GOB Chemistry Study Guide: Elements, Measurements, and Chemical Changes

Measurement and Significant Figures

Exact Numbers and Significant Figures

Understanding measurements and significant figures is essential in chemistry for accurate calculations and reporting.

• Exact Numbers: Numbers that are counted or defined, not measured. Examples include 1 inch = 2.54 cm and 1 m = 100 cm.

• Significant Figures (Sig Figs): Digits in a measurement that are known with certainty plus one estimated digit.

• Rules for Significant Figures:

  • All nonzero digits are significant.

  • Zeros between nonzero digits are significant.

  • Leading zeros are not significant.

  • Trailing zeros in a decimal number are significant.

• Scientific Notation: Only the coefficient's digits are considered significant.

• Example: In 0.00406 g, there are three significant figures.

Additional info: When multiplying or dividing, the result should have the same number of significant figures as the measurement with the fewest significant figures. When adding or subtracting, the result should have the same number of decimal places as the measurement with the fewest decimal places.

Conversion Factors and Dimensional Analysis

Conversion factors are used to change units in measurements. Dimensional analysis ensures that units cancel appropriately.

• Conversion Factor: A ratio that expresses how many of one unit are equal to another unit.

• Example: $1~\text{in} = 2.54~\text{cm}$

• Dimensional Analysis: A method to convert units using conversion factors.

• Formula: $\text{Quantity in new units} = \text{Original quantity} \times \frac{\text{Conversion factor}}{1}$

Classification of Matter

Pure Substances and Mixtures

Matter can be classified based on its composition and uniformity.

• Pure Substance: Matter with a fixed composition; includes elements and compounds.

• Mixture: A combination of two or more substances that retain their individual properties.

• Types of Mixtures:

  • Homogeneous Mixture: Uniform composition throughout (e.g., salt water).

  • Heterogeneous Mixture: Non-uniform composition (e.g., salad).

• Elements: Pure substances consisting of only one type of atom.

• Compounds: Pure substances composed of two or more elements chemically combined.

Physical and Chemical Changes

Changes in matter can be classified as physical or chemical.

• Physical Change: Change in state or appearance without altering composition (e.g., melting, boiling).

• Chemical Change: Change that produces new substances (e.g., rusting, combustion).

• Examples:

  • Sublimation: Solid to gas (e.g., dry ice).

  • Deposition: Gas to solid.

Elements, Groups, and Periods

Periodic Table Organization

The periodic table organizes elements by increasing atomic number and recurring chemical properties.

• Groups (Families): Vertical columns; elements in a group have similar properties.

• Periods: Horizontal rows; properties change progressively across a period.

• Alkali Metals: Group 1 elements (e.g., sodium, potassium); highly reactive.

• Halogens: Group 17 elements (e.g., chlorine, fluorine); very reactive nonmetals.

• Noble Gases: Group 18 elements (e.g., helium, neon); inert gases.

Atomic Structure

Atoms consist of protons, neutrons, and electrons.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus.

• Electron: Negatively charged particle orbiting the nucleus.

• Atomic Number ($Z$): Number of protons in an atom.

• Mass Number ($A$): Sum of protons and neutrons.

• Isotope: Atoms of the same element with different numbers of neutrons.

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

Example: Carbon has atomic number 6, mass number 12 (most common isotope), and 4 valence electrons.

Chemical Compounds and Formulas

Types of Compounds

Chemical compounds are formed by the combination of elements.

• Ionic Compounds: Made of metals and nonmetals; consist of ions held together by electrostatic forces.

• Covalent Compounds: Made of nonmetals; atoms share electrons.

• Polyatomic Ions: Ions composed of two or more atoms covalently bonded (e.g., $\text{NO}_3^-$, $\text{SO}_4^{2-}$).

Naming Compounds and Writing Formulas

Systematic rules are used to name compounds and write their chemical formulas.

• Ionic Compounds: Name the cation first, then the anion (e.g., sodium chloride).

• Covalent Compounds: Use prefixes to indicate the number of atoms (e.g., carbon dioxide).

• Polyatomic Ions: Memorize common names and formulas.

Temperature and Energy

Temperature Scales and Conversions

Temperature is measured in different units and can be converted using formulas.

• Celsius (°C): Water freezes at 0°C and boils at 100°C.

• Fahrenheit (°F): Water freezes at 32°F and boils at 212°F.

• Kelvin (K): Absolute temperature scale; $0~\text{K}$ is absolute zero.

• Conversion Formulas:

  • $T_{\text{C}} = \frac{5}{9}(T_{\text{F}} - 32)$

  • $T_{\text{F}} = \frac{9}{5}T_{\text{C}} + 32$

  • $T_{\text{K}} = T_{\text{C}} + 273.15$

• Example: Normal body temperature is 37°C, which is 98.6°F.

Summary Table: Classification of Matter

The following table summarizes the classification of matter:

Type	Description	Examples
Element	Pure substance, one type of atom	Hydrogen, Oxygen
Compound	Pure substance, two or more elements chemically combined	Water (H2O), Carbon dioxide (CO2)
Homogeneous Mixture	Uniform composition throughout	Salt water, air
Heterogeneous Mixture	Non-uniform composition	Salad, granite

Study Tips

• Review all PowerPoint slides and dry erase board notes.

• Practice solving problems, especially those involving conversion factors and significant figures.

• Memorize key formulas, names, and medical uses as indicated in your course materials.

• Test yourself by completing homework and practice questions.

• Read ahead before class to reinforce understanding.