Class notes for Chapter 4: Molecules and Compounds in the class General Chemistry: Macroscopic Investigations and Reaction Principles (CHEM 130) at the University of Michigan. Topics covered include chemical bonding, chemical formulas, molecular models, bonding, mathematical formulas relating to co...
Hydrogen, Oxygen, and Water
●Hydrogen is an explosive gas, oxygen is a gas in our atmosphere, and both have extremely low boiling points
●When you combine hydrogen and oxygen you get a very different substance with very different characteristics
●When two or more elements combine to form a compound, an entirely new substance results
●Most substances we see in daily life are compounds
●Compounds are made up of elements combined in fixed, definite proportions, whereas mixtures have elements mixed in any proportion randomly
○Hydrogen and Oxygen mixture → can have any ratio of hydrogen to oxygen, stays a gas
○Hydrogen and Oxygen bonded together → molecule → liquid water
Chemical Bonds
●Chemical Bond: force that holds atoms together in compounds
○Form to reduce the potential energy of charged particles in atoms
■Valence shell becomes filled → lowers potential energy
●Steps:
○When atoms approach each other the electrons of one atom are attracted to the nucleus of the other and vice versa
○Electrons and protons of each atom repel each other
○Complex set of interactions between many charged particles
○If these interactions → lower potential energy, a bond forms ●Ionic Bond: chemical bond between two oppositely charged atoms that are attracted to each other by electrostatic forces → ionic compound
○Metal and nonmetal combine
■Metals tend to lose electrons; nonmetals tend to gain
○Electrons are transferred between atoms
■The nonmetal “steals” the electron
○Crystal lattice structure
●Covalent Bond: chemical bond between two atoms that share electrons that interact w both nuclei, lowering potential energy of both through electrostatic
interactions → molecular compound
○Nonmetal and nonmetal
■Nonmetals have high ionization energy → hard to take away electrons
○Electrons shared between nuclei
○Molecule structure A —- B
●Lattice Energy: E for the process where gas phase ions form a crystalline lattice of 𝚫
alternating cations and anions
○Large negative → large release of energy
○Lattice energy: kQ1Q2 / r
■Q1 and Q2 are charges
■r is distance between ions
○When comparing compounds, lattice energy is more negative for the compound with larger charges and/or smaller radii
Chemical Formulas and Molecular Models
●Chemical Formula: symbolic representation of a compound that indicates the elements present in the compound and the relative number of atoms of each
○Contains the symbol for each element and a subscript indicating relative number of atoms of each element
○Usually list more metallic/positive element first
○EX: H2O, NaCl, CO2
Types of Chemical Formulas ●Empirical Formula: simplest whole number ratio of the atoms
○Indicates relative number of atoms of each element
○EX: water = H2O, hydrogen peroxide = HO ●Molecular Formula: exact, actual number of each atom in the molecule
○EX: water = still H2O, hydrogen peroxide = H 2O2
○Molecular formula always a whole-number multiple of the empirical formula
●Structural Formula: shows connectivity of atoms and may include geometry information
(how they are bonded together)
●Single, Double, Triple Bonds: how many pairs of shared electrons
○Double bonds generally shorter and stronger than single bonds
Molecular Models
●Molecular Model: more accurate/complete way to specify a compound
●Ball-And-Stick Model: representation of the arrangement of molecules, represents atoms as balls and bonds as sticks
○Balls color-coded to specific elements
■Typically, C is black, H is white, N is blue, O is red
●Space-Filling Model: atoms fill the space between each other
○More closely represents estimate for how molecule may appear if scaled to visible size
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