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Course lecturer :

Jasmin Šutković

21th October 2015

Organic Chemistry – FALL 2015

Lecture 2 (book chapter 4):Introduction to covalent bounding

Contents International University of Sarajevo

4.1 Introduction to Covalent Bonding4.2 Lewis Structures4.3 Exceptions to the Octet Rule4.4 Resonance4.5 Naming Covalent Compounds4.6 Molecular Shape4.7 Electronegativity and Bond Polarity4.8 Polarity of Molecules4.9 FOCUS ON HEALTH & MEDICINE:Covalent Drugs and Medical Products

Introduction

Most compounds that we come in contact with in our daily lives are covalent compounds, formed by sharing electrons between atoms.

Body – formed by water Most Drugs Industrial Chemical products ; nylon , gasoline,

pesticides,

COVALENT BONDS are created by SHARING electrons between two atoms (elements)

Hydrogen is called a diatomic molecule because it contains just two atoms.

In addition to hydrogen, six other elements exist as diatomic molecules:

nitrogen (N2), oxygen (O2), fluorine (F2),chlorine (Cl2), bromine (Br2), and iodine (I2).

Covalent bounding in the periodic table

RULEs : Covalent bond are formed by two

nonmetals Ionic bonds are formed by metal and

nonmetal! Covalent bond are also formed when a

metalloid binds to nonmetal Examples : Methane (CH4), Ammonia (NH3) and H2O

Methane – natural gas- simplest Alkane Ammonia – agricultural fertilizer –

colorless, specific smell, building block of many drugs

Water ………………………………………………………………

FOCUS ON THE HUMAN BODYCOVALENT MOLECULES AND THECARDIOVASCULAR SYSTEM

Besides water, our bodies proteins, that compose muscle, the carbohydrates that are metabolized for energy, stored fat, and DNA, the carrier of genetic information, are all covalent molecules.

Some covalent compounds related to the chemistry of the heart include water, the most prevalent covalent compound in the body; oxygen, which is carried by the protein hemoglobin to the tissues; glycine, a building block of the proteins that compose heart muscle; and nitroglycerin, a drug used to treat some forms of heart disease.

LEWIS STRUCTURES

Molecular formula shows the number of atoms in a compound, but it does not tell us what atoms are bound to each other.

A Lewis structure, in contrast, shows the connectivity between the atoms, as well as where all the bonding and nonbonding valence electrons reside.

Book page 98,99 and 100 – see examples

Multiple bonds

Examples of double Bonds

O2

CO2

C2H4

Examples of triple Bonds

N2

C2H2

EXCEPTIONS TO THE OCTET RULE

Most of the common elements in covalent compounds—carbon, nitrogen, oxygen, and the halogens—generally follow the octet rule.

Hydrogen is a notable exception, because it accommodates only two electrons in bonding.

Additional exceptions include elements such as boron in group 3A, and elements in the third row and later in the periodic table, particularly phosphorus and sulfur.

Group 3A elements

Elements in group 3A of the periodic table, such as boron, do not have enough valence electrons to form an octet in a neutral molecule.

A Lewis structure for BF3 illustrates that the boron atom has only six electrons around it. There is nothing we can do about this! There simply aren’t enough electrons to form an octet.

3th ROW elements in PT

Phosphorous and Sulfur

Resonance

Resonance Structures Structures that occur when it is possible to

write two or more valid electron dot structures for a molecule.

Environmental Ozone

O3 molecule

Ions (Book Chapter 3)

An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge.

In chemical terms, if a neutral atom loses one or more electrons, it has a net positive charge and is known as a cation. If an atom gains electrons, it has a net negative charge and is known as an anion.

Naming cations

It is common to add the word “ion” after the name of the metal cation to distinguish it from the neutral metal itself.

For example, when the concentration of sodium in a blood sample is determined, what is really measured is the concentration of sodium ions (Na+).

Cations of main group metals are given the name of the element from which they are formed.

If a metal is able to form two different cations (Fe2+ and Fe3+) then we need method to distinguish the cations.

Naming anions

PHYSICAL PROPERTIES OF IONIC COMPOUNDS

Ionic compounds are crystalline solids composed of ions,positive charged cations and negative charged anions

When a compound melts to form a liquid, energy is needed to overcome some of the attractive forces of the ordered solid, to form the less ordered liquid phase!!

Ionic compounds have very high melting points. For

example, the melting point of NaCl is 801 °C.

When an ionic compound dissolves in water, the ions are separated, and each anion and cation is surrounded by water molecules, as shown in Figure 3.4.

The interaction of the water solvent with the ions provides the energy needed to overcome the strong ion–ion attractions of the crystalline lattice.

POLYATOMIC IONS

Sometimes ions are composed of more than one element. The ion bears a charge because the total number of electrons it contains is different from the total number of protons in the nuclei of all of the atoms.

Examples

Nonmetal polyatomic ions

WRITING FORMULAS FOR IONIC COMPOUNDS WITH POLYATOMIC IONS

Unequal charge example

FOCUS ON HEALTH & MEDICINEUSEFUL IONIC COMPOUNDS

Ionic compounds are found in drugs! Examples include calcium carbonate (CaCO3), the antacid in Tums;

magnesium hydroxide [Mg(OH)2], on of the active components in the antacids Maalox and milk of magnesia; and iron(II) sulfate (FeSO4), an iron supplement used to treat anemia.

Naming covalent compounds

MOLECULAR SHAPE

We can now use Lewis structures to determine the shape around a particular atom in a molecule.

Consider the H2O molecule. The Lewis structure tells us only which atoms are connected to each other, but it implies nothing about the geometry.

What does the overall molecule look like? Is H2O a bent or linear molecule?

ELECTRONEGATIVITY AND BOND POLARITY

H2O

The two charges are present with a negative charge in the middle (red shade), and a positive charge at the ends (blue shade).

POLAR MOLECULE

Bond polarity

In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment.

Molecular polarity is dependent on the difference in electronegativity between atoms in a compound and the asymmetry of the compound's structure.

Polarity underlies a number of physical properties including surface tension, solubility, and melting- and boiling-points.

Atoms with high electronegativities — such as fluorine, oxygen, and nitrogen — exert a greater pull on electrons than atoms with lower electronegativities.

In a bond, this can lead to unequal sharing of electrons between atoms, as electrons will be drawn closer to the atom with the higher electronegativity.

Hydrogen-fluoride

In a molecule of hydrogen fluoride (HF), the more electronegative atom (fluoride) is shown in yellow.

Because of the electrons spend more time by the fluorine atom in the H-F bond, the red represents partially negatively charged regions.

POLAR MOLECULE

In the ozone, O3, molecule the two O–O bonds are nonpolar (there is no electronegativity difference between atoms of the same element).

Readings

Book Chapter 3 and 4. Chapter 3 follow according these slides and chapter 4 read completely.