Metric System and Conversions

Metric System

The metric system is a standardized system of measurement used in science. It is based on units of ten and includes base units such as meter (length), kilogram (mass), and second (time).

• Base Units: meter (m), kilogram (kg), second (s), mole (mol), kelvin (K), ampere (A), candela (cd)

• Prefix Multipliers: Used to express multiples or fractions of base units (e.g., kilo-, centi-, milli-).

Conversions

• Kelvin to Celsius: Temperature conversions are common in chemistry.

Formula:

• Prefix Multipliers: Examples include 1 kilometer (km) = 1000 meters (m), 1 milligram (mg) = 0.001 grams (g).

Example: Convert 25°C to Kelvin.

Density

Definition and Calculations

Density is the mass of a substance per unit volume. It is a physical property used to identify substances.

• Formula:

• Units: Commonly g/cm3 or kg/m3

Example: If a block has a mass of 50 g and a volume of 20 cm3, its density is:

Precision and Accuracy

These terms describe the quality of measurements.

• Precision: How close repeated measurements are to each other.

• Accuracy: How close a measurement is to the true or accepted value.

Example: If you weigh a sample three times and get 1.01 g, 1.00 g, and 1.02 g, your measurements are precise. If the true mass is 1.00 g, they are also accurate.

Atoms, Subatomic Particles, and Isotopes

Subatomic Particles

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

• Electrons: Negatively charged particles orbiting the nucleus.

Isotopes

Isotopes are atoms of the same element with different numbers of neutrons.

• Notation: where A = mass number, Z = atomic number, X = element symbol.

Example: is a carbon isotope with 6 protons and 8 neutrons.

Ions: Anions and Cations

Ions are atoms or molecules 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).

Example: Na+ is a cation; Cl- is an anion.

The Periodic Table

Atomic Number and Atomic Weight

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

• Atomic Weight: Weighted average mass of an element's isotopes.

Groups and Classification

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

• Metals vs Nonmetals: Metals are typically shiny, conductive, and malleable; nonmetals are more variable in appearance and properties.

The Mole and Avogadro’s Number

The mole is the SI unit for amount of substance. Avogadro’s number is the number of particles in one mole.

• Avogadro’s Number: particles/mol

• Mass-Mole Conversions:

Example: 18 g of H2O is mole of water molecules.

Mixtures vs Compounds

• Mixtures: Physical combinations of two or more substances; components retain their properties and can be separated physically.

• Compounds: Chemical combinations of elements in fixed ratios; properties differ from constituent elements and can only be separated chemically.

Example: Air is a mixture; water (H2O) is a compound.

Chemical Bonds: Ionic vs Covalent

• Ionic Bonds: Formed by transfer of electrons from one atom to another (typically between metals and nonmetals).

• Covalent Bonds: Formed by sharing of electrons between atoms (typically between nonmetals).

Example: NaCl is ionic; H2O is covalent.

Diatomic (Molecular) Elements

Certain elements exist naturally as molecules composed of two atoms.

• Diatomic Elements: H2, N2, O2, F2, Cl2, Br2, I2

Charges on Groups 1, 2, and 7

Elements in these groups form ions with characteristic charges.

Acids

Acids are substances that release hydrogen ions (H+) in solution.

• Common Acids: HCl, H2SO4, HNO3

Example: HCl (hydrochloric acid) dissociates in water to form H+ and Cl-.

Formula Mass Calculations

The formula mass (or molecular mass) is the sum of the atomic masses of all atoms in a chemical formula.

• Formula:

Example: For H2O:

Chemical Equations and Balancing

Chemical equations represent chemical reactions. Balancing ensures the same number of each atom on both sides.

• Law of Conservation of Mass: Matter is neither created nor destroyed in a chemical reaction.

• Steps to Balance: 1) Write unbalanced equation, 2) Count atoms, 3) Add coefficients, 4) Check balance.

Example:

Stoichiometry

Limiting Reagents

The limiting reagent is the reactant that is completely consumed first, limiting the amount of product formed.

• Find moles of each reactant.

• Use mole ratios from the balanced equation.

• The reactant that produces the least product is limiting.

Percent Yield

Percent yield compares the actual yield to the theoretical yield.

• Formula:

Example: If the theoretical yield is 10 g and the actual yield is 8 g: