What are compounds and how they form bonds?

Octet Rule and IonsAn octet• is 8 valence electrons.• is associated with stability of noble

gases.

He is stable with 2 valence electrons (duet).

valence electronsHe 2, 2Ne 2, 8, 8 Ar 2, 8, 8, 8

Kr 2, 8, 14, 8 8

What are ionic and covalent bonds?Atoms form octets• to become more

stable. • by losing, gaining,

or sharing valence electrons.

• by forming ionic bonds or covalent bonds.

Metals Form Positive IonsMetals form positive ions• by a loss of their valence

electrons.• electron configuration of

nearest noble gas.• have fewer electrons than

protons. Group 1A metals ion

1+

Group 2A metals ion 2+

Group 3A metals ion 3+

2, 8, 1 2, 8

With the loss of its valence electron,sodium ion has a +1 charge.

Sodium atom Sodium ion 11p+ 11p+

11e- 10e-

0 1+

Formation of Mg2+

Magnesium achieves an octet by losing its two valence electrons.

With the loss of two valenceElectrons, magnesium

forms apositive ion with a +2

charge.

Mg atom Mg2+ ion 12p+ 12p+

12e- 10e-

0 2+

2, 8, 2 2, 8

• charge of a positive ion is equal to its Group number.

Group 1A(1) = 1+Group 2A(2) = 2+Group 3A(3) = 3+

Formation of Negative Ions

In ionic compounds, nonmetals

• achieve an octet arrangement.

• gain electrons.• form negatively charged

ions with 3-, 2-, or 1- charges

• gaining one electron, chloride ion has a -1 charge.

Chlorine atom Chloride ion 17p+ 17p+

17e- 18e-

0 1–

2, 8, 7 2, 8, 8

• charge of a negative ion obtained by subtracting 8 or 18 from its Group number.

Group 6A = 6 - 8 = 2-

What are Ionic Compounds?

Ionic compounds

• consist of positive and negative ions.

• have attractions called ionic bonds between positively and negatively charged ions.

• have high melting and boiling points.

• are solid at room temperature.

Sodium chloride or “table salt” is an example of an ionic compound.

How to write Ionic Formulas?An ionic formula

• consists of positively and negatively charged ions.

• is neutral.

• has charge balance. total positive

charge = total negative charge

In NaCl,

• a Na atom loses its valence electron.

• a Cl atom gains an electron.

• symbol of metal is written first followed by symbol of nonmetal.

Examples of charge balances!

In MgCl2,

• a Mg atom loses two valence electrons.

• two Cl atoms each gain one

electron.In Na2S.

• two Na atoms lose one valence electron each.

• one S atom gains two electrons.

• subscripts indicate number of ions needed to give charge balance.

Writing Ionic Formulas from Charges using cross your heart method!Charge balance is used to write the formula forsodium nitride, a compound containing Na+ and

N3−.

Na+

3 Na+ + N3− = Na3NNa+

3(+1) + 1(3-) = 0 We can also use the “cross your heart”

methodWhere cation and anion ionic charges are crossed to determine the subscript

Formula from Ionic Charges Write the ionic formula of the compound with

Ba2+ and Cl.

• Write the symbols of the ions.Ba2+ Cl

• Balance the charges. Ba2+ Cl two Cl- needed

Cl

• Write the ionic formula using a subscript 2 for two chloride ions that give charge balance.

BaCl2

Writing Formulas

Write a formula for potassium sulfide.

1. Identify the cation and anion. potassium = K+

sulfide = S2−

2. Balance the charges. K+ S2−

K+

2(1+) + 2(1-) = 0

3. 2 K+ and 1 S2− = K2S

Naming Ionic Compounds with Two Elements: binary compounds To name a compound

that contains two elements,

• identify the cation and anion.

• name the cation followed by the name of the anion.

Examples of Ionic Compounds with Two Elements

Formula Ions Name

cation anion

NaCl Na+ Cl- sodium chlorideK2S K+ S2- potassium sulfide

MgO Mg2+ O2- magnesium oxideCaI2 Ca2+ I- calcium iodide

Al2O3 Al3+ O2- aluminum oxide

Transition Metals form Positive Ions transition metals

and form 2 or more positive ions. use a Roman numeral after name of metal to indicate ionic charge.

Lead

Pb2+ lead(II)

Pb4+ lead(IV)

Naming FeCl2

To name FeCl2

1. Determine the charge of the cation using the charge

of the anion (Cl-).Fe ion + 2 Cl- = Fe ion + 2- = 0Fe ion = 2+

2. Name the cation by the element name and add a

Roman numeral in parenthesis to show its charge.Fe2+ = iron(II)

3. Write the anion with an ide ending. FeCl2 = iron(II) chloride

16

Writing Formulas

Write a formula for iron(III) chloride.1. Identify the cation and anion. iron (III) = Fe3+ (III = charge of

3+) chloride = Cl−

2. Balance the charges. Fe3+ Cl−

Cl− = (3+) + 3(1-) = 0

Cl− 3. 1 Fe3+ and 3 Cl− = FeCl3

Ionic compounds containing polyatomic Ions

A polyatomic ion • group of atoms. • has an overall ionic charge.

Some examples of polyatomic ions are

NH4+ ammonium OH− hydroxide

NO3−

nitrate NO2−

nitrite

CO32− carbonate PO4

3− phosphate

HCO3− hydrogen carbonate

(bicarbonate)

Some Names of Polyatomic Ions

The names of common polyatomic anions • end in ate. NO3

− nitrate PO43−

phosphate• with one oxygen less end in ite.

NO2− nitrite PO3

3−

phosphite• with hydrogen attached use the prefix

hydrogen (or bi).HCO3

− hydrogen carbonate(bicarbonate)HSO3

− hydrogen sulfite (bisulfite)

Naming Compounds with Polyatomic Ions• The positive ion is named first followed

by the name of the polyatomic ion.NaNO3 sodium nitrate

K2SO4 potassium sulfate

Fe(HCO3)3 iron(III) bicarbonate

or iron(III) hydrogen carbonate

(NH4)3PO3 ammonium phosphite

Writing Formulas with Polyatomic IonsThe formula of an ionic compound • containing a polyatomic ion must have a

charge balance that equals zero (0).Na+ and NO3

− NaNO3

• with two or more polyatomic ions has the polyatomic ions in parentheses.Mg2+ and 2NO3

− Mg(NO3)2

subscript 2 for charge balance

What are covalent bonds?

Covalent bonds form

• when atoms share electrons to complete octets.

• between two nonmetal atoms.

• between nonmetal atoms from Groups 4A, 5A, 6A, and 7A.

Forming Octets in Molecules

A hydrogen molecule• is stable with two

electrons (helium).• has a shared pair of

electrons.

In a fluorine, F2,,

molecule, each F atom• shares one electron.• attains an octet.

Carbon forms 4 covalent bonds

In a CH4, methane, molecule

• a C atom shares 4 electrons to attain an octet.

• each H shares 1 electron to become stable like helium.

Multiple Bonds

In nitrogen molecule, N2,

• each N atom shares 3 electrons.

• each N attains an octet.

• the bond is a multiple bond called a triple bond.

• the name is the same as the element.

Naming Covalent Compounds

name covalent compounds

• STEP 1: =name first nonmetal as an element.

• STEP 2: name second nonmetal with an ide ending.

• STEP 3: Use prefixes to indicate number of atoms (subscript) of each element.

Table4.12

Naming Covalent Compounds

What is the name of SO3?

1. The first nonmetal is S sulfur. 2. The second nonmetal is O named oxide.3. The subscript 3 of O is shown as the prefix

tri.SO3 sulfur trioxide

The subscript 1 (for S) or mono is

understood.

Naming Covalent Compounds

Name P4S3.

1. The first nonmetal P is phosphorus. 2. The second nonmetal S is sulfide.3. The subscript 4 of P is shown as tetra. The subscript 3 of O is shown as tri.

P4S3 tetraphosphorus trisulfide

Guide to Writing Formulas

The prefixes in the name are used to write the formula.

STEP 1: Write the symbols in the order of the elements in

the name.

STEP 2: Write any prefixes as subscripts.

Example: Write the formula for carbon disulfide.

STEP 1: Elements are C and S

STEP 2: No prefix for carbon means 1 C

Prefix di = 2

Formula: CS2

What is electronegativity?

• Indicates attraction of an atom for shared electrons.

• increases from left to right going across a period on periodic table.

• is high for nonmetals with fluorine as highest.

• is low for metals.

Polar and non-polar Covalent BondsA non-polar covalent bond

• occurs between nonmetals.• is an equal or almost equal sharing of

electrons.

A polar covalent bond

• occurs between nonmetal atoms.• unequal sharing of electrons