Ionic Compound

Elements combine together to form compounds. Chemical compounds are of many types, depending upon the bond present in them. Corresponding to the two ways by which any two atoms rearrange to form a compound, two types of bonds are formed.

1. Ionic bond
2. Covalent bond.

Ionic bond or electrovalent bond is established by the transfer of one or more valence electrons from one atom to the other.

Thus, ionic bond is a chemical bond formed between two atoms by the transfer of one or more valence electrons from one atom to the other.

This bond is also called as a polar bond.

Formation of an ionic bond:

Formation of an ionic bond can be explained using the following example:

Consider an atom A , which has two electrons in its outermost shell. Another element B, has 6 electrons in its outermost shell.

The atom A has two electrons in excess, to make it to the Noble gas electronic configuration, while atom B has two electrons less to make it to that level.

Now, atom A gives two of the excess electron to atom B, and by this, atom A attains a completely filled shell, while atom B, having attained the required amount of electrons, also reaches the noble gas configuration.

They therefore form an ionic bond between themselves.

List of Ionic Compound Formula-

Lists of ionic-compounds with their formula are:

S.No	Ionic-compound	Formula
	Sodium chloride	NaCl
2.	Potassium iodide	KI
3.	Lithium iodide	LiI
4.	Aluminium oxide	Al2O3

Ionic Compound Example-

Some examples of ionic compound are:

Magnesium oxide - MgO – Mg²⁺, O^2-

Calcium Fluoride – CaF₂ – Ca²⁺, 2F^-

Aluminium Fluoride – AlF₃ – Al³⁺, F^3-

Properties of an Ionic Compound-

1. Ionic-compounds are three dimensional solids, with well-defined geometrical pattern.
2. Ionic solids conduct electricity when they are in water solution or in the fused state (molten state).
3. They are quite hard, have low volatility and high melting and boiling point.
4. Ionic solids are soluble in polar solvents, due to dissociation of their ions.
5. Ionic solids are very stable and have very high density.

Is NaCl an Ionic Compound-

Sodium chloride, NaCl is ionic in nature.

The formation of sodium chloride is as follows:

Na has an electronic configuration of 2, 8, 1. The last electron, in the third shell, has to be removed, for it to attain noble gas configuration.

Chlorine has a configuration of 2, 8, 7. It needs one extra electron, which it gains from Sodium, thereby getting the required magic number of ‘8’.

Is Salt an Ionic Compound–

Sodium chloride is also called as common salt. Other than this, most of the compounds, commonly known as salts are formed from the neutralization reaction of an acid and a base. They are all ionic-compounds, because they dissociate into ions in their solution.

Bonding and molecular structure

Syllabus : 

Valency electrons, the octet rule. Electrovalent and covalent bonds with examples. Properties of electrovalent and covalent compounds. Limitation of octet rule (examples), coordinate covalent bonds (examples).

Directionality of covalent bonds, shapes of polyatomic molecules (examples), concept of hybridization of atomic orbitals (qualitative pictorial approach): sp³, sp² and sp hybridizations with typical examples. Tetrahedral space model of-carbon atom, single-bond, double- bond and triple - bond involving carbon atom with examples a and 7t bonds.

Valence shell Electron Pair Repulsion (VSEPR) concept (elementary idea) - shapes of H₂0, H₂S,
CH₄, NH₃, C0₂, N0₂ and S0₂ molecules. Concept of resonance (elementary idea), resonance structures (examples). Elementary idea about electronegativity, bond polarity and dipole moment. Hydrogen bonding (inter - & intra molecular structures) and its effects on physical properties (mp, gp, and solubility).

Double salts and complex salts, and coordination compounds (examples only), coordination number (examples with C.N 4 and 6 only).

Valency Electrons and the Octet rule

When details of the electronic configurations of the elements came to be known, it was found tha the arrangement of energy levels in different orbits round the nucleus was different for different elements. Chemical union between atoms to form molecules are energetically favoured only if the chemical combination leads to lowering of energy of the system i.e, if the energy of the combined atoms i.e., the product molecule is less than the sum of the energies of the reactant molecules.

Chemical combination of atoms involves electrons of the two atoms and nuclei take no active part in chemical combination. In each individual atom, the outermost electrons have the highest energy amongst all its electrons. So the lowering of energy must be through the interactions of the outermost electrons of the two interacting atoms. Thus chemical combination in all probabilities, should involve only the outermost electrons of the participating atoms and the interactions should be such that it causes lowering of energy w.r.t. to the initial condition when the atoms lay separated from each other.

A close study of chemical properties of the different classes of elements led us to the fact that the noble gases were the most chemically inactive species amongest all the elements. They had no tendency to combine with themselves or with other elements. They were so inert that they even did not like to form molecules by the combination of two atoms. Inert gas molecules are monatomic — or in other words their atoms do not form molecules. They are devoid of any chemical affinity. mSo it may be seen in the light of our discussion above that their outer electronic configurations are most stable amongest all the elements and no further stabilization is possible by further interaction among themselves or with electrons in other elements.

The extra nuclear electrons present in n successive inert gases are 2,10,18, 36,54 and 86. These numbers were termed magic numbers as the presence of electrons in any one of these numbers in an atom gives special stability to the atom. This stability is lost if this number is changed even by 1 unit on either side.

The arrangement of the electrons in the inert gases can be described as follows :
He — 2
Ne — 2-8
Ar — 2-8-8
Kr —2-8-18-8
Ne —2-8-18-18-8

Molecular mass of polymers

Introduction :  
Several simple organic molecules of one or two types combine with each other by chemical bonds forming macro molecules the product is called a polymer. Simple organic molecules which can form polymers by chemical bonding are called monomers while this process of chemical combination is called polymerisation. In any sample or monomers there are very large number of molecules of lower masses same molecular weights and similar physical and chemical properties.

Polymers:

Ehereas in polymer sample having same molecular weights the number of molecules are very small. The polymers have comparatively very high molecular weight but all molecules do not have comparatively very high molecular weight but all molecules do not have identical molecular weights. Polymers prepared from the same monomer in different conditions do not have all the properties identical. Depending on reaction conditions polymer products have different proportions of molecules of lower and higher masses.

Molecular mass of Polymers:
Overall molecules of ethene combine with each other by addition reaction and give polyethene. This reaction at the first stage two molecules of ethene monomer combine together giving a dimer. A third molecule of ethene combines with this dimer giving a trimer and a forth molecule combine further gives a tetramer. This way monomer molecules go on joining and chain becomes longer. As result very large chain is former which is called macro molecule or polymer.
CH₂ = CH₂ `stackrel(CH_2=CH_2)(->)` CH₃-CH₂-CH = CH₂ `stackrel(CH_2=CH_2)(->)`
CH₃-CH₂-CH₂-CH₂-CH=CH₂ `stackrel(CH_2=CH_2)(->)`
CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH=CH₂
`stackrel(nCH_2=CH_2)(->)`  [-CH₂-CH₂-]_n

The polymer chain can be lengthen upto certain limit at laboratory cantons. The tendency of this long chain then decrease to combine with further monomers. Thus in any condition polymers resulting from monomers do not increase in weight more than a certain limit. Generally any polymer asmple contains varying chain-lengths, its molecular mass is always an average molecular mass. The molecular mass of a polymer is expressed as number average molecular mass `barM_n` or weight average molecular mass `barM_w`.
`barM_n = (sumN_tM_t)/(sum tN_t)`
`barM_w = (sumN_tM^2_t)/(sumtN_tM_t)`
Where N_t = number of molecules
M_t = molecular mass.

Molecular Orbital Theory

Introduction :
Molecular orbital theory is a polycentric region in space, defined by its size and shape, associated with two or more atoms in a molecule and each has a capacity of two electrons with opposite spins. Thus, in a molecular orbital, electrons are revolving in the field of more than one nucleus. The molecular orbital to explain formation of chemical bond, relative bond strengths, paramagnetic or diamagnetic nature.

Feature of Molecular Orbital Theory:

• The main features of the Molecular Orbital Theory are:
• The atomic orbital of the combine atoms partly cover to form new orbital, called molecular orbital.
• As a result of this, the atomic orbitals lose their being identity.
• Thus in a Molecular Orbital Theory, electrons revolves in the field of more than one nucleus.
• The number of Molecular Orbital is produced is equal to the number of overlap atomic orbitals.
• Maximum capacity of a Molecular Orbital is two electrons with opposite spins.
• Only those atomic orbitals can come together to form Molecular Orbital Theory which has analogous energies as well as proper orientations.
• Molecular orbital theory obtained addition of wave functions of atoms involved,`Psi`(MO)=`Psi`_A + `Psi` _B   is called bonding molecular orbital.
• Molecular orbital theory obtained by subtraction of wave functions of atoms involved  `Psi`  ^*(MO)=`Psi`_A - `Psi`_B is called antibonding molecular orbital.
• Probability of bonding molecular orbital formation greater than that of antibonding molecular orbital formation.
• Molecular theory gives the electron probability distribution around a group of nuclei just gives the electron probability distribution around nucleus.
• The shape of the molecular orbital theory produced depends on the type of the combining atomic orbitals.
• Inner molecular orbital theories which do not take part in bond formation are called non-bonding molecular orbital theory.

 Conditions for the formation of molecular orbitals:

• Any two atomic on combination do not form molecular orbitals.

• In fact, there are certain limitations to the combination of atomic orbitals.

• The energies of combining atomic orbitals should of similar magnitude.

• Thus, a homonuclear diatomic molecule will not be formed. if 1s orbital of one atom overlaps with 2s-orbital of another atom.

• Combination of atomic orbitals takes place only, if overlapping takes place to a considerable extent, since greater the overlapping of atomic orbitals, the greater is the build-up of the charge between the nuclei.

• The combining atomic orbitals should have power over the same symmetry about the molecular axis.

Chemical equilibrium animations

Introduction :

The experimental observations of chemical equilibrium tell us that most of the chemical reactions when carried out in closed vessels do not go to completion. Under these a conditions, a reaction starts by itself or by initiation, continues for some time at diminishing rates and ultimately appears to stop. The reactants may still be present but they do not appear to change into products any more. What happens in such case is that the products of the reaction start reacting at the same rate as the reactants. In other words, the rate of the back reaction becomes equal to the rate of the forward reaction.

Characteristic features of chemical equilibrium

Thus, in a given time as much of the products are formed as react back to give the reactants. The composition of the reaction mixture at a given temperature is the same irrespective of the initial state of the system, i.e., irrespective of the fact whether we start with the reactants or the products. The reaction in such conditions is said to be in a state of equilibrium.

The attainment of equilibrium can be recognized by noting constancy of observable properties such as pressure, concentration, density or color whichever may be suitable in a given case.

The relationship between the quantities of the reacting substances and the products formed can be worked out readily with the help of the law of mass action.

The Laws of Mass Action of Chemical equilibrium

The laws of mass action states that the driving force of a chemical reaction is proportional to the active masses of the reacting substances. Assuming that the driving force determines the reaction rate, the law may be stated as follows: The rate at which a substance reacts is proportional to its active mass and the rate of the chemical reaction is directly proportional to the product of the active masses of the reacting substance.

Consider a general reversible chemical reaction

aA + bB ↔ mM + nN

According to the law of mass action, assuming that active masses are equivalent to molar concentrations,

The rate of the forward reaction, r_f α [A]^a [B]^b = K_f[A]^a [B]^b

The rate of the reverse reaction, r_r α [M]^m[N]ⁿ = K_r[M]^m [N]ⁿ

Where Kf and Kr are proportionally constants and square brackets represent the molar concentrations of the entities enclosed. At equilibrium, the rate of the frontward reaction is equal to the rate of the reverse reaction, that is, Kf[A]a [B]b = Kr[M]m [N]n

K_f /K_r = K_eq = [M]^m [N]ⁿ / [A]^a [B]^b

Historical development of the periodic table

We now know more than 100 elements, the elements were classified as metals, metalloids and non metals. Metals are good conductors of heat and electricity, they are shining and they are malleable and ductile. It would be difficult to study individually the chemistry of all the elements and their numerous compounds. The periodic table provides a systematic and extremely useful framework for organizing a lot of information available on the chemical behavior of the elements into a few simple and logical patterns. This gave rise to the necessity of classifying the elements into various groups or families having similar properties.

Introduction to the historical development of the periodic table 

There were many chemists who has contributed their theories for the historical development of periodic table.  Some of them are:
Dobereiner’s Triads
Lother-Meyer’s Arrangement
Newlands Law of Octaves

Dobereiner’s Triads contribution for the historical development of periodic table

In 1829, John Dobereiner (German Chemist) classified elements having similar properties into groups of three. These groups were called triads. According to this law when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the other two elements. For example lithium, sodium and potassium constituted one triad. However, only a limited number of elements could be grouped into traids.

Newlands Law of Octaves contribution for the historical development of periodic table
In 1865, John Newlands (English Chemist) observed that if the elements were arranged in order of their increasing atomic weights, the eighth element starting from a given one, possessed properties similar to the first, like the eighth note in an octave of music. He called it the law of octaves. It worked well for the lighter elements but failed when applied to heavier elements.

Lother-Meyer’s Arrangement contribution for the historical development of periodic table

In 1869, J. Lother-Meyer in Germany gave a more detailed and accurate relationship among the elements. Lother-Meyer plotted atomic volumes versus atomic weights of elements and obtained a curve. He pointed out that elements occupying similar positions in the curve possessed similar properties.

Modern periodic law

Introduction:
Of the 109 elements known today, 92 are natural and the remaining are man made. As it was very difficult to study the chemistry of each and every element and its related compounds, in the 19th century  Chemists like Dobereiner, Newlands, Dinitri Mendeleef and Moseley thought of a proper classification of elements and did monumental work in analysing properties of these elements and classifying them .

Modern Periodic Law
" The physical and chemical properties of elements are periodic functions of their atomic numbers " .
It is a fact that atomic number is more related to the nucleus and it gives the number  of protons in the nucleus whereas  a few physical and many chemical properties of elements are related to their electronic configuration but not to the number  of electrons merely. Hence, periodic law states as

" The physical and chemical properties of elements are periodic functions of their electronic configurations "
If elements are arranged in the increasing order of their atomic numbers, the physical and chemical properties are repeated according to their electronic configurations at regular intervals.
For example: hydrogen (H0, lithium (Li), sodium (Na), potassium (K), rubidium  (Rb) are their electronic configurations as shown in table below. The atomic numbers (Z) of these elements are  1,3,11,19,37 respectively.

Alkali Metals and their electronic configurations

H ( Z = 1 )           1s1 Li ( Z = 3 )           1s2 , 2s1 Na ( Z = 11 )        1s2 , 2s2 , 2p6 , 3s1 K ( Z = 19 )          1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 4s1 Rb ( Z = 37 )         1s2 , 2s2 , 2p6 , 3s2 , 3p6 , 3d10 , 4s2 , 4p6 , 5s1

Since these elements have similar electronic configurations in their  outer  shells, ns¹ ( 'n'   is the principle number ) , they show similar physical  and chemical properties. They lose electron to get stability  and show a common  oxidation state of +1.  They are all monovalent. Their oxides, hydroxides, halides, etc.., show similar chemical properties. Thus, the properties of elements are  periodic  functions of their electronic configurations.

Present form of peridic table

Present form of periodic table is also known as Long form of periodic table.
Neils Bohr constructed the long form of the periodic table based on the electronic configurations of the elements. In the table, the vertical columns are  termed as "groups" and the horizontal rows as "periods". There are 18 groups and 7 periods in the periodic table.  All the elements are arranged in the increasing order of their atomic numbers and the atomic number increases by one unit from one element to the immediate next element as we move, from left to right in the periodic table .

Differentiating electron :
The electron by which the electron configuration of the given element differs from that of its preceding element is called differentiating electron is the last entering electron of its atom .
Periods in the long form of the periodic table : In each period, the differentiating electron enters the s-orbital in the element p-orbital in the last element . When s and p orbitals are completely filled with electrons as ns²np⁶  , the element is the noble gas with stable configuration of octet .

Periods and electronic configurations

Period	Energy Levels	Number of elements in the period
1st period 2nd period 3rd period 4th period 5th period 6th period 7th period	1s 2s                                   2p 3s                                   3p 4s                         3d      4p 5s                         4d      5p 6s            4f          5d      6p 4s            5f          6d       -	2 8 8 18 18 32 -

                        Table : Periods and energy levels in periodic table

Period	First element	Electronic configuration	Last element	Electronic configuration
1 2 3 4 5 6 7	H Li Na K Rb Cs Fr	1s1 [He] 2s1 [Ne] 3s1 [Ar] 4s1 [Kr] 5s1 [Xe] 6s1 [Rn] 7s1	He Ne Ar Kr Xe Rn -	1s [He] 2s 2p [Ne] 3s 3p [Ar] 3d 4s 4p [Kr] 4d 5s2 5 [Xe] 4f 5d 6s 6p -

Each electron in the atom is distinguished by four quantum numbers . Among the four quantum numbers, the principle quantum number ( n 0 defines the main energy levels of the shell. In the periodic table, as the shells are filled with electrons in the order n=1, n=2 , ...... , the periods are formed serially .
In the 1st period as the 1st energy level is filled with two  electrons, there are only two elements. They are hydrogen and helium. In the second period, the differentiating electron enters the 2s orbital in lithium and the second energy level is completely filled 2s²2p⁶ with electrons in neon. Thus the second energy level is now completely filled with eight electrons  and hence the second period has eight elements . In the third period , the differentiating electron enters 3s orbital in sodium. After 3s, then 3p gets filled with differentiating electron upto argon. Then the differentiating electrons does not enter 3d orbitals but enters 4s. Hence the 3rd period also has only eight elements .
In the 4th period , the energy level 4s is 1st filled i.e. from potassium. After 4s is filled the differentiating electron enters 3d orbitals from scandium to zinc. and these ten elements are called transition elements. then the differentiating electron enters into the 4p orbitals upto krypton. Krypton attains stability due to completely filled orbitals. Thus, the 4th period has 18 elements .
In the fifth period, the energy  levels are 5s, 4d, 5p are filed in a similar order with differentiating electrons as in the 4th period. The period starts with rubidium and ends wiyth xenon. The fifth period has 18 elements which include second transition series.
In the sixth period, the energy levels 6s, 4f, 5d, 6p get filled with differentiating electrons in the same order. After 6s is filled, the differentiating  electrons enter the 4f orbitals to form the 1s inner transition series with an exception of La that gets the differentiating e^-1 into 5d. 14 elements from cerium to lutetium get their differentiating electrons into 4f orbitals. These elements are therefore called 'f' block elements. Then 5d get filled, to give the third transition series and next 6p orbitals get filled upto radon. The 6th period is the longest period with a total of 32 elements. The 7th period is incomplete and has about 20 elements. The period includes the second inner transition series from thorium  to Lawrencium.