrostatic attraction between oppositely charged ions.

• Hydrogen Bonds: Weak interaction between a highly electronegative atom and hydrogen.

• Van der Waals Interactions: Very weak, transient attractions.

Ionic Bonding

Ions: Anions vs. Cations

Ions are atoms or molecules with a net electrical charge, resulting from the gain or loss of electrons. Anions are negatively charged ions (gain of electron), while cations are positively charged ions (loss of electron).

• Anion: More electrons than protons.

• Cation: Fewer electrons than protons.

Ionic Bonds

Ionic bonds are electrical attractions between oppositely charged ions. The transfer of electrons fills the valence shells of both atoms, creating charges and forming compounds like sodium chloride (NaCl).

• Example: NaCl formation from Na+ and Cl-.

Hydrogen Bonding

Definition and Biological Importance

Hydrogen bonds are interactions between a highly electronegative atom (such as F, O, or N) and a hydrogen atom. Individually, hydrogen bonds are weak, but collectively, they can be strong and are crucial in biology, affecting the properties of water and the structure of macromolecules.

• Example: Water molecules, DNA structure.

Additional info: The notes above expand on brief points and fill in missing context using standard academic knowledge of atomic structure, chemical bonding, and their relevance to biological systems.