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علم الكيمياء : علم الكيمياء : كيمياء عامة :

Covalent Bonds

المؤلف: University of Missouri System

المصدر: Introductory chemistry

الجزء والصفحة: .................

10-11-2020

1910

Covalent Bonds

Ionic bonding typically occurs when it is easy for one atom to lose one or more electrons and another atom to gain one or more electrons. However, some atoms won’t give up or gain electrons easily. Yet they still participate in compound formation. How?

There is another mechanism for obtaining a complete valence shell: sharing electrons. When electrons are shared between two atoms, they make a bond called a covalent bond.

Let us illustrate a covalent bond by using H atoms, with the understanding that H atoms need only two electrons to fill the 1s subshell. Each H atom starts with a single electron in its valence shell:

H-H

The two H atoms can share their electrons:

H-H-2

We can use circles to show that each H atom has two electrons around the nucleus, completely filling each atom’s valence shell:

H-H-3

Because each H atom has a filled valence shell, this bond is stable, and we have made a diatomic hydrogen molecule. (This explains why hydrogen is one of the diatomic elements.) For simplicity’s sake, it is not unusual to represent the covalent bond with a dash, instead of with two dots:

H–H

Because two atoms are sharing one pair of electrons, this covalent bond is called a single bond.

As another example, consider fluorine. F atoms have seven electrons in their valence shell:

F-F

These two atoms can do the same thing that the H atoms did; they share their unpaired electrons to make a covalent bond.

F-F-2

Note that each F atom has a complete octet around it now:

F-F-3

We can also write this using a dash to represent the shared electron pair:

F-F-4

There are two different types of electrons in the fluorine diatomic molecule. The bonding electron pair makes the covalent bond. Each F atom has three other pairs of electrons that do not participate in the bonding; they are called lone electron pairs. Each F atom has one bonding pair and three lone pairs of electrons.

Covalent bonds can be made between different elements as well. One example is HF. Each atom starts out with an odd number of electrons in its valence shell:

H-F

The two atoms can share their unpaired electrons to make a covalent bond:

H-F-2

We note that the H atom has a full valence shell with two electrons, while the F atom has a complete octet of electrons.

Example 4

Use Lewis electron dot diagrams to illustrate the covalent bond formation in HBr.

Solution

HBr is very similar to HF, except that it has Br instead of F. The atoms are as follows:

H-Br

The two atoms can share their unpaired electron:

H-Br-2

Test Yourself

Use Lewis electron dot diagrams to illustrate the covalent bond formation in Cl₂.

Answer

Cl-Cl

More than two atoms can participate in covalent bonding, although any given covalent bond will be between two atoms only. Consider H and O atoms:

H-O

The H and O atoms can share an electron to form a covalent bond:

H-O-2

The H atom has a complete valence shell. However, the O atom has only seven electrons around it, which is not a complete octet. We fix this by including a second H atom, whose single electron will make a second covalent bond with the O atom:

H-O-3

(It does not matter on what side the second H atom is positioned.) Now the O atom has a complete octet around it, and each H atom has two electrons, filling its valence shell. This is how a water molecule, H₂O, is made.

Example 5

Use a Lewis electron dot diagram to show the covalent bonding in NH₃.

Solution

The N atom has the following Lewis electron dot diagram:

It has three unpaired electrons, each of which can make a covalent bond by sharing electrons with an H atom. The electron dot diagram of NH₃ is as follows:

N-H

Test Yourself

Use a Lewis electron dot diagram to show the covalent bonding in PCl₃.

Answer

Cl-P

There is a simple set of steps for determining the Lewis electron dot diagram of a simple molecule. First, you must identify the central atom and the surrounding atoms. The central atom is the atom in the center of the molecule, while the surrounding atoms are the atoms making bonds to the central atom. The central atom is usually written first in the formula of the compound (H₂O is the notable exception). After the central and surrounding atoms have been identified, follow these steps:

Count the total number of valence electrons. Add extra if the species has negative charges and remove some for every positive charge on the species.
Write the central atom and surround it with the surrounding atoms.
Put a pair of electrons between the central atom and each surrounding atom.
Complete the octets around the surrounding atoms (except for H).
Put the remaining electrons, if any, around the central atom.
Check that every atom has a full valence shell.

Let us try these steps to determine the electron dot diagram for BF₄⁻. The B atom is the central atom, and the F atoms are the surrounding atoms. There is a negative sign on the species, so we have an extra electron to consider.

1. Count the total number of valence electrons. B has 3, each F has 7, and there is one extra electron: 3 + 7 + 7 + 7 + 7 + 1 = 32.

2. Write the central atom surrounded by surrounding atoms.

F-B

3. Put a pair of electrons between the central atom and each surrounding atom. This uses up eight electrons, so we have 32 − 8 = 24 electrons left.

F-B-2

4. Complete the octets around the surrounding atoms (except for H). This uses up 24 more electrons, leaving 24 − 24 = 0 electrons left.

F-B-3

5. Put the remaining electrons, if any, around the central atom. There are no additional electrons to add to the central atom.

6. Check. The B atom has eight electrons around it, as does each F atom. Each atom has a complete octet. This is a good Lewis electron dot diagram for BF₄⁻.

Sometimes, however, these steps don’t work. If we were to follow these steps for the compound formaldehyde (CH₂O), we would get the following:

C-H-O

The H and O atoms have the proper number of electrons, but the C atom has only six electrons around it, not the eight electrons for an octet. How do we fix this?

We fix this by recognizing that two atoms can share more than one pair of electrons. In the case of CH₂O, the O and C atoms share two pairs of electrons, with the following Lewis electron dot diagram as a result:

C-H-O-2

By circling the electrons around each atom, we can now see that the O and C atoms have octets, while each H atom has two electrons:

C-H-O-3

Each valence shell is full, so this is an acceptable Lewis electron dot diagram. If we were to use lines to represent the bonds, we would use two lines between the C and O atoms:

C-H-O-4

The bond between the C and O atoms is a double bond and represents two bonding pairs of electrons between the atoms. If using the rules for drawing Lewis electron dot diagrams don’t work as written, a double bond may be required.

Example 6

What is the proper Lewis electron dot diagram for CO₂?

Solution

The central atom is a C atom, with O atoms as surrounding atoms. We have a total of 4 + 6 + 6 = 16 valence electrons. Following the rules for Lewis electron dot diagrams for compounds gives us

C-O

The O atoms have complete octets around them, but the C atom has only four electrons around it. The way to solve this dilemma is to make a double bond between carbon and each O atom:

C-O-2

Each O atom still has eight electrons around it, but now the C atom also has a complete octet. This is an acceptable Lewis electron dot diagram for CO₂.

Test Yourself

What is the proper Lewis electron dot diagram for carbonyl sulfide (COS)?

Answer

C-S-O

It is also possible to have a triple bond, in which there are three pairs of electrons between two atoms. Good examples of this are elemental nitrogen (N₂) and acetylene (C₂H₂):

Triple-Bond

Acetylene is an interesting example of a molecule with two central atoms, which are both C atoms.

Polyatomic ions are bonded together with covalent bonds. Because they are ions, however, they participate in ionic bonding with other ions. So both major types of bonding can occur at the same time.

الاكثر قراءة في كيمياء عامة

موازنة حرق البيوتان

Photochemical Reactions: Alkene Isomerization

تسمية المركبات الأيونية Ionic Compounds

ترتيب عناصر الجدول الدوري حسب العائلات(الزمر) والفترات (الدورات)

Molecules and Chemical Nomenclature

حمض الخليك والأحماض الضعيفة الأخرى

Lewis Electron Dot Diagrams

تسمية المركبات الجزيئية Molecular Compounds

الصيغ الأولية Empirical Formulas

النيوترون The Neutron

Electron Configuration Energy Diagrams

Oxidation-Reduction Reactions

العدد الذري وعدد الكتلة والنظائر Atomic Number, Mass Number and Isotopes

اشباه الفلزات

فهم الأحماض والقواعد القوية والضعيفة

اخر الاخبار

مواضيع ذات صلة

تسمية المركبات الجزيئية Molecular Compounds

تسمية المركبات الأيونية Ionic Compounds

تسمية المركبات Naming Compounds

صيغ المركبات الأيونية Formula of Ionic Compounds

الصيغ الأولية Empirical Formulas

الصيغ الجزيئية Molecular Formulas

الأيونات Ions

الجزيئات Molecules

الجدول الدوري The Periodic Table

العدد الذري وعدد الكتلة والنظائر Atomic Number, Mass Number and Isotopes

النيوترون The Neutron

البروتون والنواة The Proton and the Nucleus

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Covalent Bonds

Example 4

Example 5

Example 6

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