Chemistry : Chemical Formulae And Valency

Chemical Formulae And Valency

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Chemical Formulae And Valency

Symbols Of Element

In chemistry, there are two ways to identify an element,
These two ways are;
a] Symbol and,
b] Valency.
Each element has a chemical symbol and a valency,

Let start with “symbol”
Each element has a symbol that represents one atom of the element,
Some atom are represented by the short-hand notation of its name while some are represented by abbreviation of its latin name.
E.g magnesium as Mg or carbon as C, top↑
Some elements symbols are the abbreviation of their latin names e.g
Gold is Au which has its latin name as Aurum, Iron is represented as Fe from the latin word Ferrum and Copper which is represented as Cu from the word Cuprum

These are the list of common elements

note : If you see anything on this table that you don’t understand, don’t panic, just scroll down to the end of the table for explanation.top↑

Symbols Of First 23 Elements

                                                 
Elements       |Symbols|Valency|
1] Hydrogen       H          +1, +2 (vary)
2]  Helium          He           zero
3]  Lithium         Li          +1 (LiAl(OH)4)
4]  Beryllium      Be               +2
5]  Boron            B                 +3
6]  Carbon          C           -2, -4 (vary)
7]  Nitrogen        N          -2, -4 (vary)
8]  Oxygen          O          -2, -1 (vary)
9]   Fluorine        F                  -1
10]  Neon.           Ne            Zero
11]  Sodium        Na              +1
12]  Magnesium    Mg          +2
13]  Aluminum       Al           +3
14]  Silicon              Si          +4
15] Phosphorus      P          +5
16] Sulphur              S        +4,+6(vary)
17] Chlorine             Cl         -1
18] Argon                 Ar         zero
19] Potassium         K            +2
20] Calcium             Ca          +2
21]  Iron                    Fe       +2,+3
22]  Copper              Cu         +2
23]  Zinc                   Zn         +2

Explanation

Now let explain why some have variable charge and why some have zero charge.top↑

Let start with hydrogen, will know hydrogen is always +1, yes, but in some compound like hydrogen peroxide, hydrogen is +2 not +1, how?

Hydrogen peroxide (H2O2)
If you find the oxidation state of hydrogen in that compound, it is two,

H2O2 = zero
Oxygen has a charge of -2, multiply by 2
-2 × 2 = -4, if the -4 shift to the right side,
H2 = +4
4 divided by the 2 in front of hydrogen, that will be +2, hence Hydrogen is +2 in hydrogen peroxide,
Same thing also applied to Oxygen, Oxygen is always -2 but  -1 in all peroxide, I hope you know what a peroxide is? Some student make mistake and take H2O2 as Peroxide, but capital NO, H2O2 is hydrogen peroxide,
Peroxide is when an oxide contain high proportion of oxygen than ordinary oxides. I will explain better when will get to oxides. top↑
Other examples of peroxide are;
Na2O2, CaO2and BaO2 etc
The oxidation state of oxygen in all this compound(peroxide) is always -1 not -2, I will also explain Oxidation state better when will get there, which is probably our next topic but for now, let know the meaning of some simple terms.

Second Explanation

Why is helium, neon and argon zero? helium, neon and argon are rare/noble/inert gases, and inert gases are chemically unreactive. You know, every element undergoes chemical reactions in order to obey duplect or octect rule, but rare gases as already have their outermost shell filled with 8 electrons, that is why they are called group zero element.
top↑

Third Explanation

Lithium is +1, this compound often comes out in examinations be it Jamb, post jamb or Weac,
They ask this question that you should name this compound;
LiAl(OH)4
That compound is named,         
Lithium tetra hydrido aluminate iii

Others

The oxidation state of an element in a compound depends on the type of the compound in which the element is present in,
For example, No2 is different from No, so is the oxidation number of Nitrogen,
In the first compound No2 (Nitrogen iv oxide) the oxidation state of nitrogen is -4, while in No (Nitrogen ii oxide) the oxidation state of nitrogen is -2, etc.
Do the same for other elements, to put yourself to test, whether you understand.top↑

Note : No2 and No are radicals, just use it as an example, nitrogen oxides are mostly radicals, No2 and No, etc, but in the case of N2O (nitrogen i oxide) the oxidation state of nitrogen is +1, how?
N2O = zero
2(N) + O = zero
N = ?
O = -2
2(N) + ( -2) = zero
2N = +2
N = +2/2
N = +1

There are two common iron, Fe2+ and Fe3+ depend on the one in a particular compound.top↑

Valency

Valency is the combining power of an element. Before you can write the correct formula of any compound you must be able to identify the valency of each element present in that compound.
E.g Caco3 (calcium trioxocarbonate iii)
In Caco3, calcium which is represented as Ca has a valency of 2 and trioxocarbonate which is represented by co3 has a valency of 2, to form Caco3, the two elements have to exchange their valency, which will simply cancel out and Caco3 will be formed.

Note : valency has no charge, e.g oxygen has oxidation state of -2, its valency is 2,
Calcium has oxidation number of +2 its valency is 2 not +2 because it is the oxidation number that have charge.top↑

But in some cases like, Na2O,
Sodium (Na) has a valency of 1 and Oxygen has a valency of 2, in exchange sodium will have 2 and oxygen will have 1 which leads to the formation of Na2O (Sodium oxide).

Radicals and formulae of compound

Radicals Of Compounds

Radical : Radical are group of atoms which behave as one unit in a compound, that is, they do not exist separately outside a compound.

Types Of radicals

Radicals are divided into two parts namely

Cations And Anions

Cations are the positive metallic radicals while anions are the negative non-metallic radicals.

note some non-metallic radicals contain more than one atom e.g trioxocarbonate (iii) Co3 ion which has a valency of 2 or tetraoxosulphate (iv) ion So4 which has a valency of 2top↑

Formulae of Compounds

Valency serves as the key point for writing the correct formulae of a compound.
E.g magnesium chloride,
to write the correct formula of any compound, you must first know,

the name of the element in the compound?

The correct symbols of these elements?

And the valency of each of these elements.

The name of these two elements present in magnesium chloride is magnesium and chlorine

Their correct symbols are Mg and Cl

The next is their valency,
Mg = 2 ; Cl = 1.

After knowing these three things the next step is to join the two elements together.top↑

When doing this write the symbol of the metal first with its valency, then write the symbol of the non-metal next with its valency.
Now let join the two elements together

Mg2 Cl 1
Before magnesium and chlorine could form Magnesium chloride, the two elements must interchange their valencies.
When this happened, then we will have,
Mg 1 Cl 2 to form Mgcl2.

note valency has no charge.

Another example

H2SO4
Elements or radicals
Present: Hydrogen : tetraoxosulphate (iv)
Symbols: H : So4
Valency: 1 : 2
When they interchange;
The valency of hydrogen becomes 2 while the valency of tetraoxosulphate (iv) becomes 1

Then will have H2SO4.top↑

note : SO4 is a radical not an element or a compound.

List Of Common Radicals

Cation            Oxidation          Valency

K+                         +1                             1

Na+                       +1                           1

Mg2+                     +2                         2

Ca2+                       2+                        2

Al3+                       +3                         3

NH4+                      +1                        1

Fe2+                        +2                       2

Fe3+                         +2                      2

Cu2+                        +2                      2

Zn2+                        +2                      2

Pb2+                         +2                      2
Anion          Oxidation           Valency

Cl-                        -1                             1

NO3-                    -1                             1

OH-                      -1                             1

SO42-                   -2                           2

SO32-                   -2                           2

CO32-                   -2                          2

MnO4-                  -1                          1

PO43-                   -3                          3

ClO3-                    -1                          1

Let stop here,till next class.

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Next Topic : Atoms And Molecules

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