Introduction to Chemistry

What Is Chemistry?

Chemistry is the science that seeks to understand the behavior of matter by studying the behavior of atoms and molecules. The central goal of chemistry is to understand the substances around us by understanding the atoms and molecules that compose them.

• Matter: Anything that occupies space and has mass.

• Atoms: Fundamental building blocks of ordinary matter.

• Molecules: Groups of atoms bonded together in specific geometrical arrangements.

• Structure determines properties: Small differences in atoms and molecules can result in large differences in the substances they compose.

• Example: Water molecule consists of two hydrogen atoms and one oxygen atom, forming H2O.

Matter and Its Classification

States of Matter

Matter exists in three main states: solid, liquid, and gas. Solids can be crystalline (ordered structure) or amorphous (disordered structure).

• Solid: Definite shape and volume.

• Liquid: Definite volume, indefinite shape.

• Gas: Indefinite shape and volume.

Mixtures and Pure Substances

Matter can be classified as a mixture or a pure substance.

• Mixture: Combination of two or more substances; can be homogeneous (uniform composition) or heterogeneous (non-uniform composition).

• Pure Substance: Consists of only one type of atom or molecule; can be an element or a compound.

• Element: Pure substance made of only one type of atom.

• Compound: Pure substance composed of two or more elements chemically combined.

Physical and Chemical Changes

Changes in matter are classified as physical or chemical.

• Physical Change: Alters the state or appearance but not the composition (e.g., melting ice).

• Chemical Change: Alters the composition of matter (e.g., burning wood).

Energy and Properties of Matter

Energy Concepts

Energy is the capacity to do work. It exists in two main forms:

• Kinetic Energy: Energy of motion.

• Potential Energy: Energy due to position or composition.

• Law of Conservation of Energy: Energy cannot be created or destroyed, only transformed.

Intensive vs. Extensive Properties

• Intensive Properties: Independent of the amount of substance (e.g., density, temperature).

• Extensive Properties: Dependent on the amount of substance (e.g., mass, volume).

Measurement and Units

Standard Units and SI System

Measurements in chemistry use standard units. The International System of Units (SI) is based on the metric system.

• Length: Meter (m)

• Mass: Kilogram (kg)

• Time: Second (s)

• Temperature: Kelvin (K)

• Amount of substance: Mole (mol)

• Electric current: Ampere (A)

• Luminous intensity: Candela (cd)

Temperature Scales

Three common temperature scales are Fahrenheit, Celsius, and Kelvin. Conversion formulas:

SI Prefix Multipliers

Prefix multipliers change the value of a unit by powers of ten.

Derived Units: Volume and Density

• Volume: (e.g., , L)

• Density:

Dimensional Analysis and Unit Conversion

Dimensional analysis uses units as a guide to solving problems. Conversion factors are used to change from one unit to another.

• Unit equation:

• Conversion factor: or

• General form:

Significant Figures, Precision, and Accuracy

Significant Figures

Significant figures reflect the precision of a measurement. Rules for counting significant figures:

• All nonzero digits are significant.

• Interior zeroes (between nonzero digits) are significant.

• Leading zeroes (to the left of the first nonzero digit) are not significant.

• Trailing zeroes after a decimal point are significant.

• Trailing zeroes before a decimal point (and after a nonzero number) are significant.

• Trailing zeroes before an implied decimal point are ambiguous; use scientific notation to clarify.

Significant Figures in Calculations

• Multiplication/Division: Result carries the same number of significant figures as the factor with the fewest significant figures.

• Addition/Subtraction: Result carries the same number of decimal places as the quantity with the fewest decimal places.

• Rounding: Round down if the digit dropped is four or less; round up if five or more.

• Multistep Calculations: Round only the final answer.

Precision and Accuracy

• Accuracy: How close a measured value is to the actual value.

• Precision: How close a series of measurements are to one another.

• Random error: Equal probability of being too high or too low.

• Systematic error: Tends toward being either too high or too low.

Atomic Theory and Structure

History and Laws Leading to Atomic Theory

The development of atomic theory was based on several key laws:

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

• Law of Definite Proportions: All samples of a given compound have the same proportions of their constituent elements.

• Law of Multiple Proportions: When two elements form different compounds, the masses of one element that combine with 1 gram of the other can be expressed as a ratio of small whole numbers.

Dalton's Atomic Theory

• Each element is composed of tiny, indestructible particles called atoms.

• All atoms of a given element have the same mass and properties.

• Atoms combine in simple, whole-number ratios to form compounds.

• Atoms of one element cannot change into atoms of another element.

Discovery of Subatomic Particles

• Electron: Discovered by J.J. Thomson using cathode ray experiments; negatively charged, low-mass particle present in all atoms.

• Proton: Positively charged particle in the nucleus.

• Neutron: Neutral particle in the nucleus; similar mass to proton.

Structure of the Atom

• Most of the atom's mass and all positive charge are in the nucleus.

• Electrons are dispersed in empty space around the nucleus.

• Atoms are electrically neutral: number of protons equals number of electrons.

Subatomic Particles Table

Elements, Isotopes, and the Periodic Table

Atomic Number and Chemical Symbols

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

• Chemical Symbol: One- or two-letter abbreviation for each element.

Isotopes

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

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

• Notation: or X-A (e.g., Ne-20).

• Natural Abundance: Relative amount of each isotope in a sample.

Ions

• Cation: Positively charged ion (loss of electrons).

• Anion: Negatively charged ion (gain of electrons).

• In neutral atoms, number of electrons equals number of protons.

• During chemical changes, atoms can lose or gain electrons to form ions.

The Periodic Table and Classification of Elements

Periodic Law and Table Structure

The periodic table organizes elements by increasing atomic number. Elements with similar properties are grouped together.

• Groups (columns): 18 vertical columns; elements in a group have similar properties.

• Periods (rows): 7 horizontal rows.

• Main-group elements: Groups 1, 2, and 13–18.

• Transition elements: Groups 3–12.

Classification of Elements

• Metals: Good conductors, malleable, ductile, shiny, tend to lose electrons.

• Nonmetals: Poor conductors, not ductile or malleable, tend to gain electrons.

• Metalloids: Exhibit mixed properties; some are semiconductors.

Special Groups

• Noble Gases (Group 8A): Unreactive, stable.

• Alkali Metals (Group 1A): Highly reactive, tend to lose one electron.

• Alkaline Earth Metals (Group 2A): Fairly reactive, tend to lose two electrons.

• Halogens (Group 7A): Very reactive nonmetals, tend to gain one electron.

Ions and the Periodic Table

• Main-group metals form cations with charge equal to group number.

• Main-group nonmetals form anions with charge equal to group number minus eight.

• Transition elements may form various ions with different charges.

Atomic Mass and Mass Spectrometry

Atomic Mass

Atomic mass is the average mass of an element's isotopes, weighted by their natural abundance.

• Formula:

• Example: Chlorine atomic mass calculation:

Mass Spectrometry

Mass spectrometry is used to determine the masses and abundances of isotopes.

• Percent abundance is calculated from the intensity of peaks.

• Example: If Ag-107 has intensity 100.0% and Ag-109 is 92.90%, then Ag-107 abundance =

Summary of Key Concepts

• Modern Atomic Theory and the three laws (conservation of mass, definite proportions, multiple proportions).

• Structure of the atom: protons, neutrons, electrons.

• Atomic mass unit (amu), atomic number (Z), chemical symbols.

• Isotopes, mass number (A), natural abundance.

• Calculation of atomic mass from isotope masses and abundances.

• Periodic table structure, groups, periods, classification of elements.

• Properties of metals, nonmetals, metalloids.

• Formation of ions and their charges based on periodic table position.

• Measurement, units, significant figures, precision, and accuracy.