Chemistry: Atoms and Molecules

• Chemistry is the study of matter, its properties, and how it changes.

Atoms

• The basic building blocks of all matter are atoms
• Atoms are the smallest particle of an element
• Atoms are composed of protons, neutrons, and electrons.
• Protons possess a positive charge
• Electrons possess a negative charge
• Neutrons possess no charge
• The number of protons defines the element
• Isotopes are atoms of the same element with differing neutron counts
• Atomic number defines the number of protons
• Protons and neutrons combined, dictate an atom's mass number.
• Atoms are neutral, having equal proton and electron counts
• Electrons are arranged in energy levels or shells around the nucleus.
• Valence shell: the outermost shell
• Valence electrons: electrons in the outermost shell.
• Valence electrons dictate an atom's chemical properties.
• Atoms bond to form molecules or compounds

Molecules

• Molecules consist of two or more atoms bonded by chemical bonds.
• Molecules can comprise identical elements (e.g., O₂) or varying elements (e.g., H₂O).
• Chemical formulas denote the quantity and type of atoms present (e.g. H₂O = 2 hydrogen atoms and 1 oxygen atom).
• A molecule's shape influences its properties.
• Intermolecular forces act as attractions between molecules.

Elements

• Elements are pure substances of atoms with identical proton numbers
• Elements cannot be chemically broken down
• The periodic table organizes elements by atomic number.
• Elements in the same group share similar chemical properties
• Elements are categorized as metals, nonmetals, or metalloids.
• Metals are shiny, conductive, and malleable.
• Nonmetals are dull, nonconductive, and brittle.
• Metalloids exhibit properties of both metals and nonmetals.

Compounds

• A substance of chemically bonded elements in a fixed ratio forms a compound.
• A compound's properties differ from its constituent elements.
• Compounds can be ionic or covalent.
• Ionic compounds form via electron transfer, creating ions.
• Ions are atoms or molecules with a net electric charge from losing or gaining electrons.
• Cations are positively charged ions
• Anions are negatively charged ions
• Covalent compounds are formed by the sharing of electrons between atoms.
• A compound's chemical formula indicates its atomic composition.
• Compounds are named systematically

Chemical Reactions

• Rearrangement of atoms and molecules forms new substances through chemical reaction.
• Chemical formulas and symbols in chemical equations represent chemical reactions.
• The law of conservation of mass dictates matter cannot be created or destroyed in a chemical reaction.
• Balanced equations have equal atom counts on both sides.
• Chemical reactions involve chemical bond formation and breakage.
• Energy is either absorbed or released during a chemical reaction.
• Exothermic reactions release energy, typically as heat.
• Endothermic reactions absorb energy.
• Activation energy is the minimum energy for a reaction.
• Catalysts accelerate reaction rates by reducing activation energy.
• Reaction rates are affected by temperature, concentration, surface area, and catalysts.