Covalent bond

Covalent Bonds Are Formed by Sharing a Pair of Electrons

     Instead of giving up or acquiring electrons to achieve a filled outer shell, an atom can achieve a filled outer shell by sharing a pair of Electrons. For example- two fluorine atoms can each attain a filled second shell by sharing their unpaired valence electrons.

A bond formed as a result of sharing electrons between two nuclei is called a covalent bond.

A covalent bond is commonly shown by a solid line rather than as a Pair of dots.

Two hydrogen atoms can form a covalent bond by sharing electrons. As a result of covalent bonding, each hydrogen acquires a stable, filled first shell

Similarly, hydrogen and chlorine can form a covalent bond by sharing electrons. In doing so, hydrogen fills its only shell, and chlorine achieves an outer shell of eight electrons.

We just saw that because hydrogen has one shell electron and chlorine has seven valence electrons, each can achieve a filled outer shell by forming one covalent bond.

Newlands' Octaves

     

                           Newlands

   British chemist Newlands (1837 - 1898)  was born in London and studied at Royal college of chemistry.  He set up a practice as an analytical chemist  in 1864 and in 1868 became chief chemist in a sugar refinery.  Later he left the refinery  and worked as an analyst. 

   After the failure of Donbereiner's triads, the next attempt to classify elements was done by Newlands.  By this time 56 elements were discovered. Newlands arranged all these elements in an increasing order of their atomic masses.  He found that every eighth element had properties similar to that of the first.  He compared this to the octaves found in music.  Therefore his classification was known as 'Newlands' Octaves'. 

      Newlands law states that  "When the elements are arranged in an increasing order of their atomic masses, the properties of the eighth element are similar to the first." 

     

• Some features of Newlands table 

🔹Newlands could arrange elements only up to calcium out of total 56 elements known.

🔹 After calcium every eighth element did not posses properties similar to that of the first.

🔹 At the time of Newlands only 56 elements were known but later several elements were discovered.

🔹 In order to fit the existing elements Newland. Places two elements in the same position which differed in their properties.

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Donbereiner's Triads

   

          Johann Wolfgang Donbereiner

    Johann Wolfgang Donbereiner  a German scientist (1780 - 1849) studied as a pharmacist at Munchberg in Germany and then studied chemistry at  Strasbourg.  He later became professor of chemistry and pharmacy at the University of  Jena.

    In 1829 he found some groups of three elements which showed similar properties. These groups were called as a triads. ln this triads, atomic masses of the middle elements was approximately the mean of the atomic masses of the other two elements. Also these elements showed similarities in properties. The table below shows four triads arranged vertically. 

         In the above table, take the triads of Lithium, Sodium and Potassium. The Atomic mass of sodium (23) is the mean of the atomic masses of Lithium and Potassium.

    Similarly, you can verify the atomic masses of Strontium, Bromine and Selenium from the other triads.

    The triads were known as Donbereiner's Triads. 

    Donbereiner could identify only some triads  from the elements known other triads did not obey  Donbereiner's rule. Hence, the system of Triads was not useful.

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Aromatic compounds

   Many  compounds obtained from natural sources like resins, balsams, oil of wintergreen etc. Possessing pleasant fragrance (aroma) were named as Aromatic compounds.  In greek the word aroma means pleasant smell. Benzene is one such important aromatic compound.

     Benzene and compounds that resemble benzene in their chemical behaviour are nowday called as aromatic compound. Many compounds that resemble benzene do not have pleasant odour e.g. anthracene.

 Many compounds that have pleasant odour do not resemble benzene e.g. methyl acetate, chloroform, ethyl acetate etc. Therefore, early relation of aromatic compounds with odour no longer exists.

Benzene (C6H6) :  

    Benzene is parent compound of most of the Aromatic compounds. It is a colourless liquid having a characteristic odour, boiling point 353K. coaltar is an important source of benzene. Benzene is also present in petroleum. Benzene was synthesized by Berthelot (1870) from acetylene. Benzene was originally called phene and hence C6H5 was called phenyl group. Aromatic hydrocarbons are also called arenes.  Aryl group is represented as -Ar.

 * Characteristic of Aromatic compounds

  All Aromatic compounds exhibit following three important characteristics.

i) Higher percentage of carbon

 These compounds contain Higher percentage of carbon content than corresponding aliphatic hydrocarbon. As a result, these compounds burn with sooty flame.

ii) Thermal stability 

  All Aromatic compounds are exceptionally stable as compared with alkanes. Hence on combustion, aromatic compounds liberate comparatively lesa amount of heat.

iii) Chemical behaviour 

  a) Despite the presence of unsaturation, aromatic compounds do not undergo addition reaction similar to alkenes and alkynes under normal condition. For example, benzene does not decolouries alkaline potassium permanganate.

b) Despite the presence of a number of  double bonds, Aromatic compounds undergo substitution reactions, which are characteristic of aromatic compounds. For example, benzene undergo halogenation like alkanes.

  Types of aromatic compounds :   Aromatic compounds are of two types.

 i) Benzenoid :  these compounds contain one or more benzene rings in their structure. e.g. benzene, benzaldehyde, benzoid acid, naphthalene, anthracene etc.

ii) Non - Benzenoid :  these compounds do  not contain any benzene rings in their structure e.g. furan, thiophene, pyridine etc. 

.              Some Aromatic compounds

Bohr's Model of Atom

 

 Neils Bohr (1913) was the first to explain quantitative the general features of the structure of hydrogen atom. The model explained the atomic spectrum of the hydrogen atom and the stability of the atom 

• Electrons moves around the nucleus With fix path and fix energy. Fix path are called Orbits / stationary State / energy state / energy level.

• Energy of electron does not Change. when electron Absorbs energy It moves from Lower stationary State to higher stationary state. 

  When elecron moves From higher stationary state to lower stationary state It emits energy in the form of EMR 

• 

  Energy of radiation, absorbed or emitted is equal to difference two energy state / energy of to stationary state

∆E = Ef - Ei          ∆E = E2 - E1

∆E = energy of radiation Absorbs or emitted

Ef = final energy state

Ei = initial energy state 

• there are stationary orbits for electron. As long as electrons revolve in these stationary orbits, they do not radiate energy. The energies of electrons remain constant in stationary orbits.

Different stationary levels or stationary orbits or shells are represented by capital letters K, L, M, N  with corresponding shell number 1, 2, 3, 4, 5 etc. 

• Angular momentum of electron is quanties

  

Angular momentum = m×v×r

M= mass of electron

V=  velocity of electron

R=  radius of Orbits 

   mvr = n(h/2π)

Angular momentum Of integral multiple of h/2π

• radius of Orbit

 As the elecron of an atom revolve in one of the stationary orbital, the coulombic force of attraction of nucleus of electrons, i.e. ze2/r2 must be equal to centrifugal force of the electron mv2/r2

Hence, 

       Ze2/r2 = mv2/r ____(1)

  R= Ze2/mv2.  ______(2)

According to the Bohr's angular momentum of electron mvr is integral multiple n of h/2π

Hence, mvr = nh/2π _____(3)

                  V = nh/ 2πmr _____(4) 

  Substituting v in equation (2) 

        r = n2h2/4π2mZe2

Substituting values of h,m,z=1 for hydrogen and e the electronic charge r may be caculated as 

   r = 0.529 × n2 A° 

 Thus, 

     n = 1.        r = 0.529A°

     n= 2.          r= 2.116A°

     n= 3.          r = 4.761A°

     n= 4.          r = 8.464A°

     n= 5.          r = 13.225 A°

And so on. This also suggest that electron takes up only discrete energy levels.

Hund's rule

before we can discuss atoms containing six or more electrons, we need to define Hund's rule.

  Hund's rule states that 

 When there are two or more atomic orbitals with the same energy, an electron will occupy an empty orbital before it will pair up with another elecron.

   In this way, elecron repulsion is minimized.

     

The sixth elecron of a carbon atom, therefore, goes into an empty 2p orbitals, rather than pairing up with the elecron already occupying a 2p orbital see in the table.

.   The electronic configuration of the smallest atom

There is one more empty 2p orbital, so that is where nitrogen's seventh electron goes. The eighth electron of an oxygen atom pairing up with an electron occupying a 2p orbital rather than going into the higher-energy 3s orbital.

       The location of the electrons in the remaining elements can be assigned using these three rules.

    The elecrons in inner shells (those below the outermost shell) are called core electrons.  Core electrons do not participate in chemical bonding.the electrons in the outermost shell are called valence electrons.

    Carbon has two core electrons and four valence electrons Lithium and Sodium each have one valence electron. If you examine the periodic table, you will see that and Sodium are in the same column. Elements in the same column of the periodic table have the same number of valence electrons. Because the number of Valence electrons in the major factor determining an element's chemical properties, elements in the same column of the periodic table have similar chemical properties. Thus, the chemical behaviour of an element depends on its electronic configuration.

   

* Some important notes

• core electrons are electrons in inner shells.

• Valence electrons are electrons in the outermost shell.

 • The chemical behaviour of an element depends on its electronic configuration.

How The Electrons In An Atom Are Distributed

    Elecrons were perceived to the particals immesurable "planets" that orbit the nucleus of an atom. In 1924, however a French physicist named Louis de Broglie  showed that electrons also have wavelike properties. He did this by combining a formula developed by Albert Einstein 

 that relates mass and energy with a formula developed by Max Planck  that relates Frequency and energy. The realization that electrons have wavelike properties spurred physicist to propose a mathematical concepts know as Quantum mechanics.

    Quantum mechanics uses the same mathematical equations that distribe the wave motion of a guitar string to characterize the motion of an electron around a nucleus. The version of quantum mechanics most useful to chemist was proposed by Erwin Schrodinger in 1926.

     According to Schrodinger, the elecrons in an atom can be thought of an occupying a set of concentric shells that surround the nucleus. The first shell is the one closest to the nucleus. The second shell lies farther from the nucleus. The third and higher numbered shell lie ever farther out 

   Each shell contain subshell known as atomic orbitals. Each atomic orbital has a characteristic shape and energy and occupies a characteristic volume of space.

    The first shell consists only of an s atomic orbital; the second shell consists of s and p atomic orbitals; the third shell consists of s,p, and d atomic orbitals; and the fourth and higher shells consist of s,p,d, and f atomic orbitals.

             

                    

.         Distribution of electrons in the first

            Four shell that surround the nucleus