Chemistry

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Part 2: Covalent bonds and molecules

Learning Objectives

 Explain the difference between a covalent bond and an ionic

bond. (4.6)

 Use electronegativity of elements to determine bond

polarity. (4.7)

 Classify a covalent bond as polar or nonpolar. (4.7)

 Predict the number of bonds formed by common nonmetals

. (4.8)

 Name and write formulas for covalent compounds. (4.6)

 Write Lewis formulas for simple molecules (4.10)

 Explain how shape and composition change the properties

of molecules

Chapter 4 : Chemical bonds_Part 2

N'vida E. Houndonougbo

Chapter 4 : Chemical bonds

Outline Chapter 4_part 2

I. Molecule and covalent bond

II. Polar Covalent Bonds and Electronegativity

III. Lewis Structure

IV. Molecular shapes and Properties

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Covalent bonds and Molecules

 Many nonmetal elements react by sharing electrons rather than by gaining or losing electrons.

When two atoms share a pair of electrons, a covalent bond is formed.

Example: Formation of the molecule of hydrogen (H2)

The two electrons travel about both nuclei

N'vida E. Houndonougbo

Covalent Bonding and Molecules

Single covalent bond : sharing

of a pair of electrons.

Double covalent bond: sharing

of two pairs of electrons.

Triple covalent bond: sharing of

three pairs of electrons.

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Polar Covalent Bonds and Electronegativity

• Electrons in a covalent bond occupy the region between the

bonded atoms.

• If the atoms are identical, as in H2 and Cl2, electrons are

attracted equally to both atoms and are shared equally.

• If the atoms are not identical, however, as in HCl, the

bonding electrons may be attracted more strongly by one

atom than by the other and thus shared unequally. Such

bonds are known as polar covalent bonds.

N'vida E. Houndonougbo

In a polar covalent bonds partial charges are represented

by a d- on the more negative atom and d+ on the more

positive atom.

.

Polar Covalent Bonds and Electronegativity

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• The ability of an atom to attract electrons is called the

atom’s electronegativity.

• Fluorine, the most electronegative element, assigned a

value of 4, and less electronegative atoms assigned lower

values.

N'vida E. Houndonougbo

• The difference in electronegativity between two bonded atoms can be used to determine the type of bond.

• As a rule of thumb, electronegativity differences

Example: in class

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Drawing a Lewis Structure

 A Lewis structure shows the bonds between atoms and helps us visualize the arrangement of atoms in a molecule.

 Some Lewis structures can be assembled like a puzzle. Each piece an atom, fitting together by connecting bonding sites. Others require more hints.

1. Determine the number of valence electrons in the molecule

• For a cation, subtract one electron for each positive charge on it

• For an anion, add one electron for each negative charge on it.

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Drawing a Lewis Structure

2. Determine the connectivity of atoms.

3. Connect the atoms by single bonds.

4. Show bonding electrons as a single line; show nonbonding

electrons as a pair of Lewis dots.

5. In a single bond, atoms share one pair of electrons; in a

double bond, they share two pairs, and in a triple bond they

share three pairs.

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Drawing a Lewis Structure: practice

in class

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Exceptions to the Octet Rule

• The octet “rule” is a useful guideline, but it has numerous exceptions.

• Boron has only 3 valence electrons it can share and thus forms compounds in which it has only 3 covalent bonds.

• Elements in the third row and below have vacant d orbitals that can be used to make more than 4 bonds.

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Naming Binary Molecular Compounds

• Binary molecular compounds are

composed of only two elements

• The formulas of binary molecular

compounds are written with the less

electronegative element first.

1. Name the first element in the formula,

using a prefix to indicate the

number of atoms.

• The prefix mono- is omitted for the

first element

N'vida E. Houndonougbo

Naming Binary Molecular Compounds

2. Name the second element in the formula, using the

ending -ide along with the prefix needed.

• 3. If the prefix ends with a or o and the name of the element begins with a vowel, the two successive vowels are often elided into one.

Example in class

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Some molecules are known by their common name or generic name

H2O ( water / dihydrogen monoxide)

H2S ( hydrogen sulfide / dihydrogen monosulfide

CH4 (methane / carbon tetrahydride)

NH3 (ammonia / nittrogen trihydride)

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Molecular Shapes: The VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory

predicts the shape of molecules and polyatomic ions based on

repulsions of electron pairs on central atoms.

The molecular shape indicates the arrangement of atoms

around the central atom as a result of electron repulsion.

Practice in class: Use the table 4.6: bonding and the shapes

of molecules to predict the shape of a molecule

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Molecular Shapes: The VSEPR Theory

N'vida E. Houndonougbo

Shapes and Properties: Polar and Nonpolar

Molecules

In order for a molecule to be polar, two conditions must

be met:

1. It must have polar bonds.

2. The bonds must be arranged such that a

separation of charge exists.

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Shapes and Properties: Polar and Non

polar Molecules

Why water is polar molecule and carbon dioxide, CO2, is not?

In water, H2O, the central O atom is surrounded by two nonbonding pairs and two bonding pairs.

Thus, the molecular shape is bent

N'vida E. Houndonougbo

Shapes and Properties: Polar and Non

polar Molecules

• In carbon dioxide, CO2, the central C atom is surrounded by

four bonding pairs.

• According to VSEPR theory, the electron pairs will repel each

other and they will be at opposite sides of the C atom.

• Thus, the molecular shape are both linear.

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Molecular Polarity

• Polarity has a dramatic effect on the physical properties of molecules, particularly on melting points, boiling points, and solubility.

• Example :

The polarity of the bond in water molecule profoundly affects the property of water including what dissolves in it.

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Like Dissolves Like

 We can predict whether a solid will dissolve in a liquid by applying the like dissolves like rule

 Ionic compounds such as sodium chloride, NaCl and Polar compounds, such as table sugar (C12H22O11), are soluble in polar solvents such as water, H2O.

 Polar solvents such as water have large dipole moments (“partial charges”); they contain bonds between atoms with very different electronegativities, such as oxygen and hydrogen.

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Hydrophilic molecules

Polar molecules such as Methanol, CH3OH that

dissolve easily in water are called Hydrophilic

molecules

Methanol is soluble in water because its molecular

structure (-OH groups form hydrogen bonds).

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Hydrophilic molecules

 Hydrogen bonding is a weak attraction

between Molecules.

 It is an interaction between a hydrogen atom and an

electronegative atom such as oxygen, nitrogen and

fluorine.

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The hydrophobic effect

 Nonpolar molecules such as

(methane ) do not dissolve in water

because they cannot form hydrogen

bonds.

 Methane is nonpolar and when

forced into water, the polar water

molecules form a cage-like

structure.

N'vida E. Houndonougbo

The hydrophobic effect

• Nonpolar molecules such as

methane, CH4 that do not mix with water

are called Hydrophobic molecules.

• When hydrophobic molecules are placed

in water, the nonpolar molecules group

together to minimize their interaction

with the solvent. This is called

the hydrophobic effect.

 Nonpolar molecules CH4 are soluble in

nonpolar solvents such as hexane C6H14