Section 2.1 Chemistry and the Elements

Elements and Atomic Symbols

Chemistry is the study of matter, its properties, and the changes it undergoes. Elements are the simplest substances that cannot be broken down by chemical means. Each element is represented by a unique chemical symbol.

• Element: A pure substance consisting of only one type of atom.

• Elemental Symbols: One- or two-letter abbreviations for elements (e.g., H for hydrogen, O for oxygen).

• Periodic Table: A systematic arrangement of elements based on atomic number and properties.

Example: The element sodium is represented by the symbol Na.

Section 2.2 Elements and the Periodic Table

Classification of Elements

The periodic table organizes elements by increasing atomic number and groups them based on similar chemical properties.

• Metals, Nonmetals, Metalloids: Elements are classified as metals (good conductors, malleable), nonmetals (poor conductors, brittle), or metalloids (properties intermediate between metals and nonmetals).

• Groups and Periods: Vertical columns are groups (families) with similar properties; horizontal rows are periods.

• Main Group Elements: Elements in groups 1, 2, and 13–18.

• Transition Metals: Elements in groups 3–12.

Example: Iron (Fe) is a transition metal, while oxygen (O) is a nonmetal in the main group.

Section 2.3 Some Common Groups of Elements and Their Properties

Element Groups and Periodic Trends

Certain groups of elements share characteristic properties and reactivity.

• Alkali Metals (Group 1): Highly reactive, soft metals (e.g., sodium, potassium).

• Alkaline Earth Metals (Group 2): Reactive metals, harder than alkali metals (e.g., magnesium, calcium).

• Halogens (Group 17): Very reactive nonmetals (e.g., chlorine, fluorine).

• Noble Gases (Group 18): Inert gases with very low reactivity (e.g., helium, neon).

• Transition Metals: Elements with variable oxidation states and colored compounds.

Example: Chlorine (Cl) is a halogen, while neon (Ne) is a noble gas.

Section 2.4 Conservation of Mass and the Law of Definite Proportions

Fundamental Laws of Chemistry

Two foundational laws govern chemical reactions and the composition of compounds.

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

• Law of Definite Proportions: A chemical compound always contains the same elements in the same proportion by mass.

Equation:

Example: Water (H2O) always contains hydrogen and oxygen in a 2:1 ratio by number of atoms and about 1:8 by mass.

Section 2.5 Atomic Structure: Electrons

Discovery of the Electron

Experiments in the late 19th and early 20th centuries revealed the existence of subatomic particles.

• Thomson's Cathode Ray Tube Experiment: Discovered the electron as a negatively charged particle.

• Millikan's Oil Drop Experiment: Measured the charge of the electron.

Example: The electron has a charge of coulombs.

Section 2.6 Atomic Structure: Protons and Neutrons

Discovery of the Nucleus

Further experiments established the structure of the atomic nucleus.

• Rutherford's Gold Foil Experiment: Demonstrated that atoms have a small, dense, positively charged nucleus.

• Protons: Positively charged particles in the nucleus.

• Neutrons: Neutral particles in the nucleus.

Example: A carbon atom has 6 protons and (usually) 6 neutrons in its nucleus.

Section 2.8 Atomic Numbers

Atomic Number and Isotopes

The atomic number defines the identity of an element, while isotopes are atoms of the same element with different numbers of neutrons.

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

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

• Isotopes: Atoms with the same atomic number but different mass numbers.

Equation:

where = mass number, = atomic number, = number of neutrons.

Example: Carbon-12 and Carbon-14 are isotopes of carbon.

Section 2.9 Atomic Mass (Weight) and the Mole

Atomic Mass and Avogadro's Number

Atomic mass is the weighted average mass of an element's isotopes. The mole is a counting unit for atoms and molecules.

• Atomic Mass Unit (u or amu): 1/12 the mass of a carbon-12 atom.

• Average Atomic Mass: Weighted average based on isotopic abundance.

• Avogadro's Number (): entities per mole.

• Mole: The amount of substance containing Avogadro's number of particles.

Equation:

Example: 1 mole of carbon-12 has a mass of 12 grams and contains atoms.

Section 2.10 Mixtures and Chemical Compounds: Molecules and Covalent Bonds

Classification of Matter

Matter can be classified as mixtures or pure substances. Compounds are pure substances composed of two or more elements chemically combined.

• Mixture: Physical combination of two or more substances (e.g., air, saltwater).

• Compound: Substance formed when two or more elements are chemically bonded (e.g., H2O).

• Molecule: Two or more atoms covalently bonded together.

• Covalent Bond: Chemical bond formed by the sharing of electron pairs between atoms.

Example: Water is a compound and a molecule; air is a mixture.

Section 2.11 Ions and Ionic Bonds

Ions and Ionic Compounds

Ions are charged particles formed when atoms gain or lose electrons. Ionic bonds are formed by the electrostatic attraction between oppositely charged ions.

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

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

• Ionic Compound: Compound composed of cations and anions (e.g., NaCl).

Example: Sodium chloride (NaCl) consists of Na+ and Cl- ions.

Section 2.12 Naming Chemical Compounds

Nomenclature of Compounds

Chemical compounds are named according to systematic rules based on their composition and structure.

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

• Binary Covalent Compounds: Use prefixes to indicate the number of each atom (e.g., CO2: carbon dioxide).

• Polyatomic Ions: Ions composed of more than one atom (e.g., SO42-: sulfate).

• Acids: Compounds that release H+ ions in solution (e.g., HCl: hydrochloric acid).

Example: The formula for calcium nitrate is Ca(NO3)2.

Summary Table: Classification of Elements

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Chemistry curriculum.