Chemical bonding 1

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Chemical Bonding

Chemical Bonding-By Aditya Abeysinghe

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Why are bonds formed?Many elements and ions in nature are unstable and sometimes they are highly reactive. However any element or ion tend to increase their stability by achieving the noble gas configurations.For example, a sodium atom which has a valence electron may bind with an electron deficient chlorine atom to form sodium chloride. Thus the sodium atom will achieve the noble gas configuration and the chlorine atom will achieve the noble gas configuration of argon and be stable.


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Proof that formation of bonds make the participant atoms stable

1. Most atoms in nature do not exist as ‘free’ atoms but as compounds.

2. Emission of energy when bonds are formed and the absorption of energy in the creation of bonds.(According to the Gibb’s equation any element becomes more stable when it possesses less energy and is of high disorder)

3. Most rare gases are reluctant to form compounds(However, some compounds of xenon and argon have already being discovered)


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Overlapping

When two orbitals are ‘on each other’ overlapping occurs. However the percentage of overlapping changes depending on the type of orbital. For example the overlapping of two s orbitals is considered fully overlapped since the percentage overlapped remains constant regardless of the direction.(due to the spherical shape of the orbitals)In contrast, the overlapping of other orbitals such as p, d, and f orbitals change because of their shape.

The overlapping percentage remains constant in s orbitals regardless of direction

The overlapping percentage of p orbitals changes according to the direction in which they overlap


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What is a bond?A bond is the process by which two orbitals of the two atoms overlap to form a molecular orbital and produce a chemical connection between the two atoms.The bond is caused by the electrostatic force of attraction between two oppositely charged poles.


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Types of bonds

Type of bond(Primary Bonds)

Covalent bonds

Polar

Non-polar

CoordinateIonic or

electrovalent bonds

Metallic bonds


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Co-valent bonds(Co+ Valent bonding)

Covalent bonds are bonds formed by sharing electrons between the participating atoms. Here overlapping of two or more orbitals produce molecular orbital(s) and form the bond as a whole.As mentioned earlier there are three types of bond by physical nature1. Non-polar covalent bonds2. Polar covalent bonds3. Coordinate covalent bondsCovalent bonds can also be classified by the no. of bonds existing between any two bonded atoms as4. Single bond covalent formation5. Double bond covalent formation6. Triple bond covalent formation Chemical Bonding-

By Aditya Abeysinghe

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Non polar covalent bonding

In non-polar covalent bonding,1. The electronegativity difference between the atoms is

less than 0.4 and sometimes 0.2. The dipole moment of such bonds may be negligible.E. g: CH4 The electronegativity

of the C-H bond is 0.35. However the

polarity of any three C-H bonds is neutralized

by the other bond due to the tetrahedral shape of methane. Thus the molecule as a whole is non polar


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Polar covalent bonding

Polar bonding results when two different non-metals unequally share electrons between them.Polar covalent bonds usually exist in molecules where1. There is an imbalance of the electro

negativities in a molecule as a whole.2. The electro negativity between the atoms in

a specific bond lies between 0.4 and 1.6.


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E. g: When one hydrogen of methane is being replaced by a hydroxyl radical(-OH) the formation of methanol will result in a non-polar molecule due to the imbalance or the residue of the electro negativities in the molecule as a whole

Methanol(CH3- OH)


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Coordinate covalent bonding(Dative covalent/ Dipolar bond)

Coordinate covalent bonds result when both electrons in a specific bond is ‘gifted’ by a donor(by a single element unlike the sharing of electrons provided by both elements in the bond)A dipolar bond is formed when a Lewis base donates a pair of electrons to a Lewis acid(for a detailed description of Lewis acids and Lewis bases see the next slides).For example the Ammonium ion is formed by donating a pair of electrons to the electron deficient Hydrogen ion by the Ammonium compound.Chemical Bonding-


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+ [H]+

NH3 + H+ = NH4

+

Similarly Boron Tetraflouride is formed when the electron deficient Boron Tri fluoride atom gains a pair of electrons from a donor fluoride ion.BF3 + F- = BF4

- .

Finally not only ions can act as electron donors or receptors but also compounds.For example the following compound is formed by the donation of a pair of electrons from a ammonia compound to a boron trifluoride compound.


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NH . B

F

+


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Lewis acidA Lewis acid is an electron pair acceptor.A Lewis acid may be formed by 1. By compounds in which central atom is electron

deficient2. When multiple bonds exist between different

atoms3. Atoms having sextet configuration.4. By compounds containing a central atom with an

unfilled d- orbital.E. g: Various metal cations, such as Mg2+ and metal compounds such as AlCl3 are Lewis acids because they have unfilled valence orbitals. Chemical Bonding-


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Lewis base

Lewis base is a compound with a pair of non bonding electrons.Thus H2O with its two pairs of non bonding electrons on oxygen acts as a Lewis base by donating an electron pair to an H+ thus forming the hydronium ion, H3O+ .

Lewis bases can also be formed in similar methods as that in Lewis acid formation.


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Ionic bonding

Ionic bonding usually occurs when a bond is formed by two or more elements with electro negativity difference of 1.7 or more.Ionic bonds are also called electrovalent bonds since the two charges in any ionic bond have electrostatic properties and a valency unlike that in any covalent compound.In 1916, W. Cosol mentioned that ‘ atoms tend to achieve the nearest noble gas electronic configuration when participating in a chemical reaction’.According to his theory, NaCl may be formed as follows:

Na- [Ne] 3s1 Na+-[Ne]

Cl- [Ne] 3s2 3p5 Cl–- [Ar] NaCl


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Electro-valency

Electro-valency is the process by which one or more electrons are drifted between atoms and result in the valency of an element.The magnitude of the valency of an atom depends upon the no. of electrons received or donated in the process of electro-valency.For example Mg2+ means that the magnesium atom has donated two electrons either to a recipient atom/compound or to an aqueous solution. Also N3- means that nitrogen has either gained three electrons from a donor ion/ compound or from an aqueous solution. Chemical Bonding-


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Characteristics of metals

1. Luster and reflection of incident light2. High electrical and thermal conductivity properties3. High density4. Malleability and elasticity5. High melting and boiling points and also high evaporation heats6. Photoelectric effect7. Formation of cations/positive ions in chemical reactions Chemical Bonding-


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Drude and Lorentz theoryThe model proposed by Drude was developed by Lorentz and stated a new theory stemming away from the structures of covalent and ionic bonding.The model states that a metal is composed of a rigid positive ion lattice and the electrons in that lattice do not form any attractions with the positive ions and exist as a ‘cloud’ covering the lattice.


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Explaining the metallic bonding using the hypotheses of Drude

1. Electrons which are at a lower level may promote to a higher level when given energy to the atom. On retuning to a lower energy level the energy is emitted mainly as heat and light. Thus luster is visible to be reflected over a specific metallic surface when exposed to a light or radiation source.

2. The availability of free electrons makes it easier for electrical and thermal conductions throughout the metal. In contrast, due to the sharing of electrons in a covalent bond tight attractions between bonding atoms make its electron mobility extremely low. Thus metals transfer the heat and electricity through motion and vibration of electrons within the metal.Chemical Bonding-


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3. High density of the metal may be due to the close of electrons and the positive charges within the metal.4. Malleability and elasticity As shown in the next page if we pull opposite sides along A-B of a metal and of an ionic compound, the metallic bond forms attractive forces while the ionic compound forms repulsive forces.


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Metallic CompoundIonic Compound

A B

Chemical Bonding- By Aditya Abeysinghe

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Covalent compounds too break up when applied with stress due to weak intermolecular forcesE.g.: S, I, NaphthaleneHowever, giant covalent molecules like diamond do not break easily. If broken, they do not form small sheets or rods as in the case of metals

Due to the increase in the no. of mobility electrons in a given area, the density of alkaline earth metals (Mg, Be) is greater than that of alkaline metals(Li, Na).


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5. High melting, boiling points and evaporation heats can be described in a similar way to that of the high density.Similarly the photoelectric effect can be explained by the luster given off by a metal due to the incidence of light.6. Formation of positive ions in a chemical reactionDue to the presence of high mobility electrons the metal may tend to release one or more electrons to a recipient element to make positive ions equal in charge to the no. of electrons released to make the ion.


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For more information about the intermolecular attractions and other information on chemical bonding visit the presentation

‘Chemical Bonding-2By Aditya Abeysinghe’


Education

Chemical bonding 1