chemical formulas & naming compounds

the basics:

- when we have been showing substances, we have been using chemical formulas, which show qualitative and quantitative information about the substance

- the chemical symbols = elements present

- the subscripts = how many atoms of each element

types of formulas:

empirical formula - the simplest ratio in which atoms combine to form a compound

ex.: KCl, CaO, Al2O3, CH2O

molecular formula - the formula that tells you what is part of a molecule (the smallest unit of a covalent compound)

ex.: H2O, C6H12O6

this does NOT have to be the simplest ratio of elements

oxidation states/oxidation #:

an atom’s oxidation state = its hypothetical charge if you assume all its bonds were fully ionic

ex.: Na has an oxidation state of +1; you can interpret this as Na forms an ion with a 1+ charge in an ionic bond

unless otherwise shown, you should use the top number, which is the most common oxidation state!

naming compounds (pt. 1):

binary ionic compounds (2 parts)

- rules for writing formulas: elements with positive oxidation #’s combine with elements with negative oxidation #’s (USE THE TOP # FOR NOW)

- the algebraic sum of all the charges that you have must be zero to reach a compound that is overall neutral

- write the formula starting with the element with the positive oxidation #, then the one with the negative # number

“cross & drop”

1. write the elements and their oxidation states above the respective elements in the appropriate order (positive, then negative)

2. take the number of the oxidation state (not the charge!) of one element and write it as the subscript of the other for both elements

3. scale the numbers down to the simplest whole number ratio possible

giving compounds names

for binary ionic compounds, we do the following

1. keep the first element (the positive one) as is

2. use the suffix “–ide” instead of the normal name of the second element

- ending in -ine, -ium or -on: swap the suffix out for “i-de” directly (“carbide,” “chloride,” “telluride”)

- ending in –gen: drop the vowel preceding the suffix as well before you swap for “-ide” — ”hydride” “oxide”

weird ones: sulfur = “sulfide,” phosphorus = “phosphide,” antimony = “antimonide,” arsenic = “arsenide”

“what happens if your metals have multiple oxidation states?”

the stock system! - used to indicate specific oxidation states for metals that have multiple oxidation states

- positively charged elements only

- for elements that have multiple oxidation states, a roman numeral in parentheses is added to the name to show the one it has in the compound being shown

ex.: FeCl3—iron (III) chloride

remember—all compounds should end up with 0 charge, so use the charge of the other element to help you figure out what you have!

compounds with polyatomic ions:

recall: polyatomic ions are groups of atoms that can behave like single atom ions

ex.:

NaCl - regular ionic compound

NH4Cl - polyatomic ionic compound

NH4NO3 - only polyatomic ions

when multiple of these are involved in forming a compound, they need to be shown as part of a single group (we do so by using parentheses to indicate the polyatomic ions where more than one is needed)

ex.: (NH4)2SO4

naming covalent compounds:

FIRST RULE: DO NOT SIMPLIFY CHEMICAL FORMULAS

we will only be dealing with binary molecular compounds

prefix naming system - know your prefixes and their associated #’s

1—mono

2—di

3—tri

4—tetra

5—penta

6—hexa

7—hepta

8—octa

9—nona

10—deca

general rules are like naming binary ionic compounds

- first element keeps its original spelling

- second element is changed to end in –ide

- prefixes are required for BOTH elements to show how many of each atom is in the compound

- the exception is using mono- for the first element (not necessary)

weird naming quirk: for oxide, drop any ending “a” or “o”