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II. CHEMICAL BONDS

In their chemical interactions the atoms of different

elements tend to achieve a stable rare gas configuration 1s2

or ns2np6. Interactions that occur between atoms are called

chemical bonds.

1. Strong chemical bonds:

a) ionic bond (between metals and nonmetals);

b) covalent bond (between nonmetals);

c) metallic bond (between metallic atoms).

2. Weak chemical bonds:

a) van der Waals forces (dipole-dipole attraction)

b) hydrogen bonding

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1. Strong chemical bonds

a) Ionic bond = a type of chemical bond based on the

electrostatic attraction forces between ions having opposite

charges.

Ionic bond forms between electropositive and electronegative

elements, e.g. between metal and non-metal ions.

II. CHEMICAL BONDS

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The metal, with a few electrons on the last shell, donates one

or more electrons to get a stable electron configuration and

forms positively charged ions (cations). These electrons are

accepted by the non-metal to form a negatively charged ion

(anion) also with a stable electron configuration. The

electrostatic attraction forces between the anions and cations

causes them to come together and form a bond.

II. CHEMICAL BONDS

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Example: the formation of ionic bond between Na and Cl

Ionic bond formation in NaCl

Na: 1s22s22p63s1 Na loses one e- from its outer shell Na+: 1s22s22p6

Cl: 1s22s22p63s23p5 Cl gains one e- Cl-: 1s22s22p63s23p6

II. CHEMICAL BONDS

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When sodium and chlorine react, the outer

electron of the sodium atom is transferred to the

chlorine atom to produce sodium ion Na+ and chlorine

ion Cl- , which are held together by the electrostatic

force of their opposite charges. NaCl is an ionic

compound.

II. CHEMICAL BONDS

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NaCl formation may be illustrated showing the outer

electrons only (Lewis symbol):

In a similar way, a calcium atom may lose two electrons

to two chlorine atoms forming a calcium ion Ca2+ and two

chloride ions Cl-, that is calcium chloride CaCl2 :

II. CHEMICAL BONDS

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In sodium chloride, the ionic bonds are not only

between a pair of sodium ion Na+ an chlorine ion Cl-, but

also between all the ions. These electrostatic interactions

have as a result the formation of NaCl crystal.

We write the formula of sodium chloride as NaCl, but

this is the empirical formula. The sodium chloride crystal

contains huge and equal numbers of Na+ and Cl- ions pocket

together in a way that maximizes the electrostatic forces of

the oppositely charged ions.

II. CHEMICAL BONDS

8Sodium chloride crystal

9Lithium bromide crystal

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b) Covalent bonds = is a type of chemical bond formed by

sharing pairs of electrons between atoms.

When two electronegative atoms react together,

ionic bonds are not formed because both atoms have a

tendency to gain electrons. In such cases, an stable

electronic configuration may be obtained only by sharing

electrons. First, consider how chlorine atoms Cl react to

form chlorine molecules Cl2 :

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Each chlorine atom shares one of its electrons with the

other atom. The electron is shared equally between both

atoms, and each atom in the molecule has in its outer shell 8

electrons – a stable electronic configuration corresponding to

that of Ar.

The sharing of a single pair of electrons results in a

single covalent bond, often represented by a dash sign, so

chlorine molecule may be written as follow: Cl — Cl

II. CHEMICAL BONDS

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If two pairs of electrons are shared we have a double

covalent bond

Ex: the oxygen molecule O2, each oxygen atom shares two

electrons O ═ O

If three pairs of electrons are shared we have a triple

covalent bond

Ex: the nitrogen molecule N2, each nitrogen atom shares

three electrons. N ≡ N

II. CHEMICAL BONDS

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In a similar way a molecule of carbon tetrachloride CCl4

is made up of carbon and four chloride atoms. The

carbon atom shares all its four electrons and the chlorine

atoms share one electron each. The carbon atom forms 4

covalent bonds with 4 chlorine atoms. In this way, both

the carbon and all four chlorine atoms attain a stable

electronic structure.

II. CHEMICAL BONDS

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A molecule of ammonia NH3 is made up of one nitrogen and

three hydrogen atoms:

Coordinate bond

The nitrogen atom forms three bonds and the hydrogen

atoms one bond each. In this case, one pair of electrons is

not involved in bond formation and this is called a lone pair

of electrons.

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It is possible to have a shared electron pair in which

the pair of electrons comes just from one atom and not from

both. Such bond is called coordinate covalent bond.

Even though the ammonia molecule has a stable

configuration, it can react with hydrogen H+ by donating the

lone pair of electrons, forming the ammonium ion NH4+:

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In the chlorine molecule Cl – Cl the pair of electrons

of the covalent bond is shared equally between both

chlorine atom. Because there is not a charge separation

between the chlorine atoms, Cl2 molecule is nonpolar.

Partial ionic character of covalent bonds

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On the contrary, in HCl molecule, there is a shift of

electrons toward the chlorine atom which is more

electronegative than hydrogen. Such molecule, in which a

charge separation exists is called a polar molecule or

dipole molecule.

The polar molecule of hydrochloric acid

+ +e-

H Cld

+ -

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The separation between the positive and negative

charges is given by the dipole moment μ. The dipole

moment is the product between the magnitude of the charges

(δ) and the distance separating them (d):

μ = δ · d

The symbol δ suggests small magnitude of charge,

less than the charge of an electron (1.602 · 10-19 C).

The unit for the dipole moment is Debye (D):

1D = 3.34 · 10-30 C m

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C...

d20

12

30

1053210136

10343031

The charge δ for HCl molecule represents about 16%

of the electron charge (1.602 10-19 C). We can say that the

covalent H – Cl bond has about 16% ionic character.

Dipole moment values for some molecules:

Carbon dioxide CO2 μ = 0 D

Carbon monoxide CO μ = 0.112 D

Water H2O μ = 1.85 D

Hydrochloric acid HCl μ = 1.03 D

dist. between H and Cl atoms is d = 136 pm (136 10-12 m)

We can calculate the charge δ for HCl molecule:

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c) Metallic bond

The metallic bond represents the electromagnetic

attraction forces between delocalized electrons and the metal

nuclei. The metallic bond is a strong chemical bond, as

indicated by the high melting and boiling points of metals.

A metal can be regarded as a lattice of positive metal

“ions” in a “sea” of delocalised electrons.

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Metal atoms contain few electrons in their outer shells.

Metals cannot form ionic or covalent bonds.

Sodium has the electronic structure 1s22s22p63s1.

When sodium atoms come together, the electron from the 3s

atomic orbital of one sodium atom shares space with the

corresponding electron of a neighbouring atom to form a

molecular orbital. All the 3s orbitals of all the atoms overlap

to give a vast number of molecular orbitals which extend over

the whole piece of metal. There is a huge numbers of

molecular orbitals because any orbital can only hold two

electrons.

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The electrons can move freely within these molecular orbitals and so each

electron becomes detached from its parent atom. The electrons are called

delocalized electrons. The “free“ electrons of the metal are responsible

for the characteristic metallic properties: ability to conduct electricity and

heat, malleability (ability to be flattened into sheets), ductility (ability to be

drawn into wires) and lustrous appearance.

Crystal structure of sodium