Ions and Ionic Compounds

Worksheet 6.3, 6.4 - Ions and Ionic Compounds

For group A representative elements, the group number gives the number of electrons in the outermost energy level.  These electrons are the ones that participate in chemical bonding and are called valency electrons.

1.  How many valency electrons do each of the following elements have?

a)  potassium                            b)  sulfur                       c)  nitrogen

d)  calcium                                e)  iodine                      f)  neon

An ion is a charged atom. If an atom loses electrons, it will have more protons than electrons and therefore have a positive charge forming a cation.  If an atom gains electrons, it will have more electrons than protons and therefore have a negative charge forming an anion. 

Remember that metals tend to lose electrons and nonmetals to gain electrons to achieve the octet or 8 electrons in the outer energy level of a noble gas because this is very stable.  Metals will then lose their valency electrons forming cations and nonmetals will gain enough electrons to complete an octet forming anions.

 

Examples:  Write the chemical shorthand (chemical symbol with the charge as a superscript) for the most common ions of the following elements.  The charge on an ion refers to the number of electrons lost or gained.

sodium – Group 1A, loses 1 valency electron so Na+  (a “1” is understood in front of the +).

chlorine– Group 7A, has 7 valency electrons, gains 1 electron to make 8 so Cl–   (a “1” is understood in front of the  –)

magnesium– Group 2A, loses 2 valency electrons so Mg2+

phosphorus– Group 5A, has 5 valency electrons, gains 3 electrons to make 8 so  P3–

 

2.  Write the chemical shorthand  for the ions of

a)  potassium                            b)  sulfur                       c)  nitrogen

d)  calcium                                e)  iodine                     

3.  Would neon normally form ions?  Why or why not?

 

Formulas of ionic compounds:

The atoms in molecules bond to one another through "sharing" of electrons. Ionic compounds on the other had have atoms or molecules that bond to one another through their mutual attraction between positive and negative charges:

Positively charged cations and negatively charged anions attract one another. Conversely, cations repel one another as do anions.

Electrostatic attraction is indiscriminate. That is a cation can attract more than one anion and visa versa. The result is that cation-anions attractions form a large array that we call an ionic compound or "salt". The bonds holding these ions together are called "ionic" bonds. However, this array has a very specific composition completely dictated by the charges on the cations and anions.

Formulas of ionic compounds:

The composition of ionic compounds is determined by the requirement that the compounds must be electrically neutral. That is that the charges of the cations and anions must balance or 'cancel" out one another. For example consider sodium cations (Na+) and Chlorine anions (Cl–). Sodium has a positive 1 charge and chloride has a negative 1 charge. Thus one sodium cation cancels one chloride anion (+1 + –1 = 0) resulting in the formula Na1Cl1 or NaCl. This formula is called the formula unit since it represents only one unit of the vast NaCl array or lattice.

 

 

A small representative bit of a NaCl lattice looks like this:

If you look at the diagram carefully, you will see that the sodium ions and chloride ions alternate with each other in each of the three dimensions.

This diagram is easy enough to draw with a computer, but extremely difficult to draw convincingly by hand. We normally draw an “exploded” version which looks like this:

Naming an ionic compound:

In the name of an ionic compound, the cation is listed first and the anion, second.  Anionsr usually have an –ide suffix.  Therefore NaCl is named sodium chloride.

Example:  Give the formula of the ionic compound, magnesium phosphide.

As shown before, the ions of magnesium and phosphorus are Mg2+ and P3–. To form a neutral formula unit, we would need:

            Mg2+                                                               P3–

Mg2+                                                               P3–

Mg2+

total cation charge = (3)(+2) = +6                 total anion charge (2)(–3) = –6

total charge on one formula unit = (6+) + (6–) = 0

Since we used three Mg2+ and two P3– the formula is Mg3P2. The subscripts give the number of each ion used.

Example:  Give the name of the ionic compound whose formula is MgO.

Write the name of the cation first and then anion with –ide suffix so magnesium oxide

 

Transition elements can form ions with several different charges so these must be looked up on a chart (see Table 1 at the end of the handout).  For example, iron can be Fe2+ and Fe3+, called iron (II) ion and iron (III) ion respectively. Note that the Roman numeral following the metal name is the same as the charge of the ion.

 

Example:  Give the formula of the ionic compound, iron (III) chloride.

            Fe3+                                                                 Cl–

                                                                        Cl–

                                                                        Cl–

total cation charge = (1)(+3) = +3                 total anion charge (3)(–1)= –3

total charge on one formula unit = (+3) + (–3) = 0

 

 

Example:  Give the name of the ionic compound with the formula CuCl2.

From the subscripts, the compound contains one copper ion and two chloride ions.

Chlorine is in group 7A, has 7 valency electrons and gains 1 (8-7=1) to form Cl– ions.

2  Cl–  ions have a charge of (2)( –1) = –2. 

1 copper ion must then have a charge of +2 to make a neutral compound, (–2) + (+2) = 0

The compound name is copper (II) chloride.  The Roman number in parentheses is the charge on the copper ion.

 


Below are other examples of simple ionic compounds made up of single atom (monatomic) ions:

 

Polyatomic ions and compounds:

The ionic compounds above were made up of monatomic ions. Molecules can also be ions - polyatomic ions. Most polyatomic ions are anions with one notable exception - the ammoniun cation (NH4+). The composition of salts with polyatomic ions is determined by the same rule as with monatomic ions. A few example are given below:

 

Note that when there is two or more polyatomic ions in the formula the ion is enclosed with parenthesis "()".


Formulas and names of ionic compounds

 

Summary

 


4.  Write the formulas for the following ionic compounds. 

 

name of compound

cation

anion

formula

sodium sulfide

 

 

 

calcium iodide

 

 

 

beryllium phosphide

 

 

 

aluminum chloride

 

 

 

cobalt (II) oxide

 

 

 

lead (IV) sulfide

 

 

 

lithium floride

 

 

 

tin (IV) chloride

 

 

 

manganese (III) oxide

 

 

 

lead (II) sulfide

 

 

 

 

5.  Write the formulas for the following ionic compounds containing polyatomic ions. 

name of compound

cation

anion

formula

  lithium carbonate

 

 

 

calcium nitrate

 

 

 

ammonium sulfide

 

 

 

sodium sulfate

 

 

 

sodium thiosulfate

 

 

 

mercury (II) chlorate

 

 

 

potassium dichromate

 

 

 

tin (IV) phosphate

 

 

 

lead (II) nitrite

 

 

 

mercury (I) sulfide

 

 

 

 

6.  Write the names for the following ionic compounds given the formula. 

formula

cation

anion

name of compound

 NH4Cl

 

 

 

Fe(NO3)3

 

 

 

TiBr3

 

 

 

Cu3P

 

 

 

SnSe2

 

 

 

GaAs

 

 

 

Pb(SO4)2

 

 

 

Be(HCO3)2

 

 

 

Mn2(SO3)3

 

 

 

Al(CN)3

 

 

 


Table 1: Metal cations with more than one common charged form

Fe2+

iron(II) ion

Fe3+

iron(III) ion

Cu+

copper(I) ion

Cu2+

copper(II) ion

Hg22+

mercury(I) ion

Hg2+

mercury(II) ion

Pb2+

lead(II) ion

Pb4+

lead(IV) ion

Sn2+

tin(II) ion

Sn4+

tin(IV) ion

Cr2+

chromium (II) ion

Cr3+

chromium (III) ion

Mn2+

manganese (II) ion

Mn3+

manganese (III) ion

Co2+

cobalt (II) ion

Co3+

cobalt (III) ion

Table 2:  Common polyatomic cations, arranged by family.  (alternate names in italics)

carbon

nitrogen

sulfur

chlorine

CO32–

carbonate

HCO3–

hydrogen carbonate
(bicarbonate)

NO3–

nitrate

NO2–

nitrite

SO42–

sulfate

SO32–

sulfite

S2O32–

thiosulfate

HSO4–

hydrogen sulfate
(bisulfate)

HSO3–

hydrogen sulfite
(bisulfite)

ClO4–

perchlorate

ClO3–

chlorate

ClO2–

chlorite

ClO–

hypochlorite

 

phosphorus

cyanide

cations

metal oxyanions

PO43–

phosphate

HPO42–

hydrogen phosphate

H2PO4–

dihydrogen phosphate

CN–

cyanide

OCN–

cyanate

SCN–

thiocyanate

NH4+

ammonium

H3O+

hydronium

Hg22+

mercury(I)

CrO42

chromate

Cr2O72–

dichromate

MnO4–

permanganate

 

oxygen

organics

OH–

hydroxide

O22–

peroxide

C2H3O2–

acetate

 

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