Ionic Compounds

There are over 100 elements in the PT Thousands of different compounds are formed when these elements combine.

How can we name these compounds?

How can we write formulas to represent them?

We have seen from past discussions that

The PT and a knowledge of the electronic structure could be used to predict ionic charge of elements

Ionic charges (or valences) of some elements in the PT

Naming Ionic Compounds

1.

2.

The name of the metal first, followed by name of the of the nonmetal.

The ending of the name of the nonmetal changes and ends with “ ide ”

Names and Ionic Charges of some nonmetals Name of element Symbol Ionic Charge Name in compound Fluorine Chlorine Bromine Iodine Oxygen Sulfur Nitrogen Phosphorus F Cl Br I O S N P 1 1 1 1 2 2 3 3 fluoride chloride bromide iodide oxide sulfide nitride phosphide

Names and Formulas for Atoms with More Than One Ionic Charge Some metals are able to form more than one kind of ion.

For example, the element copper forms two completely different compounds when it reacts with chlorine One of the compound is white: the other is yellow

Ionic charge on the copper in the white compound is 1+ . Its chemical formula is CuCl The ionic charge on the copper in the yellow compound is 2+, its formula is CuCl 2

Name of element Symbol Ionic charges Roman numeral

copper iron lead tin Cu Fe Pb Sn 1+,2+ 2+,3+ 2+,4+ 2+,4+ I,II II,III II,IV II,IV

We have come across compounds such as

•

Calcium carbonate

•

Sodium bicarbonate

•

Calcium hydroxide, and copper sulfate These names do not fit the naming so far

What are these compounds

?

Polyatomic ions

•

They are pure substances

•

Involve combinations of metals with polyatomic ions

•

Groups of atoms that tend to stay together and carry an overall ionic charge

When a compound containing this ion is dissolved in water, the positive metal ion and the nitrate ion separate from each other but the nitrate ion itself stays together as a unit surrounded by water molecules An example is The nitrate ion

Writing Formulas for Polyatomic Compounds

The ionic charges of polyatomic ions makes it possible for them to form ionic compounds

Common Polyatomic ions and Their Ionic charges Name of polyatomic ion Ion formula Ionic charge

nitrate hydroxide bicarbonate chlorate carbonate sulfate phosphate NO 3 OH HCO 3 ClO 3 CO 3 2 SO 4 2 PO 4 3 1 1 1 1 2 2 3-

When a polyatomic ion such as nitrate or sulfate combines with other elements

We follow the same rules for writing formulas

What is the formula for the ionc compound formed by sodium and a sulfate ion?

Rule 1: write the symbols of the metal and of the polyatomic group

Na SO

4 Rule 2: write the ionic charges 1+ Na 2 SO 4

Crisscross rule: crisscross the ionic charges 1+ 2 Na The formula is

Na

2

SO

4 SO 4

Note that polyatomic ions do not ”reduce” . Formula cannot be simplified Na 1 SO 2 because SO 4 is a group

Try this: what is the formula of lead(IV) carbonate?

There are many types of polyatomic ions, but one special group is known as the Oxyacids Oxyacids are compounds formed when hydrogen combines with polyatomic ions that contain oxygen. Ionic charge for hydrogen in these compounds is 1+ Ion name

nitrate nitrite chlorate carbonate sulfate sulfite phosphate

Ion formula

NO 3 NO 2 ClO 3 CO 3 2 SO 4 2 SO 3 2 PO 4 3-

Ionic charge

1 1 1 2 2 2 3-

Oxyacid formula

HNO 3 HNO 2 HClO 3 H 2 CO 3 H 2 SO 4 H 2 SO 3 H 3 PO 4

Oxyacid name

Nitric acid Nitrous acid Chloric acid Carbonic acid Sulfuric acid Sulfurous acid Phosphoric acid

Molecular Compounds

Imagine that you find an unlabelled container of solid white crystals in the kitchen.

You are sure the crystals are either salt or sugar A simple taste test will tell you what the crystals are.

But imagine you find the same crystals in the lab. A taste is too dangerous.

What do you do?

Dissolve the crystals in water and test for conductivity.

If it conducts electricity, the compound must contain ions Salt or sodium chloride is an ionic compound

In ionic compounds, metals with 1, 2, or 3 electrons in their outer shell lose electrons to nonmetals, which often have 5, 6, or 7 electrons in their outer shell.

If the solution does not conduct electricity, it must be a different kind of compound

Most compounds you encounter every day do not contain ions.

Rather, they contain neutral groups of atoms called molecules.

Sugar is a molecular compound. It is made up of molecules in which nonmetal atoms, such as hydrogen and oxygen share electrons to form stable arrangements.

Water and carbon dioxide are also molecular compounds, whether in a gas, a liquid, or a solid state, the particles in ionic and molecular compounds are different as shown Salt is an example of an ionic compound made up of ions of opposite charge. Ice (H 2 O) is an example of a molecular compound made up of neutral molecules

Hydrogen gas is a molecule formed when two hydrogen atoms combine. Each hydrogen atom has one electron.

For the two hydrogen atoms to become stable, both must gain an electron.

They do this by sharing a pair of electrons, one from each atom

The result is a covalent bond--- a shared pair of electrons held between two nonmetal atoms that holds the atoms together in a molecule.

Many nonmetals form molecules in this way. For example chlorine gas is a molecule that consists of two chlorine atoms held together with a covalent bond. Each chlorine atom has 7 electrons in its outer orbit and needs to gain electron to be stable

Many nonmetallic elements exist as covalently bonded molecules. Table below lists elements that form diatomic molecules.

Name of element Hydrogen Oxygen Nitrogen Fluorine Chlorine Bromine Iodine Chemical symbol H O N F Cl Br I Formula and state at RT H 2 (gas) O 2 (gas) N 2 (gas) F 2 (gas) Cl 2 (gas) Br 2 (liquid) I 2 (solid)

Molecular compounds are all around us a bottle of soda contains water molecules, sucrose, glucose, or fructose

Writing formulas for Molecular Compounds Formulas can be written using a method similar to the one used for ionic compounds.

The number of electrons that metals and nonmetals transfer to become stable ions can be a clue to the formula of an ionic compound.

Similarly, the number of electrons that a nonmetal needs to share to become stable is a clue to the number of covalent bonds it can form The combining capacity of a nonmetal is a measure of the number of covalent bonds that it will need to form a stable molecule

Table 1: Combinig Capacities of Nonmetal Atoms 4 C Si 3 N P As 2 O S Se 1 H F Cl Br I

Carbon has four electrons in its outer(valence) orbit. If it lost 4 electrons, it would form a positive ion. If it gained 4 electrons, it would have the electron arrangement of neon and would form a negative ion

It turns out that carbon cannot form either ion. Instead it “gains” 4 electrons by sharing: carbon has a combining capacity of 4.

For example, when carbon shares one of its outer orbit electrons with each of four different hydrogen atoms, as shown in figure, the result is methane CH 4, the major component of natural gas

As a result of forming covalent bonds through sharing electrons, the atoms end up with a stable arrangement in their orbit similar to that of a noble gas.

You can use the combining capacity to write the formulas of molecular compounds without having to consider the electronic structure

How would you write the formula for a compound formed between Carbon and Sulfur?

Rule 1 : Write the symbols, with the left hand element from Table 1 with the combining capacities

4 C 2 S Rule 2 : Crisscross the combining capacities to produce subscripts 4 2 The formula is C 2 S 4 C S

Rule 3

: Reduce the subscripts if possible The formula C 2 S 4 is reduced to C 1 S 2

Rule 4

: Any “1” subscript is not needed.

The correct formula is CS 2

Naming Molecular Compounds

Many molecular compounds have simple names. The compound H 2 S is called hydrogen sulfide, much as if it is ionic. Other molecular compounds have names that are very familiar to us even though they do not follow a system

Common names have been used for centuries for water (H 2 O): ammonia (NH 3 ), hydrogen peroxide (H 2 O 2 ) and methane (CH 4 )

The names of molecular compounds often contain prefixes. These prefixes are used to count the number of atoms when the same two elements form different combinations.

For example , the gas that you exhale is carbon dioxide (CO 2 ) while the poisonous combination of carbon and oxygen that can be formed in automobiles is carbon monoxide

The prefixes “di” and “mono” differentiate between the two molecules

Table 2: Prefixes in Molecular Compounds Prefix Number Example (formula) mon(o) di tri tetra pent(a) 1 2 3 4 5 carbon monoxide(CO) carbon disulfide (CS 2 ) sulfur trioxide (SO 3 ) carbon tetrafluoride (CF 4 ) Phosphorus pentabromide (PBr 5 )