General Chemistry Study Guide: Matter, Measurement, Atoms, Molecules, and Ions

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Matter is anything that has mass and occupies space. It can be classified based on its composition and uniformity.

Pure Substances: Matter with a fixed composition and distinct properties. Examples: water (H2O), carbon dioxide (CO2).
Mixtures: Physical combinations of two or more substances. Can be separated by physical means.
Elements: Substances that cannot be decomposed into simpler substances by chemical means. Example: O2.
Compounds: Substances composed of two or more elements chemically combined in fixed proportions. Example: NaCl.
Homogeneous Mixture (Solution): Uniform composition throughout. Example: saltwater.
Heterogeneous Mixture: Non-uniform composition. Example: sand and iron filings.
Law of Constant Composition (Law of Definite Proportions): A given compound always contains the same proportion of elements by mass.

Properties describe the characteristics and behavior of matter.

Physical Properties: Can be observed without changing the substance's identity (e.g., color, melting point, density).
Chemical Properties: Describe how a substance reacts to form other substances (e.g., flammability, reactivity).
Physical Change: Alters appearance but not composition (e.g., melting ice).
Chemical Change: Alters the composition, forming new substances (e.g., rusting iron).
Intensive Properties: Independent of the amount of substance (e.g., density, boiling point).
Extensive Properties: Depend on the amount of substance (e.g., mass, volume).

Measurements are expressed using standard units. The SI (International System of Units) is commonly used in science.

Basic SI Units:
- Length: meter (m)
- Mass: kilogram (kg)
- Time: second (s)
- Temperature: kelvin (K)
- Amount of substance: mole (mol)
- Volume: cubic meter (m3)
Derived Units: Combinations of base units (e.g., density = kg/m3).
Exact Numbers: Defined values or counted quantities (e.g., 1 dozen = 12).
Inexact Numbers: Measured values, subject to uncertainty.
Metric Prefixes: Indicate powers of ten (e.g., kilo- = 103, milli- = 10-3).

All measurements have some degree of uncertainty, reflected in significant figures.

Precision: How closely repeated measurements agree with each other.
Accuracy: How closely a measurement agrees with the true value.
Significant Figures: Digits that reflect the precision of a measurement.
Rules for Significant Figures:
- Nonzero digits are always significant.
- Zeros between nonzero digits are significant.
- Leading zeros are not significant.
- Trailing zeros are significant only if there is a decimal point.
Rounding Off: Round to the correct number of significant figures based on the operation performed.

Dimensional analysis uses conversion factors to convert between units.

Conversion Factors: Ratios derived from the equality between two units (e.g., 1 inch = 2.54 cm).
Process: Multiply by conversion factors so units cancel appropriately.
Example: Convert 25.0 cm to inches:

Atoms are composed of three main subatomic particles:

Each element is represented by a unique one- or two-letter symbol (e.g., H for hydrogen, Na for sodium).
Common elements to know: H, He, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca, Fe, Cu, Zn, Br, I, etc.

Atomic Number (Z): Number of protons in the nucleus; defines the element.
Mass Number (A): Total number of protons and neutrons in the nucleus.
Isotope Symbol (Nuclide Symbol): Written as , where X is the element symbol.
Example: has 6 protons and 8 neutrons.

Ion: Atom or molecule with a net electric charge due to loss or gain of electrons.
Cation: Positively charged ion (loss of electrons).
Anion: Negatively charged ion (gain of electrons).
Isotope: Atoms of the same element (same Z) with different numbers of neutrons (different A).
Example: , , and are isotopes of carbon.

Atomic Mass Unit (amu): Defined as 1/12 the mass of a carbon-12 atom.
Measuring Atomic Mass: Atomic masses are measured relative to the carbon-12 standard.
Average Atomic Mass: Weighted average of the masses of all naturally occurring isotopes.

Main-group elements tend to form ions with predictable charges based on their group number.
Group 1: +1, Group 2: +2, Group 17: -1, Group 16: -2, etc.

Rows: Called periods.
Columns: Called groups or families.
As you move across a period (left to right), the atomic number increases by one for each element.
Elements in the same group have similar chemical properties.
Special Groups:
- Group 1: Alkali metals
- Group 2: Alkaline earth metals
- Group 17: Halogens
- Group 18: Noble gases
- Transition elements: Groups 3-12
- Main-group elements: Groups 1, 2, 13-18
- Inner transition elements: Lanthanides and actinides
Metals, Metalloids, Nonmetals:
- Metals: Left and center of the table
- Metalloids (semimetals): Border between metals and nonmetals (e.g., Si, B, As)
- Nonmetals: Upper right of the table
Diatomic Elements ("Big Seven"): H2, N2, O2, F2, Cl2, Br2, I2

Molecular Compounds: Composed of molecules formed by covalent bonds between nonmetals.
Ionic Compounds: Composed of cations and anions held together by electrostatic forces (usually metal + nonmetal).
Classification: NaCl (ionic), H2O (molecular).