Primary Bonds1.pdf

7/27/2019 Primary Bonds1.pdf

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Primary Bonds


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Lecture 3

Ionic Bond

Covalent Bond

Metallic Bonding

Bond Length & Bond Energy


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What atoms want?

Atoms are most stable when they have no partially-filled electron shells.

All chemical bonds involve electrons and atoms stay close together if they have a

shared interest in one or more electrons; hence they form a chemical bond.

electromagnetic attractive force between opposite charges: between electrons and

nuclei of both atoms that form a bond

If an atom has only a few electrons in a shell, it will tend to lose them to empty theshell. These elements are metals.

When an atom has a nearly full electron shell, it will try to find electrons from another

atom so it can fill its outer shell. These elements are usually described as nonmetals.

OUTERMOST SHELL CLOSED: 8e-


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Ionisation energy

Ionization energy(ionization potential) is the energy necessary to remove an electron

from the neutral atom.

It is lowest for the alkali metals, which have a single valence electron.Increases across a period, as the number of valence electrons increases.

Maximum for the noble gases, which have closed shells.

Tends to decrease in a group.

eXX g

E

g

I


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Electron affinity

Electron affinityis a measure of the energy change when an electron is added to a

neutral atom to form a negative ion.

Highest for non-metals (17th group), as they need only one electron to fill the shell.

Generally increases in a period.

Decreases in a group.

Basically zero for noble gases.

g

E

g XeXa


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Chemical Bonds

Chemical compounds are formed by the joining of two or more atoms.

The total energy of the combined atoms is lowerthan the energy of separated atoms.

The various ways in which atoms can be bound together involve:

(1) Primary bonds - ionic, covalentor metallic bonds, which are all relatively strong

(2) Secondary bonds - Van der Waals and hydrogen bonds, which are both relatively weak


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Ionic Bonds

Ionic bond - electrostatic attraction between positive and negative charges (ions)Ionic bonds are formed between a cation = positively charged ion, which is usually a

metal, and an anion = negatively charged ion, which is usually a nonmetal.

Ionisation Energy Electron Affinity = 5.14 - 3.6 = 1.54eV ?!?!

NaClClNaClNa

CleCleNaNa

Ei=5.14 eV

Ea=-3.6 eV


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Ionic BondsU - potential energy (net force)

q - charge on each ionr- distance (separation) between the ions (n = 12)

ni r

B

r

qUrU

0

2

4

attractive part

repulsive part


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Covalent BondsCovalent bondis characterized by the sharing of pairs of electrons between atoms(commonly nonmetals), in contrast to the transfer of electrons in ionic bonds.

Such bonds lead to stable molecules (a noble gas configuration for each atom).


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Covalent Bonds

nmr

B

r

ArU

nm


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Electronegativity

Molecule is an electrically neutral group of at least two atoms held together by

covalent chemical bonds.

Electronegativityis a measure of the ability of an atom in a molecule to draw bonding

electrons to itself.

Linus Pauling has assigned the value 4.0 to fluorine (F), the most electronegative

element. Difference between electronegativities of constituent atoms is used for the

prediction of thepolarityof a chemical bond.

Covalent bonds are affected by the electronegativity of the connected atoms.Two atoms with equal or similar electronegativity will make non-polarcovalent bonds.

Otherwise, the covalent bond ispolar.


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Polar & Non-polar Covalent Bonds

Covalent bonds in which the sharing of the electron pair is unequal, with the electronsspending more time around the atom with higher electronegativity, are calledpolar

covalent bonds. If atoms have the same electronegativitynon-polar covalent bond.

In a polar covalent bond there is a charge separation with one atom being slightly more

positive and the other more negative, i.e., the bond will produce a dipole moment.


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Covalent Bonds in Polymers

A polymer is a large molecule (macromolecule) composed of repeating structural units.These subunits (monomers) are typically connected by covalent chemical bonds.

Polymerization is a process of reacting monomer molecules together in a chemical

reaction to form polymer chains.

Natural: cellulose, shellac, amber, rubber

Biopolymers: nucleic acids, proteins

Synthetic: polyethylene (PE), polypropylene(PP), polystyrene (PS), nylon, PVC, epoxy

resins, synthetic rubbers, teflon, etc.


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Ionic vs. Covalent Bonds

Property Ionic Compounds Covalent Compounds

Elements metal - nonmetal nonmetal - nonmetal

Phase (at STP) solid (in crystal lattice) solid, liquid or gas

Hardness hard and brittle (salt)brittle and weak (sugar)

or soft and waxy (butter)Melting/Boiling Points high low

Solubility mostly soluble in water solubility varies widely

Electrical Conductivitysolid - nonconductor,

liquid or aqueous solution -

conductor

insulators


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Metallic Bonding

Metallic bondis the electrostatic attractive force between the delocalized electrons, andthe positively charged metal ions (cations).

The properties of metals : conduction of heat and electricity suggest that electrons can

move freely in all directions in the lattice.

positive ions in a sea of electrons

strength varies from metal to metal

depends on the structure


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Bond Lengths & Bond Energy

The length of a chemical bondis the distance between the centres of the two bonded

atoms (the internuclear distance).

Bond lengths have traditionally been expressed in ngstrom units, but

the official unit is a picometer (1pm = 0.01 = 10-12 m).

Bond lengths depend mainly on the sizes of the atoms,

and secondarily on the bond strengths: stronger bonds are shorter.

Multiply-bonded atoms are closer together than singly-bonded ones.

Bond e- Order Strength Length

Single 2 1 weakest longest

Double 4 2

Triple 6 3 strongest shortest


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Bond Lengths & Bond Energy

Bond lengths depend mainly on the sizes of the atoms, and secondarily on the bond

strengths; stronger bonds are shorter.

Multiply-bonded atoms are closer together than singly-bonded ones.

The bond energyis the amount of work that must be done to pull two atoms

completely apart (the depth of the well in the potential energy curve).

stronger bond has higher energy and shorter length

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