Atoms and Elements

Definition and Structure of Matter

Matter is anything that occupies space and has mass. All matter is composed of chemical elements, which are pure substances consisting of only one type of atom. Atoms are the smallest units of elements and, therefore, the smallest units of matter that retain the properties of an element.

• Matter: Includes all physical substances (e.g., organisms, rocks, water).

• Chemical Element: A pure substance made of only one kind of atom.

• Atom: The smallest unit of an element, maintaining its chemical identity.

Atomic Structure

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons. These particles differ in charge, mass, and location within the atom.

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

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

• Electron: Negatively charged, orbits the nucleus in electron shells, negligible mass.

Elements of Life and the Periodic Table

Of all known elements, only a small subset is essential for life. The periodic table organizes elements by their chemical properties. The majority of living matter is composed of six elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (CHNOPS).

• Major Elements: Required in large amounts (e.g., CHNOPS).

• Trace Elements: Required in minute amounts but essential for life.

Atomic Properties

Atomic Number, Mass Number, and Atomic Mass

Each atom has unique properties that distinguish it from others:

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

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

• Atomic Mass: The weighted average mass of all isotopes of an element.

Formula:

Electron Configuration and Energy Shells

Electron Orbitals and Shells

Electrons occupy specific energy levels or shells around the nucleus. The arrangement of electrons determines the chemical behavior of an atom.

• Electron Shells: 3D regions where electrons are likely to be found.

• Valence Electrons: Electrons in the outermost shell, important for chemical bonding.

• Shell Capacities: 1st shell holds up to 2 electrons; 2nd shell up to 8 electrons; higher shells hold more.

The Octet Rule

The octet rule states that atoms are most stable when their outermost (valence) shell is fully occupied, typically with 8 electrons (except for hydrogen and helium, which are stable with 2).

• Atoms with incomplete valence shells are more reactive.

• Atoms tend to gain, lose, or share electrons to achieve a full valence shell.

Isotopes and Radioactivity

Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons, resulting in different mass numbers.

• Stable Isotopes: Do not change or decay over time.

• Radioactive Isotopes: Unstable and decay, emitting radiation.

Radioactive Decay and Half-Life

Radioactive isotopes decay at a predictable rate, described by their half-life—the time required for half of the atoms in a sample to decay.

• Applications: Used in radiometric dating (e.g., carbon-14 dating) and medical imaging.

Chemical Bonding

Introduction to Chemical Bonds

Chemical bonds are attractive forces that hold atoms together in molecules and compounds. There are two main categories: intramolecular (within a molecule) and intermolecular (between molecules) bonds.

• Molecule: Two or more atoms chemically bonded (e.g., O2).

• Compound: Molecule composed of two or more different elements (e.g., H2O).

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

Covalent Bonds

Covalent bonds involve the sharing of electron pairs between atoms. The type of covalent bond depends on the difference in electronegativity between the atoms involved.

• Nonpolar Covalent Bond: Equal sharing of electrons (atoms have similar electronegativities).

• Polar Covalent Bond: Unequal sharing of electrons (atoms have different electronegativities), resulting in partial charges (δ+ and δ−).

Electronegativity: A measure of an atom's ability to attract shared electrons (scale: 0–4).

Noncovalent Bonds

Noncovalent bonds are interactions that do not involve the sharing of electrons. These include ionic bonds, hydrogen bonds, and van der Waals forces.

• Ionic Bonds: Formed by the transfer of electrons from one atom to another, resulting in oppositely charged ions (cations and anions) that attract each other.

• Hydrogen Bonds: Weak interactions between a hydrogen atom covalently bonded to a highly electronegative atom (F, O, or N) and another electronegative atom.

• Van der Waals Forces: Weak attractions due to temporary dipoles in molecules.

Ions: Anions and Cations

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

• Anion: Negatively charged ion (gains electrons).

• Cation: Positively charged ion (loses electrons).

Hydrogen Bonding in Water

Hydrogen bonds are crucial in biology, especially in water and macromolecular structures. In water, the polar covalent bonds between hydrogen and oxygen create partial charges, allowing hydrogen bonds to form between molecules.

• Hydrogen bonds are individually weak but collectively strong.

• Responsible for water's unique properties and the structure of DNA and proteins.

Summary Table: Types of Chemical Bonds

Additional info: This guide covers foundational concepts in atomic structure, isotopes, and chemical bonding, which are essential for understanding more advanced topics in chemistry such as chemical reactions, molecular structure, and the behavior of matter.