Atoms: The Smallest Unit of Matter

Definition and Structure 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 smallest unit of an element and, therefore, the smallest unit of matter. Atoms make up both living and nonliving matter.

• Matter: Anything with mass and volume.

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

• Atom: Smallest unit of an element.

• Example: Atoms are the smallest units of matter.

Atomic Structure and Subatomic Particles

Atoms are composed of three subatomic particles, each with a characteristic charge, mass, and location within the atom:

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

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

• Electron: Negatively charged, located in electron shells, mass of ~0 AMU.

Elements and Atomic Properties

Elements of Life and the Periodic Table

Only a small subset of all known elements is found in living organisms. The periodic table arranges all known elements based on their chemical properties. Approximately 97% of the mass of most life is composed of Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, and Sulfur (CHNOPS).

• Trace Elements: Required for life in small amounts.

• Example: Periodic Table of Elements.

Atomic Number, Mass Number, and Atomic Mass

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: Weighted average total mass of all atoms of an element, considering isotopes.

• Example: Atomic properties of a carbon atom.

Electron Orbitals and Energy Shells

Electron Configuration

Electron orbitals are three-dimensional regions around a nucleus where electrons are found, often visualized as energy shells. Shells closer to the nucleus are lower in energy than distant shells, which 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: Electrons in the outermost shell.

Octet Rule and Chemical Stability

Octet Rule

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 energy shell: Holds up to 2 electrons.

• 2nd energy shell: Holds up to 8 electrons.

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

Isotopes and Atomic Mass

Isotopes

All atoms of an element have the same number of protons, but not necessarily the same number of neutrons. Isotopes are atoms of the same element that vary in the number of neutrons. Isotopes have the same atomic number but different mass numbers.

• Atomic Mass: Average mass of all isotopes.

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

Radioactive Isotopes and Half-Life

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

• Example: Radioactivity of Carbon-14.

• Half-life of Carbon-14: 5,730 years.

Chemical Bonding

Introduction to Chemical Bonding

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. The chemical formula reveals the composition and number of atoms in a molecule.

• Intramolecular Bonds: Interactions within a single molecule.

• Intermolecular Bonds: Interactions between atoms of different molecules.

Covalent Bonds

Covalent bonds are interactions between two atoms resulting from the sharing of electrons. There are two types of covalent bonds:

• Nonpolar Covalent: Equal sharing of electrons between atoms with similar electronegativities.

• Polar Covalent: Unequal sharing of electrons between atoms with different electronegativities, leading to partial (δ) charges.

• Electronegativity: Measure of an atom’s attraction to electrons (scaled from 0-4).

Noncovalent Bonds

Types of Noncovalent Bonds

Noncovalent bonds are interactions between two atoms resulting from full or partial charges. Unlike covalent bonds, there is no sharing of electrons. Several types of noncovalent bonds are commonly found in biology:

• Ionic Bonds: Electrical attractions between oppositely charged ions.

• Hydrogen Bonds: Interaction between a highly electronegative atom (F, O, or N) and a hydrogen atom.

• Van der Waals Bonds: Very weak interactions due to temporary dipoles.

Ionic Bonding

Ions: Anions vs. Cations

Ions are atoms or molecules with a net electrical charge, resulting from the gain or loss of electrons:

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

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

Ionic Bonds

Ionic bonds are electrical attractions between oppositely charged ions (cations and anions). The transfer of electrons can fill the valence shells of both atoms and create charges.

• Example: Formation of ionic bond in sodium chloride (NaCl).

Hydrogen Bonding

Hydrogen Bonds

A hydrogen bond is an interaction between a highly electronegative atom (F, O, or N) and a hydrogen atom. Individually, hydrogen bonds are weak, but collectively, they can be quite strong. Hydrogen bonds are important in biology, including the properties of water and the structure of macromolecules.

• Example: Water molecules interact through hydrogen bonds.

Key Equations and Concepts

Atomic Mass Calculation

The average atomic mass of an element is calculated by considering the masses and abundances of its isotopes:

• Formula:

Half-Life Calculation

The amount of radioactive isotope remaining after n half-lives:

• Formula: