Valency

Valency

Combining power of atoms

Valency of different elements Valency (historical meaning) - no. of

bonds to be formed by an atom.

Historically, the values are deduced from the formulas of some chemicals.e.g. H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Valency deduction exercise Deduce the valency of each kind of

atom from the following formulas.H2O, CH4, HCl, NH3, H2, HF, H2S, PH3, SiH4, HBr, Cl2

Atom H O C Cl N F S P Si Br

Valency




Valency 1




Valency 1 2 4 1 3 1 2 3 4 1

Valency of some atoms

Valency 1 2 3 4

Atom

HFClBr

OS

NP

CSi

Positions of the first 20 Elements

Positions of the first 20 Elements

Is there any connection between the position of an element in the periodic table and the valency of the atom ?

YesExcept H, the valency of an atom is the same as (8 – group no.) of the atom. i.e. Extra no. of electrons required to fill the outermost shell.

Valency of different elements Valency - no. of bond to be formed

Real meaning of valency Valency of chlorine : 1

Formula of chlorine molecule : Cl2

Covalent bond formation by electron sharing

co- means together; jointly; -valent means outermost shell

A non-metal atom can form bond by sharing outermost shell electrons with another atom.

Electrostatic nature of covalent bond

e le c tro s ta tica ttra c tio n

e le c tro s ta tica ttra c tio n

n e g a tiv eb o n d in ge le c tro n s

p o s it iv en u c le u s

p o s it iv en u c le u s

No. of electrons to be shared

No. of electrons to be shared= No. of electrons required to fill the

outermost shell= 8 - no. of electrons present= 8 - group no.

Exception :H has only 1 electron to share.


No. of electrons to be shared by group VII atom e.g. F, Cl, Br, I = 8 - 7 = 1

No. of electrons to be shared by group VI atom e.g. O, S = 8 - 6 = 2

No. of electrons to be shared by group V atom e.g. N, P = 8 - 5 = 3

No. of electrons to be shared by group IV atom e.g. C, Si = 8 - 4 = 4


Each electron to be shared can be considered as a hand to join with other atoms.

For a stable molecule, all hands have to be held together i.e. all atoms have completely filled outermost shell.

Real meaning of valency Valency of an atom

= no. of bond to be formed= no. of electrons to be shared= 8 – group no.

Not all compounds are made of molecules, some have electrically charged groups of atoms called ions.

One of these is aluminium sulphate, known as alum, whose formula is Al2(SO4)3 and which contains positive aluminium ions and negative sulphate ions, the SO4 part of the formula.

There are a small number of these poly-atomic ions whose formula we will need to know. Their names may be familiar:-

sulphate SO42-

nitrate NO3-

carbonateCO32-

hydroxideOH -

where the negative signs indicate the electrical charge

The question now is:-

“How do we know how many of each atom are needed?”.

The question now is:-

“How do we know how many of each atom are needed?”.

It turns out that atoms of each element (or poly-atomic ion) have a number associated with them which determines how they all link up together. This is sometimes called the valency or valence.

What a difficult set of numbers to learn!Or are they?

Let’s re-arrange the list

H hydrogen +1Na sodium +1K potassium +1Mg magnesium+2Ca calcium +2Al aluminium +3

Crchromium +3Fe iron +2 or +3Cu copper +2Zn zinc +2Ag silver +1

Cl chlorine -1Br bromine -1I iodine -1O oxygen -2S sulphur -2P phosphorus -3N nitrogen -3C carbon ±4Si silicon ±4

NO3 nitrate -1OH hydroxide -1CO3 carbonate -2

SO4 sulphate -2

So how do we use this table?Here are a few simple steps to follow in going from a compound’s name to its formula:-I’ll use calcium chloride as a example1 write the symbols implied by the name

Ca and Cl (compounds of two elements usually have a name ending in –ide, for three elements it’s usually -ate)2 Find the numbers on the list

Ca +2 and Cl -13 The rule now is that “The numbers in the compound must add to zero”.

one Ca (+2) and two Cl (-1) will add to 0.4 And so the formula is CaCl2

2 Now let’s try aluminium iodide1 Al and I

2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -1

2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I

2 Now let’s try aluminium iodide1 Al and I2 Al +3 and I -13 one Al will need 3 I4 formula is AlI3


3 sodium oxide


3 sodium oxide1 Na and O


3 sodium oxide1 Na and O2 Na +1 and O -2


3 sodium oxide1 Na and O2 Na +1 and O -23 this time we need two Na for one

O



O4 formula is Na2O



O4 formula is Na2O

notice that in the name and the formula, the metal or hydrogen (things which have the positive numbers) come first.

A few more examples

3 calcium sulphate 1 Ca and SO4

A few more examples


2 Ca +2 and SO4 -2

A few more examples


2 Ca +2 and SO4 -23 one Ca for one SO4

A few more examples


2 Ca +2 and SO4 -23 one Ca for one SO4 4 formula is CaSO4

A few more examples3 calcium sulphate

1 Ca and SO4

2 Ca +2 and SO4 -23 one Ca for one SO4 4 formula is CaSO4

4 magnesium hydroxide1 Mg and OH2 Mg is +2 and OH -13 one Mg for two OH4 formula is Mg(OH)2

5 Aluminium carbonate1 Al and CO3

2 Al +3 and CO3 -23 two Al for three CO3 4 formula is Al2(CO3)3



All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated.



All these examples have had a metal as one part of the compound. If two non metals combine life is a bit more complicated. We often need to just make sure that the numbers are equal for each element. It is easier than that all sounds!

Let’s work out some examples

1 Silicon dioxide


1 Silicon dioxide1,2Si 4, O 2


1 Silicon dioxide1,2Si 4, O 23 to make the values

equal we would need two O for each Si

Silicon dioxide1,2Si 4, O 23 to make the values

equal we would need two O for each Si4 SiO2, quartz

Try

Hydrogen nitride (usually called ammonia)

1,2 H 1, N 33 for equal values we need

three H for each N4 NH3, ammonia

3 Phosphorus oxide1,2 P 3 and O 23 To make these the

same for each element we need 2 P and 3 O

4 P2O3

You should now be able to write the formula for several hundred compounds. You will find that the number associated with each element is easy to remember with a bit of practice.

Writing Formulae.

Formulae of compounds can be written using either

1. The name of the compound.

“mono” means one“di” means two“tri” means three“tetra” means four“penta” means five

If any of these prefixes appears in the name of the compound we use this to write the formula.e.g. carbon monoxide is COcarbon dioxide is CO2

2. Using Valency Rules

If the name does not give any clues about the formula, we use valency rules.

e.g. sodium oxide

write symbols Na O

write valency 1 2

cross over valency 2 1

cancel if necessary

write formula Na2O

Note:- For transition metals the valency is usually written as a Roman numeral in brackets after the name of the transition metal

e.g. In iron(III) oxide, the iron has a valency of 3.

3. Formulae involving Complex (group) Ions

If the name of the compound ends in “-ate” or “ –ite” , it contains a complex ion (watch also for hydroxide). Complex ions are found in your data book (p4).

e.g. Copper(II) nitrate

write symbols or formula CuNO3

-

write valency 21

(for complex ions, this

is the charge)

cross over valency 12

Use brackets and cancel

if necessary

write formula Cu(NO3)2

4. Formulae Showing Charges

Sometimes you are asked to show the formula with charges (ionic formula)

e.g. sodium carbonate

write symbols or formula NaCO3

2-

write valency 1 2

(for complex ions, this

is the charge)

cross over valency 2 1

Use brackets and cancel

if necessary

write formula leaving charges (Na+ ) 2 CO3

2-

Write ionic formulae for the compounds listed above.

Calculations for you to try.

SO3

1. Write the formula for:-

(a) Sulphur trioxide

(b) Dinitrogen monoxide

(c) Phosphorus pentachloride

(d) Carbon terachloride.

N2O

PCl5

CCl4


(a) Sodium oxide.

(b) Aluminium iodide

(c) Iron(III) oxide

(d) Silver(I) oxide.

AlI3

Na2O

Fe2O3

Ag2O


(a) Sodium carbonate

(b) Aluminium nitrate

(c) Iron(III) hydroxide

(d) Nickel(II) sulphate

Na2CO3

Al(NO3)3

Fe(OH)3

NiSO4

4. Write the ionic formula for:-

(a) Lithium hydrogencarbonate

(b) Aluminium hydroxide

(c) Iron(II) sulphate

(d) Copper(II) nitrate.

Li+HCO3-

Al3+(OH-)3

Fe2+SO42-

Cu2+(NO3-)2

Valency

Documents

metals combine life

valency deduction exercise

aluminium iodide al

complex ions

real meaning

values equal

si br

sodium oxide