Chapter 9: Chemical Names and Formulas Section 9., Naming Ions

The language of Chemistry…..● Chemistry has it’s own distinct language● With chemical names and formulas, it can all seem a bit daunting● It is important for you to understand how Ions and ionic compounds are

named so you can understand many aspects of this ‘language’

What is a monatomic ion? ● A monatomic ion is any single ion

with a positive or negative charge● The charge comes from the loss or

gain of a valence electron● Group 4A and 8A elements don’t

tend to form ion. Why?

Properties of cations and anions ● Metallic elements tend to lose

valence electrons● Metals in Groups 1A, 2A and 3A

form cations with positive charges the same as their group number

● The name is simply the element name, followed by the term ion

● Nonmetals gain electrons and form anions. To find the charge, subtract 8 from the group number

● Their names end in ide, such as phosphide or nitride ion

Can some elements form more than one ion? Yes! Some metallic elements form more than one cation with different ionic charges; largely transition metals through group 1 and 8

The charges of the cations of many transition metals must be determined by the number of electrons lost.

For example iron is a transition element that forms two common cations: Fe 2+ and Fe 3+. Tin and Lead are also common examples of cations with more than one charge

Does this make naming more complicated? ● Yes. Two systems exist - the stock system and classical naming system● Stock system is preferred. This is where the charge of the ion is shown as a

roman numeral in parentheses after the name of the element. Example Fe2+ is Iron(II), Fe3+ is Iron(III).

● Classical system - older syste. Utilises root word and different suffixes. ○ Ferrum - latin for ion, so ferr is root for iron.

● Suffix -ous is used to name the cation with the lower charges, ● Suffix -ic is used with the higher of the two ionic charges.

○ Fe2+ is Ferrous, Fe3+ is Ferric

Stock vs Classica names

What is a polyatomic ion? ● A polyatomic ion

contains more than one atom, but but they behave as a unit and carries a charge

● The atoms in each polyatomic ion are held together by covalent bonds

●

How do you name polyatomic ions? ● Most polyatomic ions

end in -ate or ite● The -ite always has

one less oxygen than the -ate

● Example● SO3

2- = Sulfite● SO4

2- = Sulfate● Exceptions:

○ CN- = cyanide ○ OH- = Hydorxide○ NH4+ = Ammonium

What about when polyatomic ions contain hydrogen?

● When the formula for a polyatomic ion begins with H (hydrogen), you can think of the H as representing a H+ ion

● For example, HCO3- is a combination of H+ and CO3

2-

● The charge on the new ion is the sum of the ionic charges of the two ions that combine

Section 9.2 Naming and writing formulas for ionic compounds

What do these things have in common?

How do you know the formula for binary ionic compounds?● A binary compound is any compound composed of two or more elements

○ Molecular compounds can also be binary compounds

● If you know the name, you can write the formula of binary ionic compounds- how?

● You need to use your knowledge to obtain some information○ The number charge of the cation and anion (can be calculated from the number of valence

electrons

Balancing charges method● In Ionic compounds the positive charge of the cation(s) balances the negative

charge of the anion○ The net ionic charge is zero

● This can sometime be intuitive (for example K+ and Cl-) ● At other times it can be more complicated, and you need to the find the lowest

common multiple of the charges of the ions, and determine how many of each ion you need

○ Example Fe3+ and O2-

Crisscross method

Practice problems ● Identify the most likely charged ion formed by the following elements, and

write formulas formed from the following pairs ○ Ba, S ○ Li, O ○ Ca, N○ Al, O

● Which of the following formulas are incorrect? ○ AsRb3○ BeCl3○ NaF

Does it work in reverse? If you know the formula can you determine the name? ● Yes, you can.

○ To name a binary ionic compounds you place the cation name first, and then the anion name. Example, NaCl : Sodium Chloride

● What about if the element can form more than one ion?● If the element can form more than one cation, a roman numeral must be

included in the cation name - example SnF2○ This would be written as Tin(II) fluoride

● What about SnO2?● This would be Tin(IV) Oxide

Practice problems● Name the following binary ionic compounds: ● ZnS● BaO● KCl● AlF3● Cu2Se● FeS

How is it different with polyatomic ions?

● The procedure for writing formulas is very similar ● You write the symbol (or formula) for the cation followed by the symbol (or

formula) for the anion ○ Then simply add subscripts to balance the charges as needed

● Example: Calcium Nitrate, composed of Ca2+ cation and NO3- anion

○ Two nitrate anions are needed to balance the charge of the calcium cation . The nitrate ion is put in parentheses in the formula, with a subscript 2 to indicate two are required

● Ca(NO3)2 ○ As the ions are expressed in the lowest whole number ratio the formula is correct

Practice problemsWrite formulas for compounds formed from the following pairs of ions

1. Calcium ion and sulfate ion2. Sodium Ion and Sulfite ion3. What would the formula be for lithium hydrogen sulfate?

How do you name compounds with polyatomic ions? ● The first step is to identify any polyatomic ions in the formula ● If a polyatomic ion looks unfamiliar, or you are unsure, look it up (page 242)● The write the name of the cation followed by the name of the anion

○ As always, if the cation is a metal that can have more than one ionic charge, include a roman numeral in the cation name

● Example - NaClO● This contains a Sodium ion, and the polyatomic ion ClO-(hypochlorite).● So, the name is Sodium Hypochlorite.

Practice problemsName the following compounds

CaC2O4

KMnO4

Sn3(PO4)2

NaClO3

Section 9.3 Naming and Writing formulas for Molecular

Compounds

What is a Binary Molecular Compound? ● A binary molecular compound is composed of two elements, both of which

are non metals○ They are held together by covalent bonds

● Non-metallic ions can often combine in a variety of ways. For example CO and CO2,

● One of these is a poisonous gas that prevents your blood from being able to carry oxygen, the other is a common everyday gas used to make fizzy drinks

○ Both are essentially carbon oxide, but have very different properties

How do you name a binary molecular compound? ● Prefixes are used to indicate the number of each

kind of atom ○ The prefix tells you how many atoms of an element are

present in a molecule

● To write the name of a binary molecule● Write the names of the elements in the order listed in

the formula● Use prefixes to indicate the number of each kind of

atom. The only exception is for the first element, when, if there is one atom you do not need to use the prefix mono

● End the name of the second element with suffix -ide● Following these guidelines, CO is Carbon Monoxide,

and CO2 is Carbon dioxide

How do you write formulas for Binary Molecular compounds ● You can work backwards, using the prefixes to tell you subscript of each

element in the formula. Then write the correct symbols for the two elements with the correct subscripts

● Example - Tetraphosphorus trisulfide● There are two prefixes, tetra and tri, so the subscripts of Phosphorous and

Sulfur must be 4 and 3● Chemical Symbol of Phosphorous is P and Sulfur is S, so the formula is P4S3

Section 9.4 Naming and Writing Formulas for acids and bases

What is an acid? Acids are a group of ionic compounds with a set of unique properties. They must contain at least one hydrogen atom, and they must produce a hydrogen ion in a water solution

How do you name an acid? ● They are a bit harder to name than other ionic compounds

○ They are essentially an anion combined with one or more hydrogen ions

● There are 3 key rules: ○ When the anion ends in -ide, the acid begins with the prefix hydro-○ When the anion ends in -ite, the acid name is the stem of the anion with the suffix -ous.

Example H2SO3 (X=Sulfite), Sufurous acid○ When the anion name ends in -ate, the acid name is the stem of the anion with the suffix - ic.

Writing formulas for acids● As we have already discussed in other section, if you know the name of an

acid you can write the formula; you simply reverse the rules for naming the acid

● You then balance the charges as you would for any ionic compound● Example -

○ Phosphoric acid is a combination of Hydrogen ions and phosphate ions (PO4)4-

○ Four hydrogen ions are needed to balance the -4 charge of the phosphate, therefore the formula is H4PO4

What about bases? ● A base is an ionic compound that produces hydroxide ions when dissolved in

water ● They are named by following the rules for naming ionic compounds ; cation

followed by anion● Example - Sodium hydroxide (NaOH) is made from combining Na+ and OH-

ions ● When writing formulas for bases you first write the symbol for the metal cation

followed by the formula for the hydroxide ion, and balance the charges as you would for any ionic compound

Section 9.5The laws governing how compounds form

The law of definite proportions ● In different samples of a given

chemical compound, the masses of the elements are always in the same proportions

● In different samples of a compound, the masses of the elements are always in the same proportion

● This is in keeping with Dalton’s atomic theory, that atoms combine in simple whole-number ratios

Examples

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 number. (book example pg. 265).

So how do you name a chemical formula?

Naming compound tips ● Follow the rules for naming acids when the H is the first element in the

formula● If the compound is a binary ionic compound, the name generally ends with

-ide. If the compound is a molecular binary compound, use prefixes to indicate the number of atoms of each element

● When a polyatomic ion contains oxygen, the compound name generally ends in -ate or -ite

● If the compounds contains a metal cation that can have different ionic charges, use a roman numeral to indicate the numerical value of the ionic charge

So how do you write a chemical formula? Helpful tips:

● An -ide generally indicates a binary compound

● An -ite or -ate ending means a polyatomic ion that contains oxygen it it’s formula

● Prefixes generally indicate that the compound is molecular

● A roman numeral after the name of a cation shows the ionic charge of the cation