Introduction to Atoms

• Chemistry is the study of matter and its properties as well as how matter changes

Atoms

• The atom is the basic building block of matter
• Protons and neutrons exist in a central nucleus, surrounded by electrons in orbitals
• Protons have a positive charge, neutrons have no charge, and electrons have a negative charge
• The number of protons determines the element and its position on the periodic table
• Atoms are electrically neutral, meaning they have the same number of protons and electrons
• The atomic number (Z) is the number of protons in an atom
• The mass number (A) is the total number of protons and neutrons in an atom
• Isotopes are atoms of the same element with different numbers of neutrons
• Relative atomic mass is the weighted average mass of the isotopes of an element compared to 1/12th the mass of a carbon-12 atom
• Electrons are arranged in energy levels or shells around the nucleus
• The first shell can hold up to 2 electrons, the second shell can hold up to 8, and the third shell can hold up to 8 (or 18)
• The electronic configuration of an atom describes how the electrons are arranged in these shells

Molecules

• A molecule is a group of two or more atoms held together by chemical bonds
• Molecules can be elements (e.g., O2, N2) or compounds (e.g., H2O, CO2)
• Chemical bonds are formed by the sharing or transfer of electrons between atoms
• Covalent bonds involve the sharing of electrons between atoms
• These bonds typically form between two nonmetal atoms
• Ionic bonds involve the transfer of electrons from one atom to another, creating ions which are oppositely charged
• These bonds form between a metal and a nonmetal
• Metallic bonds involve the delocalization of electrons within a metal lattice
• Representing molecules requires different formulas
• Empirical formula: The simplest whole number ratio of atoms of each element in a compound
• Molecular formula: The actual number of atoms of each element in a molecule
• Structural formula: Shows the arrangement of atoms and bonds in a molecule
• Displayed formula: Shows the relative positioning of all the atoms in a molecule, and all the bonds between them

Elements

• An element is a substance that cannot be broken down into simpler substances by chemical means
• Each element is defined by its unique number of protons
• Elements are arranged on the periodic table in order of increasing atomic number
• Metals: Typically lustrous, good conductors of heat and electricity, malleable, and ductile
• Nonmetals: Typically dull, poor conductors of heat and electricity, and brittle
• Metalloids (or semi-metals): Have properties intermediate between those of metals and nonmetals, and are semiconductors

Compounds

• A compound is a substance made up of two or more elements chemically combined in a fixed ratio
• Compounds have properties that are different from those of their constituent elements
• Ionic compounds are formed from the electrostatic attraction between oppositely charged ions
• They typically have high melting and boiling points and conduct electricity when dissolved in water or melted
• Covalent compounds are formed from the sharing of electrons between atoms
• They can be solids, liquids, or gases at room temperature and typically have lower melting and boiling points than ionic compounds
• Naming compounds follows specific rules based on the elements and number of atoms present
• E.g., NaCl is sodium chloride, H2O is water, CO2 is carbon dioxide

Chemical Reactions

• A chemical reaction involves the rearrangement of atoms and molecules to form new substances
• Reactants are the substances that undergo change in the reaction, while products are the substances formed as a result of the reaction
• Chemical equations represent chemical reactions using chemical formulas and symbols
• Balanced chemical equations ensure that the number of atoms of each element is the same on both sides of the equation, following the law of conservation of mass
• State symbols indicate the physical state of each substance in the reaction: (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous solution

Types of Chemical Reactions:

• Synthesis (combination): Two or more reactants combine to form a single product
• Decomposition: A single reactant breaks down into two or more products
• Single displacement (substitution): One element replaces another in a compound
• Double displacement (metathesis): Two compounds exchange ions or groups of atoms
• Neutralization: An acid reacts with a base to form a salt and water
• Redox (oxidation-reduction): Involve the transfer of electrons between reactants
• Oxidation is the loss of electrons, while reduction is the gain of electrons
• Oxidizing agents accept electrons and get reduced, while reducing agents donate electrons and get oxidized

Energy Changes:

• Exothermic reactions release energy (heat) into the surroundings, resulting in a decrease in enthalpy (ΔH < 0)
• Endothermic reactions absorb energy (heat) from the surroundings, resulting in an increase in enthalpy (ΔH > 0)
• Activation energy is the minimum amount of energy required for a reaction to occur
• Catalysts speed up chemical reactions by lowering the activation energy without being consumed in the reaction
• Reaction rate is the speed at which reactants are converted into products
• Factors affecting reaction rate include: concentration of reactants, temperature, surface area, and presence of a catalyst