Fundamentals of Atomic Structure and Matter Composition

The material can be a mixture or a pure substance. Materials can be pure substances, which include chemical elements or chemical compounds. For example, Oxygen is a chemical element that cannot be decomposed by chemical procedures. Water is a compound ($\text{H}_2\text{O}$) that can be decomposed into oxygen and hydrogen by chemical methods (like passing an electric current).

The Atom: Building Block of Matter

An atom is the smallest unit that exists in a chemical element. A material is discontinuous and formed by tiny particles called atoms. The size of atoms is on the order of $10^{-12}$ meters (which equals 1 picometer, $\text{pm}$, or $0.000000000001\text{m}$).

Subatomic Particles

• Protons: Carry a positive electric charge ($+$). The charge is an elementary charge, the smallest known. The mass of a proton is $1.67 \times 10^{-27}\text{ kg}$.
• Neutrons: Have no electric charge and their mass is approximately equal to that of the proton. Protons and neutrons are collectively called nucleons.
• Electrons: Carry an electric charge equal in magnitude but opposite to the proton (negative). Electrons have a very small mass; the electron is about 1840 times lighter than a proton.

Atomic Identity and Notation

Atomic Number and Mass Number

• Atomic Number ($ ext{Z}$): The number of protons in an atom's nucleus.
• Mass Number ($ ext{A}$): The total number of protons and neutrons in an atom's nucleus. $\text{A} = \text{Z} + \text{N}$ (where $\text{N}$ is the number of neutrons).
• Isotopes: Atoms of the same element (same atomic number $\text{Z}$) but with a different number of neutrons $\text{N}$ (and thus a different mass number $\text{A}$).

Isotope Notation: (Standard notation is typically $\text{^A}_\text{Z}\text{X}$)

Ions and Mass Calculation

Ions

Ions: Atoms or groups of atoms with a net electrical charge (due to an excess or defect of electrons).

• Anions: Ions with a negative charge (excess electrons), e.g., $\text{Cl}^-$, $\text{S}^{2-}$.
• Cations: Ions with a positive charge (lack of electrons), e.g., $\text{H}^+$, $\text{He}^{2+}$.

Atomic and Molecular Mass

• Atomic Mass: The mass of an atom is expressed in $\text{amu}$ (atomic mass units). One $\text{amu}$ is defined as one-twelfth ($\frac{1}{12}$) of the mass of a carbon-12 atom. Thus, the mass of one atom of carbon-12 is $12\text{ amu}$.
• Molecular Mass: A molecule is a set of a well-defined number of atoms of one or more elements. The mass of a molecule is the sum of the masses of its component atoms. The average masses of atoms of each element are found in the periodic table. Example: $\text{M}[\text{H}_2\text{O}] = (2 \times 1.0079\text{ amu}) + 15.9994\text{ amu} = 18.0152\text{ amu}$.