FORMULAS OF IONIC COMPOUNDS

Lesson 22

FORMULAS OF IONIC COMPOUNDS

Key Words:

Formula of an ionic compound:

Chemical formula that tells the kinds and relative numbers of atoms of each kind in an ionic compound.

Radical:

▪Group of atoms of different elements that act together to form an ion.

Key Ideas:

The chemical formula of an ionic compound shows the kinds of atoms and the relative numbers of atoms of each kind. To write a formula an ionic compound, you need to know the charge on each ion in the compound.

•Dental hygienist helps people keep their teeth healthy.

•Hygienists often tell their patients to use fluoride toothpaste to prevent cavities.

•Fluoride is the anion of the element fluorine.

•The fluoride ions actually become part of your teeth and make them stronger.

This Photo by Unknown Author is licensed under CC BY

This Photo by Unknown Author is licensed under CC BY-NC

UNDERSTANDING CHEMICAL

FORMULAS

▪An easy way to to show a chemical compound is to write a chemical formula.

▪A chemical formula is a group of symbols used to show the kinds and numbers of atoms in a compound.

NaCL H2O PO4

Ca10(PO4)6F2 O2

▪The formula of an ionic compound uses symbol of elements to show the kinds of atoms in an ionic compound.

NaCl is an example.

▪The formula also shows the relative number of each kind of atom in a sample of the compound.

▪The formula NaCl show that there is one Na atom for every Cl atom in a sample of this compound.

SUBSCRIPT

A subscript, or number at the lower right of a symbol, tells how many atoms of that element there are compared to other atoms in the compound.

H2 shows that there are 2 atoms of the element Hydrogen present

RADICAL

A group of atoms that bond together.

A radical has electric charge and acts like an ion.

An ion is an atom that develops an electric charge when it gains or loses electrons.

PO4 is a radical

Ca10(PO4)6(OH)2

There are 6 PO4 radicals in this formula -- (PO4)6

The OH, or hydroxide, is another radical

This formula has 2 hydroxide radicals -- (OH)2

WRITING FORMULAS

Many formulas are much simpler than Ca10(PO4)6(OH)2

To figure out the formula for an ionic compound, you need to know the charge on each ion (how many electrons they have gained or lost).

The charge will be either positive (cation) or negative (anion)

The positive and negative charges in a compound must add up to zero

CHARGES OF IONS

Positive Ions Negative Ions

Na 1+ Br1-

K 1+ Cl1-

Ag 1+ F1-

Ba 2+ OH1-

Ca 2+ I1-

Mg 2+ O2-

Al 3+ S2-

WRITING FORMULASWrite the formula for the ionic compound formed by the reaction of sodium and fluorine

1st- Write the symbols for the elements in the compound.

The symbol for the cation (positively charged ion) goes first

Then the symbol for the anion (negatively charged ion).

Sodium Fluorine

Na F

2nd- Find the electric charge for each ion

Na 1+ F1-

3rd- Crisscross the numbers from top to bottom. But do not write + or - charges

Na 1+ F 1-

Na 1 F 1

•When the subscript is 1 do not use it.

• Also when the subscripts are the same in an ionic compound, don’t use them.

NaF is a neutral compound

NAMING COMPOUNDS

To name a compound

Use the name of the 1st element

Then the name of the 2nd element but end it with –ide

Fluorine become Fluoride

NaF = Sodiom Fluoride

FORMULAS OF COVALENT COMPOUNDS

Lesson 23

FORMULAS OF COVALENT COMPOUNDS

Key Words:

Molecular Formula:

Chemical formula that tells the kinds and numbers of atoms in a molecule.

Key Ideas:

Molecular formulas are chemical formulas used to show the kinds and numbers of atoms in a molecule. Since the same 2 elements may combine to form 2 or more compounds, their molecular formulas must be different.

UNDERSTANDING MOLECULAR FORMULAS

A formula show the kinds of atoms in a sample of a compound.

A formula of an ionic compound shows the relative numbers of ions in a sample of an ionic compound, but not their actual numbers

Covalent Compounds form molecules, chemical formulas give different information about them.

A molecular formula shows the kinds and actual numbers of atoms in a molecule of a covalent compound.

WRITING FORMULAS

Some molecules are formed by 2 atoms of the same element.

For instance hydrogen gas is made of molecules that contain two H atoms.

Its molecular formula is simple: H2

WRITING FORMULAS

Take the molecule formed by hydrogen (H) and fluorine (F) as an

example.

For the formulas of some covalent compounds, you can use almost the same rules as for ionic compounds.

WRITING FORMULAS

1st write down the symbols

Put first element that holds the electrons in the covalent bond less tightly.

• This element will be found farther left in the periodic table

Then write the symbol for the element that holds the electrons pair more tightly

• The one farther right in the table

Hydrogen Fluorine

H F

WRITING FORMULAS

2nd- place at the upper right the charges for each atom would have if it did form an ion (1+ for H and 1- for F).

These are not actual electric charges because these atoms don’t form ions.

Instead, they are called apparent charges.

H 1+ F1-

3rd- Crisscross the numbers from top to bottom. But do not write + or - charges

Na 1+ F 1-

Na 1 F 1

Hydrogen fluoride

WRITING FORMULAS

The apparent charges in a molecule must add up to zero (0).

This is easy to see for HF (1+ + 1- = 0)

Now consider water (H20), in which the O atom has an apparent charge of 2-.

Each H atom has an apparent charge of 1+.

Here the two charges (2x 1+ = 2+) for H plus the 2- chare for the O add up to 0.

This forms a neutral H2O molecule.

COVALENT COMPOUNDS

Most ionic compounds are formed by a cation (+) from the left side of the periodic table and an anion (-) from

the right side.

But some covalent compounds contain 2 elements from the

right side, such as sulfer dioxide (S02).

Both elements hold their valance electrons (outer

electrons) tightly.

So first write down the element that holds the

electron pair less tightly (this one is farther left on the

periodic table). And give it a + apparent charge

The second element (to the right of the table) keeps it apparent negative charge.

Follow the crisscross method from here (found on slide17)

COMPOUND NAMING

▪Of course, because N2O and NO are different compounds, they also have different names.

▪In cases like this, where both elements are from the right side of the periodic table, chemists use prefixes (shown on next slide)

▪So N2O is known as dinitrogen oxide, and NO is called nitrogen monoxide.

▪Just remember no to use mono- as part of the first word.

COMPOUNDNAMING

Prefix Meaning

mono- 1

di- 2

tri- 3

tetra 4

penta- 5

STEPS TO WRITE FORMULAS FOR COVALENT COMPOUNDS

1.

2.

3. May need prefixes to name

Write apparent

charges:

A+X B-y

Criss-cross:

Ay Bx

Element to left in periodic table goes first;

Element on right goes next

No Ions

Atoms on left gets + apparent charge

No Ions

Atoms on right gets – apparent charge

Write Symbols:

A B

Don’t use 1