Chapter 6, Section 1 - Compounds and Molecules
Compound - A substance made of two or more elements that are chemically combined.
Chemical Bond - The force that holds atoms/ions together in a molecule.
Chemical Structure - The arrangement of atoms in a molecule.
- Chemical structure can be shown by models...
In the ball-and-stick model, bonds are represented by sticks. The balls represent atoms. This model allows you to see bond angles.
Structural formulas show the molecule's
structure, as well. It is similar to the ball-
Bond Length - The distance between the nuclei of two bonded atoms.
Bond Angle - The angle formed by two bonds that involve the same atom.
A space-filling model shows the space taken up by the individual atoms. This shows the relative sizes of the atoms, but not the bond lengths.
Bonds are not rigid - they can bend, stretch and rotate.
The chemical structure of a compound determines the properties of that compound.
- Compounds with network structures are strong solids.
- Example - Quartz
- Rigid structure = strong, inflexible mineral.
- Strong bonds = very high melting/boiling points.
- Some networks are formed from positive and negative ions.
- Example - Table Salt
- Repeating sequence of Na+ and Cl- = creates a cube-shaped crystal.
- Strong attraction between ions = high melting/boiling points.
- Some compounds are made of separate molecules.
- Example - Sugar
- Sugar molecules attract each other to form sugar crystals.
- Weaker attraction between molecules = lower melting/boiling points.
- Example - Oxygen Gas
- Weak attraction between molecules = gas spreads out.
Hydrogen Bond - The weak attraction between water molecules. (The H of one is attracted to the O of another.)
- Causes water's surface tension.
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Chapter 6, Section 2 - Ionic and Covalent Bonding
- when their valence electrons interact.
- to fill their outer shells and make a stable electron configuration.
- by either sharing electrons (covalent) or transferring electrons (ionic).
Ionic Bond - The attraction (force) between oppositely charged ions. These ions form when electrons are transferred from one atom to another.
- Some atoms (usually nonmetals) have stronger attraction for electrons than other atoms (usually metals) do.
- So, electrons tend to be transferred from the metals to the nonmetals.
- Ionic compounds do not form molecules.
- The ions line up with each other. Each positive ion is surrounded by several negative ions, and vice versa.
- The chemical formula NaCl shows the lowest ratio of Na+ to Cl- ions (1:1).
- So, NaCl is a formula unit, not a molecule.
- Ionic compounds conduct electricity when melted or dissolved.
- Electricity is moving charges.
- Melting/dissolving an ionic compound allows the ions (charged particles) to move.
Covalent Bond - A bond formed when atoms share one or more pairs of electrons.
- Covalent bonds usually form between nonmetals.
- Atoms share electrons to fill their outer shells.
- Covalent bonds are the strongest type of bond.
- Atoms can share...
- two pairs of electrons to make a double bond.
- three pairs of electrons to make a triple bond.
- Atoms do not always share electrons equally.
- When two identical atoms bond, the shared electrons are equally attracted to both nucleus. This makes a nonpolar covalent bond.
- When different atoms bond, the electrons are more attracted to one nucleus than the other - they are not shared equally. This makes a polar covalent bond.
- Fluorine is the strongest electron attractor - the closer an element is to F on the periodic table, the greater its attraction for electrons.
Metallic Bond - The bond formed between positive metal ions and the electrons around them.
- Metal atom nuclei attract the electrons of neighboring atoms, pulling them in close.
- The outermost energy levels overlap, so electrons can move from atom to atom.
- This allows metals to conduct electricity well.
Polyatomic Ions - An ion made up of two or more atoms.
- Made of covalently bonded atoms - make ionic bonds to form compounds.
- The polyatomic ion in a compound acts as a single unit.
- When there is more than one of a polyatomic ion in a chemical formula, we use parentheses...
- Example - Ammonium Sulfate = (NH4)2SO4 [Two NH4 ions.]
- Negative polyatomic ions that contain oxygen often have names ending in -ite (fewer O) or -ate (more O).
- Special cases...
- Hydroxide = OH-
- Cyanide = CN-
Chapter 6, Section 3 - Compound Names and Formulas
Names of ionic compounds are made up of the names of the ions in them.
- Positive ion first, negative ion second.
- Positive ions take the name of their element.
- Al = Aluminum, Al+3 = Aluminum ion
- Negative ions change the element name, to end in -ide.
- F = Fluorine, F- = Fluoride ion
Ionic compounds must have a total charge of zero.
- Na+ + Cl- = NaCl
- Ca+2 + F- = CaF2
Elements that make multiple cations must show their charge using Roman numerals.
- Fe+2 + O-2 = FeO Iron (II) Oxide
- Fe+3 + O-2 = Fe2O3 Iron (III) Oxide
When there are only two elements involved...
- The one farther left on the periodic table goes first.
- The one farther right on the periodic table goes second and takes the -ide ending.
- Prefixes are used to show the numbers of each atom.
- Except when there is only one atom of the first element.
Empirical Formula - The formula for a compound that shows the simplest ratio of elements in that compound.
- Different compounds can have the same empirical formula.
- Formaldehyde - Empirical Formula CH20 - Molecular Formula CH2O
- Acetic Acid - Empirical Formula CH20 - Molecular Formula C2H4O2
- Glucose - Empirical Formula CH20 - Molecular Formula C6H12O6
Molecular Formula - The formula for a compound that shows the specific number and kinds of atoms in that compound, but does not show how they are arranged.
When a new compound is discovered, scientists figure out the empirical formula first by finding the mass of each element in the compound. Once the masses are known, convert to moles and find the molar ratio.
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Chapter 6, Section 4 - Organic and Biochemical Compounds
Organic Compound - A covalently bonded compound that contains carbon, except for carbonates and oxides.
- Carbon atoms form four covalent bonds in organic compounds.
- This includes double and triple bonds.
Hydrocarbon - A compound made of only carbon and hydrogen.
- Alkanes - Hydrocarbons with only single bonds between atoms.
- Alkanes in a straight line are called normal alkanes (n-alkanes).
- Alkanes with four or more carbons can have branches.
- Formulas for alkanes follow the pattern: CnH2n+2
- Except for alkanes that form rings.
- Alkenes - Hydrocarbons with at least one double bond.
- Alkynes - Hydrocarbons with at least one triple bond.
- Alcohols - Organic compounds that have a hydroxyl (-OH) group attached.
Polymer - A large molecule made up of smaller molecules (monomers) formed together in a chain.
- Examples - rubber, wood, cotton, wool, starch, protein, DNA.
- Most artificial polymers are plastics or fibers.
- Elastic polymers can be stretched, and will return to their original shape.
Biochemical Compounds - Organic compounds made by living things.
- Carbohydrates (contain C, H, and O)
- Sugars, starches, and fibers.
- Starch is a polymer made of long chains of sugars.
- Your body uses carbohydrates for energy.
- Made of amino acids.
- Amino acids have...
- amino group (-NH2)
- carboxyl group (-COOH)
- side group that gives it unique properties.
- 20 different naturally-occurring amino acids.
- DNA (contains C, H, O, N, and P)
- Made of nucleic acids, which have...
- Alternating sugar & phosphate units that make up the side chains.
- Nitrogen bases that make up the codes for living things.
- DNA has the instructions for making proteins.