Atoms: The Smallest Unit of Matter

Definition and Hierarchy of Matter

All physical substances are composed of matter, which is anything that occupies space and has mass. Matter is made up of chemical elements, and each chemical element is composed of atoms, the smallest unit of matter that retains the properties of the element.

• Matter: Anything with mass and volume (e.g., organisms, rocks, water).

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

• Atom: The fundamental unit of an element and matter.

Atoms in Living and Nonliving Matter

Atoms are the basic building blocks for both living and nonliving matter. The structure and composition of atoms determine the properties of substances, from diamonds to honey bees.

• Example: Diamond is composed of carbon atoms; honey bee structures contain carbon, oxygen, and hydrogen atoms.

Atomic Structure

Subatomic Particles

Atoms are made up of three main subatomic particles: protons, neutrons, and electrons. Each has distinct properties and locations within the atom.

• Proton: Positively charged, 1 atomic mass unit (AMU), located in the nucleus.

• Neutron: No charge, 1 AMU, located in the nucleus.

• Electron: Negatively charged, almost no mass, orbits the nucleus.

Atomic Model Example: Carbon Atom

The carbon atom consists of a nucleus containing protons and neutrons, surrounded by electrons in energy shells.

• Nucleus: Contains protons (+) and neutrons (0).

• Electron Shells: Electrons (-) orbit the nucleus.

Elements of Life and the Periodic Table

Major and Trace Elements

Of all known elements, only a subset is found in living organisms. The periodic table organizes these elements based on their chemical properties. The bulk of life’s mass is composed of six elements: Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS).

• Major Elements: Required in large amounts for life.

• Trace Elements: Required in small amounts.

Atomic Properties

Atomic Number, Mass Number, and Atomic Mass

Each atom 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 Shells and Valence Electrons

Electrons occupy energy shells (orbitals) around the nucleus. The shell closest to the nucleus is lowest in energy, while outer shells are higher in energy. Valence electrons are found in the outermost shell and determine chemical reactivity.

• 1st shell: Holds up to 2 electrons.

• 2nd shell: Holds up to 8 electrons.

• Valence Shell: Outermost shell; determines bonding.

The Octet Rule

Stability of Atoms

Atoms are most stable when their valence shells are fully occupied, typically with 8 electrons (except for the first shell, which holds 2). This is known as the octet rule.

• Octet Rule: Atoms are less reactive when their valence shells are full.

• Example: Neon is unreactive because its valence shell is full.

Isotopes

Definition and Properties

Isotopes are atoms of the same element with different numbers of neutrons, resulting in different mass numbers. Atomic mass is the average mass of all isotopes.

• Example: Carbon-12, Carbon-13, and Carbon-14 are isotopes of carbon.

Radioactive Isotopes

Some isotopes are unstable and emit energy as rays or particles. These radioactive isotopes are used in medicine and dating fossils. The half-life is the time it takes for half of the radioactive atoms to decay.

• Example: Carbon-14 is used in radiometric dating.

Introduction to Chemical Bonding

Molecules and Compounds

Chemical bonds are attractive forces that hold atoms together to form molecules and compounds. A molecule contains two or more chemically bound atoms, while a compound contains two or more different elements.

• Chemical Formula: Shows the types and numbers of atoms in a molecule (e.g., C6H12O6).

Intramolecular vs. Intermolecular Bonds

Bonds can be classified as 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 a molecule.

• Intermolecular Bonds: Weaker, between molecules.

Ionic Bonding

Ions: Anions vs. Cations

Ions are atoms or molecules with a net electrical charge, formed by the gain or loss of electrons. Anions are negatively charged (gain electrons), while cations are positively charged (lose electrons).

• Anion: More electrons than protons; negative charge.

• Cation: Fewer electrons than protons; positive charge.

Ionic Bonds

Ionic bonds are electrical attractions between oppositely charged ions. The transfer of electrons fills the valence shells of both atoms, creating charged ions.

• Example: Formation of sodium chloride (NaCl) from sodium and chloride ions.

Covalent Bonding

Types of Covalent Bonds

Covalent bonds involve the sharing of electrons between atoms. There are two main types: nonpolar covalent bonds (equal sharing) and polar covalent bonds (unequal sharing due to differences in electronegativity).

• Electronegativity: Measure of an atom’s attraction for electrons.

Noncovalent Bonds

Types and Biological Importance

Noncovalent bonds are interactions between atoms resulting from full or partial charges, without electron sharing. Types include strong electrostatic interactions (ionic and hydrogen bonds) and weak van der Waals interactions.

• Ionic Bonds: Strong electrostatic attraction.

• Hydrogen Bonds: Weak, but important in biology.

• Van der Waals: Weak, transient interactions.

Hydrogen Bonding

Definition and Role in Biology

Hydrogen bonds occur between a highly electronegative atom (F, O, or N) and a hydrogen atom. Individually weak, but collectively strong, hydrogen bonds are crucial for the properties of water and the structure of macromolecules.

• Example: Hydrogen bonding in water and nucleic acids.

Introduction to Water

Structure and Properties

Water is a small, polar molecule with two hydrogen atoms and one oxygen atom. It has partial negative and positive charges, and forms hydrogen bonds with other water molecules.

• Example: Water molecules bind via hydrogen bonds.