Introduction to Atoms, Molecules, Compounds, and Chemical Measurements

Molecules and Compounds

When atoms combine, they form molecules. A molecule is defined as a group of atoms bonded together, representing the smallest fundamental unit of a chemical substance that can participate in a chemical reaction.

Distinguishing Molecules from Compounds

It is important to distinguish between a simple molecule and a compound:

1. Molecules: Can be made of the same type of atom (e.g., $\text{O}_2$ or $\text{S}_8$).
2. Compounds: These are molecules made of two or more different types of atoms. In a compound, the relative amounts of each element remain constant.

Chemical Formulas

Chemists use a shorthand language called chemical formulas to describe the composition of substances. A formula lists the elemental symbols and uses subscripts to indicate the number of atoms of each element present.

Example: Water ($\text{H}_2\text{O}$)

• H is the symbol for Hydrogen.
• The subscript 2 indicates there are two hydrogen atoms.
• O is the symbol for Oxygen. Since there is no subscript, it is understood there is only one oxygen atom.

Exam Tip: Always remember that subscripts in chemical formulas must be whole numbers (integers) because you cannot have a fraction of an atom in a molecule.

Properties of Matter

To describe substances, chemists categorize their properties into two main groups: physical and chemical.

Physical vs. Chemical Properties

• Physical Properties: These can be measured or observed without changing the identity of the substance (e.g., color, size, mass, melting point, or density).
• Chemical Properties: These are only observable during a chemical transformation or reaction (e.g., flammability, the ability to rust, or reactivity with water).

Intensive vs. Extensive Properties

Physical properties can be further divided based on whether they depend on the amount of matter present:

• Intensive Properties: Do not depend on the size or amount of the sample. Examples include density, color, melting point, and conductivity.
• Extensive Properties: Do depend on the size or amount of the sample. Examples include mass, volume, and length.

Visualizing the Microscopic World

Because atoms and molecules are too small for light microscopes, scientists use advanced technology to "see" them.

• Scanning Tunneling Microscopy (STM): Best suited for imaging individual atoms on a surface.
• Atomic Force Microscopy (AFM): Better suited for observing larger structures, such as strands of DNA.
• Magnetic Resonance Imaging (MRI): While used by physicians to scan organs, when combined with AFM and extreme cooling, it can even detect individual spinning electrons.

Chemical Measurements and Units

Scientific accuracy relies on three aspects: numerical magnitude, a unit of scale, and precision.

The SI System

The International System of Units (SI) is the standard used by the scientific community. Key base units include:

• Mass: kilogram (kg)
• Length: meter (m)
• Time: second (s)
• Temperature: Kelvin (K)
• Amount: mole (mol)

To handle very large or very small numbers, scientists use scientific notation ($A \times 10^n$) and prefixes (such as nano- for $10^{-9}$ or kilo- for $10^3$).

Precision and Accuracy

• Precision: Refers to the reproducibility of measurements (how close repeated measurements are to each other).
• Accuracy: Refers to how close a measurement is to the true or accepted value.

Note on Dimensional Analysis: When converting units (e.g., gallons to liters), always use "unit factors" to ensure that the unwanted units cancel out, leaving only the desired unit.