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Notes on Valency and Molecular Formula

586 / May-28-2021 20:57:54 By- srijan /

1. Elements: An element is the pure form of substance which cannot further be broken down into any simpler form. 
There are 92 natural elements and the rest artificial elements. Elements can be grouped into metal,non-metal,metalloid , inert element, etc. Similarly elements can be grouped into s- block, p- block, d- block and f- block etc.

2. Atoms: An atom is the smallest indivisible particle of an element that can take part in a chemical reaction which may or may not exist independently . One substance may be composed of one or more kinds of atoms. Chlorine is made of two chlorine atoms. Water is made of hydrogen atoms and oxygen atom. Depending on the substance, the ratio of the atoms of different elements vary.
Name two elements other than chlorine whose molecule consists of two atoms.

3. Atomic Number: The total numbers of protons or electrons present in the atom is called atomic number.
Therefore, Atomic number= No.of protons + No. of neutrons.
Therefore, z = p++ e-

4. Atomic Weight
The sum of numbers of protons and numbers of neutrons present in the nucleus of an atom is atomic weight. 
It can also be called as Atomic mass and denoted by 'A'. Atomic weight = No. of protons + No. of neutrons . It makes convenient in measuring all over the world.

5. Electronic Configuration: The systematic arrangement of elements in the different shells or orbit around the nucleus of an atom is called electronic configuration. A shell is the space around the nucleus of atom where there is high probability to find electron. Shell or Orbit can be represented by K, L, M, N, O,..... with number 1, 2, 3, 4, 5,...... respectively. Every shell or orbit has a definite number of elecrtrons that can be placed. This number of electrons can be calculated by 2n2 formula . This formula help us to calculate maximum numbers of electron that can be placed in particular shell or orbit.

Shell 2n2 Max. electrons available
K = 1


K = 1


K = 1


K = 1



Table : (orbital electronic configuration)

The number of protons, electrons and neutrons in the atoms of first 20 elements along with the distribution of electrons in various shells is given in below table:::

Element Symbol Atomic Number No. of protons No. of neutrons No. of electrons Electronic Configuration
Hydrogen H 1 1 0 1 1
Helium He 2 2 2 2 2
Lithium Li 3 3 4 3 2,1
Beryllium Be 4 4 5 4 2,2
Boron B 5 5 6 5 2,3
Carbon C 6 6 6 6 2,4
Nitrogen N 7 7 7 7 2,5
Oxygen O 8 8 8 8 2,6
Fluorine F 9 9 10 9 2,7
Neon Ne 10 10 10 10 2,8
Sodium Na 11 11 12 11 2,8,1
Magnesium Mg 12 12 12 12 2,8,2
Aluminium Al 13 13 14 13 2,8,3
Silicon Si 14 14 14 14 2,8,4
Phosphorus P 15 15 16 15 2,8,5
Sulphur S 16 16 16 16 2,8,6
Chlorine Cl 17 17 18 17 2,8,7
Argon Ar 18 18 22 18 2,8,8
Potassium K 19 19 20 19 2,8,8,1
Calcium Ca 20 20 20 20 2,8,8,2

Table : (electronic configuration)

From the above table, the elements like Ne and Ar, it is seen that there are 8 electrons in the outermost shell/ orbit. These elements do not combine or react with each other.Therefore, they are also known as inert elements (gas). Such a stable shell of eight electrons in an atom is called octet.

Similarly, helium has two electrons in the first ( K ) shell. A K- shell can accomodate only two electrons. Therefore, such a shell is also stable. The stable shell of two electrons with a single shell of an atom is called duplet.

When two or more elements combine together chemically, then the combination results in the formation of a compound. Molecules are formed when atoms combine together to form a stable structure and has independent existence. The smallest particle of an element or a compound that exists independently is called a molecule. There are two types of molecules i.e. molecules of elements (O2, F2, Cl2, H2, Br2, I2, N2 only) and molecules of compounds (CO2, H2O, NH3 etc.) The stability is achieved by the elements if there is a transfer or sharing of the electrons between atoms. In this process, the combining atoms attain the configuration like that of the nearest inert gas i.e. either duplet or octet state.
Question : What is molecule? How is the molecule of a substance formed?

Octat Rule:

The atoms tend to adjust the arrangement of electrons in such a way that they achieve 8 electrons in the outermost shell is called octet rule.
Duplet Rule:

The behaviour of elements assuming as stable if they have 2 electrons in the single shell i.e. have k shell only is called duplet rule. Helium "He" is stable based on duplet rule.

Compounds: Compound is a substance which is composed of two or more elements, chemically joined which cannot be separated by separated by physical means. The properties of compounds are unique by themselves and are different from those of the constituent elements. In a compound the number of atoms of elements are combined in a definite ratio. There must be a certain force by which these atoms are glued together in forming a compound. This force of attraction that holds the atoms together in a molecule is called a bond. A bond is resulted in between the atoms by following ways:

  • By the transfer of electrons.
  • By the sharing of electrons between the atoms.
  • The transfer or sharing of electrons is determined by the number of electrons in the outermost shell. After the transfer or sharing of electrons, an atom attains a stable configuration.

Electrovalent or Ionic Combination: In this combination, one atom of metallic element or group loses electron from its outermost shell. Those electrons pass over to the outermost shell of non metallic element or group of elements. When electron transfer from one atom to another atom, an octet is left behind in the metallic element and created in the non- metal. Thus both species attain the nearest inert i.e. noble gas configuration. They differ from noble gas in such a way that the atom which has lost the electrons gains positive charge and the atom that has gained electron develops the negative charge. These particles are no more atom and called ions. When unstable atomic form changes into ionic state by losing or gaining electrons and attains the stable configuration, the release of energy occurs so that the final state is more stable than the initial state. It is something like dropping a stone from the top of a cliff to its bottom, it releases energy and comes to the stable state.
Consider a chemical reaction which occurs when the metal sodium is placed in a container with chlorine gas. A vigorous reaction takes place and sodium burns to form sodium chloride. Sodium Chloride is a typical example of electrovalent compound.

Before combination

Elements No. of protons No. of electrons electronic configuration
Sodium atom 11 11 2,8,1
Chlorine atom 17 17 2,8,7

After combination

Elements No. of protons No. of electrons electronic configuration
Sodium atom 11 11 2,8,1
Chlorine atom 17 17 2,8,7

Structure as Ne. Similarly, by receiving an electron from sodium, an atom of chlorine can attain the stable structure as argon.
When sodium atom loses electron, it will contain one more positively charged proton than electron. As a result it will form positively charged sodium ion. Similarly, by gaining one extra electron, chlorine atom develops into chloride ion.

Questions : Why is sodium ion not neutral?
Questions : Inspite of similar configuration (2,8,8), why chloride ion not called argon atom?
Questions : What is bond. Write its types?

The sodium chloride ions formed are held together by an electrostatic attraction. This attractive force is called electrovalent or ionic bond. In the solid state millions of Na + and Cl - ions are held in place in the crystalline form by these bonds.
Similarly, in the formation of ionic compound calcium chloride Ca 2+(Cl -) 2 a calcium atom has the configuration 2,8,8,2 and so attain stables configuration.

It must give away 2 electrons. Chlorine accepts only one electron per atom, so for each calcium atom to react, it requires two chlorine atoms.
Electrovalent compounds are solid and have high melting and boiling points. They dissolve in water but not in organic solvents. Their aqueous solution is a good conductor of electricity.

Covalent combination : Consider the combination of two chlorine atoms to form a chlorine molecule. Each chlorine atom has the configuration 2,8,7 which is one electron shorter than the inert gas argon. Each atom can effectively attain the stable configuration of argon by contributing one electron to a pair shared by both the atoms.

Here two atoms are being held together by the attraction of each positive nucleus for the pair shared electron. The binding force resulting from the sharing of pair of electrons between atoms is known as covalent bond and the compounds formed by this method of combination is called covalent compounds.
Examples of covalent compounds are: CH4, NH3, CO2, H2O

Covalent compounds do not contain any ions and are non- electrolytes. Simple covalent compounds are gases or low boiling liquids. Atoms are held together by a weak force of attraction and therefore they have a low melting point and a boiling point. Covalent compounds are soluble in organic solvents like alcohol, benzene, ether, etc. and insoluble in water.

Difference between O2 and 2O

2O O2
It has 2 atoms of oxygen. It has one molecule of oxygen
It is highly reactive. It is stable in electronid configuration.

Difference between Electrovalent Compound and Covalent Compound

Electrovalent Compound Covalent Compound
It is formed due to the complete transfer of electron from Basic Radical to Acidic Radical. It is formed due to mutual sharing of electrons between the constituent atoms.
It is mostly soluble in water. It is mostly insoluble in water.
It conducts electricity in solution state. It doesnot conduct electricity.
For eg. NaCl, MgCl 2, Na 2O For eg. CH 4, H 2O, NH 3

Valency The combining capacity of an element or radical is called valency. Or, the valency may be defined as the number of electrons to be lost, gained or shared to obtain the stable configuration. But there are some elements which have more than one valency. Such valency cannot be explained by above statement.
Questions: What is the valency of phosphorus in P2O5

Here are valency of some common elements::

Valency of some elements:

These elements have more than one valency i.e. variable valency.

Radicals: A radical is an atom or a group of atoms which behaves as a single unit in many chemical reactions and carries an electric charge in it. Like element, a radical also has its own valency which is equal to the number of charges present in it. It cannot exist independently but remain as free ions in aqueous solution.

Molecular Formula
The molecular formula of a compound is one which expresses the actual number of each kind of atom preset in its molecule.
So, the symbolic representation of molecules of elements is molecular formula.
Molecular Weight: The sum of atomic weight of atoms of a molecule is molecular weight.
Below are some examples of molecular formula by valency exchanging method: