Chemical Names and Formulas

Chapter 9 Naming and Writing Formulas

Classifying Compounds The system for naming an ionic compound is different from that for naming a covalent compound, so before a compound can be named, it must be classified as ionic or covalent. Classifying a compound is not an easy task, but for the purposes of naming them, we employ a simple test: If the answer is yes, use the system for naming ionic compounds. If the answer is no, use the system for naming covalent compounds. Is there a metal or a polyatomic ion present?

Naming Ionic Compounds Ionic compounds are named simply by naming the ions present. There are, however, two complicating factors: I. Some metals form more than one ion. II. Identifying polyatomic ions I. Metals that form more than one ion, such as iron, add a Roman numeral to the name to indicate the charge: Fe 2+ is called iron (II) and Fe 3+ is called iron (III) Assume a Roman numeral is required for any metal except 1. metals in groups IA and IIA on the periodic table 2. aluminum, cadmium, silver, and zinc

Stock System
Some elements form more than one ion
Use roman numerals to distinguish the ion formed
Fe 2+ Fe 3+
Iron (II) Iron (III)
Only for metals that form more than one ion

Naming using the stock system
Name of the cation + Roman numeral indicating charge then the name of the anion
CuCl 2
copper (II) chloride

Rules for Naming Polyatomic Ions
No good rules
Since most polyatomic ions contain oxygen, we look at the number of oxygen atoms
One less oxygen than – ate is an – ite
NO 3 - NO 2 -
Nitrate Nitrite

One more oxygen than –ate add per- to the beginning of the name
ClO 3 - ClO 4 -
Chlorate Perchlorate
One less oxygen than –ite add hypo- to the beginning of the name
ClO 2 - ClO -
Chlorite Hypochlorite

Naming Ionic Compounds (continued) If a Roman numeral is required, the charge on the metal ion must be determined from the charge on the negative ion. Helpful Rules to Remember Examples A metal ion is always positive. The Roman numeral indicates the charge, not the subscript. The positive and negative charges must cancel (total charge must = 0). Nonmetals are always negative & can never form more than one monatomic ion. copper (I) nitride lead (IV) sulfide iron (III) oxide iron (II) chloride Name N has a 3- charge, so the Cu must have a total charge of 3+ split equally between the 3 copper atoms, so each must have a 1+ charge Cu 3 N O has a 2- charge, and there are 3 of them for a total of 6-, so the Fe must have a total charge of 6+ split equally between the two iron atoms, so each must have a 3+ charge Fe 2 O 3 S has a 2- charge, and there are 2 of them for a total of 4-, so the Pb must be 4+ PbS 2 Cl has a 1- charge, and there are 2 of them for a total of 2-, so the Fe must be 2+ FeCl 2 Reasoning Formula

Naming Ionic Compounds (continued) II. Polyatomic ions each have specific names which must be memorized so they can be recognized on sight. (At this point, if you are asked to name any compound that contains more than two elements, it will contain at least one polyatomic ion.) A few of the more common polyatomic ions ammonium NH 4 1+ carbonate CO 3 2- bicarbonate HCO 3 1- acetate C 2 H 3 O 2 1- Name Formula hydroxide OH 1- nitrate NO 3 1- sulfate SO 4 2- phosphate PO 4 3- Name Formula

Naming Monatomic Ions
Monatomic cations – just use the name
Li +
Lithium
Monatomic anions – drop the ending and add – ide
Br Br -
Brom ine Brom ide

Naming Ionic Compounds: Examples Na 2 SO 4 sodium sulfate Fe(NO 3 ) 2 iron (II) nitrate AlCl 3 aluminum chloride PbI 4 lead (IV) iodide (NH 4 ) 3 PO 4 ammonium phosphate Mg 3 N 2 magnesium nitride AgC 2 H 3 O 2 silver acetate acetate C 2 H 3 O 2 1- * Groups I & II, Al, Zn, Cd, and Ag need no Roman numeral. sulfate SO 4 2- phosphate PO 4 3- hydroxide OH 1- nitrate NO 3 1- ammonium NH 4 1+ bicarbonate HCO 3 1- carbonate CO 3 2-

Naming Covalent Compounds Covalent compounds are named by adding prefixes to the element names. The compounds named in this way are binary covalent compounds. ‘ Binary’ means that only two atom are present. ‘ Covalent’ (in this context) means both elements are nonmetals. A prefix is added to the name of the first element in the formula if more than one atom of it is present. (The less electronegative element is typically written first.) A prefix is always added to the name of the second element in the formula. The second element will use the form of its name ending in ‘ide’.

Naming Covalent Compounds Prefixes Note: When a prefix ending in ‘o’ or ‘a’ is added to ‘oxide’, the final vowel in the prefix is dropped. penta- 5 tetra- 4 tri- 3 di- 2 mono- 1 Prefix Subscript deca- 10 nona- 9 octa- 8 hepta- 7 hexa- 6 Prefix Subscript

Naming Binary Covalent Compounds: Examples N 2 S 4 di nitrogen tetra sulfide NI 3 nitrogen tri iodide XeF 6 xenon hexa fluoride CCl 4 carbon tetra chloride P 2 O 5 di phosphorus pent oxide SO 3 sulfur tri oxide * Second element in ‘ide’ from mono 1 * Drop –a & -o before ‘oxide’ deca 10 nona 9 octa 8 heptaa 7 hexa 6 penta 5 tetra 4 tri 3 di 2

Classifying Compounds Classifying a compound using its name is not as difficult as using its formula. The names of covalent compounds will be easily recognized by the presence of the prefixes (mono-, di-, tri-, etc.). If no prefixes are present in the name, the compound is ionic. (Exception: some polyatomic ion names always contain prefixes (such as dichromate) but those will be memorized and recognized as ions.)

Writing Formulas for Ionic Compounds Formulas for ionic compounds are written by balancing the positive and negative charges on the ions present. The total positive charge must equal the total negative charge because the number of electrons lost by one element (or group of elements) must equal the number gained by the other(s). Polyatomic ion names must still be recognized from memory (e.g. ammonium nitrate), but metals will have a Roman numeral associated with them if there is the possibility of more than one ion (e.g. copper (I) chloride or copper (II) chloride). The Roman numeral indicates the charge on the ion not the number of ions in the formula.

Writing Formulas for Ionic Compounds (continued) Helpful Rules to Remember Examples A metal ion is always positive. The Roman numeral indicates the charge, not the subscript. The positive and negative charges must cancel (total charge must = 0). If more than one polyatomic ion is needed, put it in parentheses, and place a subscript outside the parentheses. Cu 3 (PO 4 ) 2 ZnCl 2 (NH 4 ) 2 SO 4 Formula Zn has a 2+ charge & Cl has a 1- charge, so 2 chloride ions are required for each zinc ion. zinc chloride Cu has a 2+ charge & PO 4 has a 3- charge, so 3 copper (II) ions are required for every two phosphate ions. copper (II) phosphate NH 4 has a 1+ charge & SO 4 has a 2- charge, so 2 ammonium ions are required for each sulfate. ammonium sulfate Reasoning Name

Writing Formulas for Covalent Compounds The names of covalent compounds contain prefixes that indicate the number of atoms of each element present.
Remember:
The compounds named in this way are binary covalent compounds (they contain only two elements, both of which are nonmetals).
When in covalent compounds, atoms do not have charges. Subscripts are determined directly from the prefixes in the name.
If no prefix is present on the name of the first element, there is only one atom of that element in the formula (its subscript will be 1). A prefix will always be present on the name of the second element. The second element will use the form of its name ending in

Writing Formulas for Binary Covalent Compounds: Examples nitrogen dioxide NO 2 diphosphorus pentoxide P 2 O 5 xenon tetrafluoride XeF 4 sulfur hexafluoride SF 6 * Second element in ‘ide’ from mono 1 * Drop –a & -o before ‘oxide’ deca 10 nona 9 octa 8 heptaa 7 hexa 6 penta 5 tetra 4 tri 3 di 2

Acids
An acid is a substance that produces positive hydrogen ions when placed in water. (H + )
The strength of an acid depends on how completely the substance ionizes. Strong acids completely ionize in water. Weak acids ionize only slightly.

Base
A base is a substance that produces negative hydroxide ions when placed in water. (OH - )
The strength of a base depends on how completely the substance dissociates into metal ions and hydroxide ions in water. Strong bases dissociate completely. Weak bases do not.

Acids and Bases
Produce H+ ions in water
Have a sour taste
Break down metals
Formula starts with H
Poisonous and corrosive to skin
pH less than 7
Produce OH- ions in water
Have a bitter taste and a slippery feel
Break down fats and oils
Formula ends with OH
Poisonous and corrosive to skin
pH greater than 7

Acids and Bases
ACIDS
Examples: vinegar, lemon juice, aspirin, stomach acid, battery acid, cola, milk
BASES
Examples: soap, shampoo, ammonia, drain cleaner, antacids

Neutral Substances
pH = 7
Safe to ingest and leave on skin
Concentrations of H + ions and OH - ions are equal
Examples: dH2O, salts, most cosmetics, lotions, eye drops, etc.

Naming Acids
If the name of the anion ends in –ide, the acid name begins with the prefix hydro- The stem of the anion is given then the suffix -ic is added and is followed by the word acid.
H 2 S
(anion sulfur)  hydro + stem + ic + acid
Hydrosulfuric Acid

Naming Acids
If the name of the anion ends in –ite, the acid name is the stem of the anion with the suffix –ous and is followed by the word acid.
H 2 SO 3
(anion Sulfite)  stem + ous + acid
Sulfurous Acid

Naming Acids
If the name of the anion ends in –ate, the acid name is the stem of the anion with the suffix –ic and is followed by the word acid.
H 2 SO 4
(anion Sulfate)  stem + ic + acid)
Sulfuric Acid

Does the formula contain Oxygen? Hydro “stem”ic acid Anion end in __ ? No Yes “ stem”ous acid “ stem”ic acid “ ate” “ ite” Naming Acids

Writing Formulas for Acids
If the name starts with “hydro”
Hydrosulfuric Acid
Write the hydrogen ion with charge.
H +1
Write the anion with the proper charge.
S -2
Balance the charges using subscripts.
H 2 S

If the name contains the suffix –ous
Sulfurous Acid
H +1
Look up the polyatomic ion (sulfite) and write it with the correct charge.
SO 3 -2
H 2 SO 3

If the name contains the suffix –ic without the prefix hydro
Sulfuric Acid
H +1
Look up the polyatomic ion (sulfate) and write it with the correct charge.
SO 4 -2
H 2 SO 4

Writing Formulas/Naming Acids
Remember the following statements…
“ I ate it and it was ic ky.” -ate becomes -ic
“ R ite ous ” -ite becomes -ous
(Righteous)

Naming Bases
Bases are named using the traditional ionic naming system.
Metal name + polyatomic ion name
Examples:
Ca(OH) 2 = calcium hydroxide
NaOH = sodium hydroxide
Al(OH) 3 = aluminum hydroxide

Writing Base Formulas
Base formulas are written using the traditional ionic system.
Look up the metal ion. Write the symbol with the proper charge. Ca +2
Look up the polyatomic ion. With bases, this will always be hydroxide, OH -1 .
Ca(OH) 2

The rules for naming and writing formulas for compounds are possible because of 2 laws:
1. The Law of Definite Proportions : samples of any chemical compound, the masses of the elements are always in the same proportions.
2. The Law of Multiple Proportions : Whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.

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Chemical Names and Formulas

by Currituck County High School on Dec 04, 2008

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Chemical Names and Formulas — Presentation Transcript