2.1: Types of Chemical Bonds

Learning Targets/ Content Objectives

I can explain the relationship between the type of bonding and the properties of the elements participating in the bond.

Understand how electronegativity values of the representative elements increase going from left to right across a period and decrease going down a group. These trends can be understood qualitatively through the electronic structure of the atoms, the shell model, and Coulomb’s law.
Describe how valence electrons shared between atoms of similar electronegativity constitute a non-polar covalent bond. For example, bonds between carbon and hydrogen are effectively non-polar even though carbon is slightly more electronegative than hydrogen.
Explain how valence electrons shared between atoms of unequal electronegativity constitute a polar covalent bond.

a. The atom with a higher electronegativity will develop a partial negative charge relative to the other atom in the bond.

b. In single bonds, greater differences in electronegativity lead to greater bond dipoles.

c. All polar bonds have some ionic character, and the difference between ionic and covalent bonding is not distinct but rather a continuum.

Explain how the difference in electronegativity is not the only factor in determining if a bond should be designated as ionic or covalent. Generally, bonds between a metal and nonmetal are ionic, and bonds between two nonmetals are covalent. Examination of the properties of a compound is the best way to characterize the type of bonding.
Understand how in a metallic solid, the valence electrons from the metal atoms are considered to be delocalized and not associated with any individual atom.

Chemical bonds

A chemical bond forms if the energy of combined atoms is lower than the energy of separated atoms.

Why do chemical bonds form?

In general a bond will form when the two atoms bonded together will be lower in energy than the atoms separated.

For example, the graph below shows how the potential energy changes when a bond is formed between two hydrogen atoms. It also shows that the potential energy of bonded hydrogen atoms is less than the potential energy of separate hydrogen atoms indicating that H2 molecule is more stable (has lower energy) than two separate H atoms. The potential energy start decreasing as the two atoms start interacting until a covalent bond is formed indicating the lowest overall energy of the system.

Types of Chemical Bonds

There are three main types of Chemical bonds, ionic bonds, metallic bonds and covalent bonds.

Ionic bonds: form between metals and nonmetals. Electrons are transferred from the metal to the non-metal. When a nonmetal and a metal react to form a binary ionic compound, the ions form so that the valence electron configuration of the nonmetal achieves the electron configuration of the next noble gas atom. The valence orbitals of the metal are emptied. Ionic bond is the attractive force between a positive ion and a negative ion. This type of attraction is a Coulombic or electrostatic attraction. Ionic bonds are stronger when the charges are larger and the ions are smaller, this is explained by Coulomb’s Law.,

Properties of Ionic Substances

Form crystals (lattice of positive and negative ions)
High melting and boiling points
Hard, Brittle, Conduct electricity when dissolved and when molten (melted).
Good insulators as a solid

Covalent bonds are bonds that form between two nonmetals. In covalent bonds, electrons are shared between atoms. When electrons are equally shared between the bonded atoms a nonpolar covalent bond is formed. On the other side, polar covalent bonds form when electrons are not equally shared between the two bonded atoms. Polar covalent bonds are formed between two atoms of different electronegativity values. Because electrons are not equally shared in polar bonds the bond will have a dipole moment.

Properties of Nonpolar and Polar Covalent Molecules (Non-metals)

Non-lustrous,
various colors
Brittle, hard or soft
Poor conductors
Nonmetallic oxides are acidic and covalent Form anions by gaining electrons

Metallic bonds form between metal atoms. It can be for just one type of metal, a pure substance, or for different types of metal, a mixture called an alloy. The metallic attractions are due to multiple metallic cations being attracted to a delocalized sea of valence electrons. The IMF is stronger when there are smaller metallic cations and when there are more valence elections.

Properties of Metallic Substances

Shiny (Luster)
Malleable and ductile
Conduct heat and electricity
Metallic oxides are basic and ionic
Lose electrons to form cations

Metallic Bonds: In metallic bonds, electrons are mobile and they form a sea of electrons

Electronegativity

(This was covered in 1.7 Periodic Trends)

Electronegativity is a measure of the ability of an atom (or group of atoms) to attract shared electrons. This can be explained by applying Coulomb’s law.

According to Coulomb's law, the attractive force between charged particles increases with an increase in charge and decreases with an increase in the distance.

Coulomb's law

On the periodic table, electronegativity generally increases across a period and decreases down a group.

Electronegativity decreases as you move down a column as there is a greater distance from the nucleus and because there is also more electron shielding.
Electronegativity increases as you move across a period on the periodic table, from left to right. This is because the atomic radius is decreasing while the number of protons (and effective nuclear charge) is increasing.

The range of electronegativity values is from 4.0 for fluorine (the most electronegative) to 0.7 for cesium (the least electronegative).

Fluorine is the most electronegative element; it has a small radius with a small amount of electron shielding, coming from the 1s electrons,to the effective nuclear charge is high. This results in a strong pull on shared electrons

Types of Bonds and Electronegativity

Whether a bond will be ionic or covalent (polar or nonpolar covalent) is determined by how strongly the atoms involved attract shared electrons. What we call electronegativity. The greater the difference in electronegativity between two atoms them more ionic character the bond will have. We call bonds that are slightly ionic “polar covalent bonds”. Polar covalent bonds mean that the atoms involved in the bond do not share the electrons equally and therefore have a positive end and a negative end or a positive pole and a negative pole.

Check for Understanding

If lithium and fluorine react, which one has more attraction for an electron? Why?

The fluorine has more attraction for an electron than does lithium. Both have valence electrons in the same principal energy level (the 2nd), but fluorine has a greater number of protons in the nucleus. Electrons are more attracted to a larger nucleus (if the principal energy level is the same).

2. In a bond between fluorine and iodine, which has more attraction for an electron? Why?

The fluorine also has more attraction for an electron than does iodine. In this case the nuclear charge of iodine is greater, but the valence electrons are at a much higher principal energy level (and the inner electrons shield the outer electrons).

3. What is the general trend for electronegativity across rows and down columns on the periodic table?

Generally speaking, the electronegativity increases in going from left to right across a period because the number of protons increases which increases the effective nuclear charge. The electronegativity decreases going down a group because the size of the atoms increases as you go down so when other electrons approach the larger atoms, the effective nuclear charge is not as great due to shielding from the current electrons present.

The table below illustrates the relationship between electronegativity and bond type.

Exercise!

4. the following bonds from most to least polar:

a) N–F O–F C–F

b) C–F N–O Si–F

c) Cl–Cl B–Cl S–Cl

4. The greater the electronegativity difference between the atoms, the more polar the bond.

a) C–F, N–F, O–F

b) Si–F, C–F, N–O

c) B–Cl, S–Cl, Cl–Cl

5. Which of the following bonds would be the least polar yet still be considered polar covalent? Mg–O C–O O–O Si–O N–O

The correct answer is N-O. To be considered polar covalent, unequal sharing of electrons must still occur. Choose the bond with the least difference in electronegativity yet there is still some unequal sharing of electrons.

6. Which of the following bonds would be the most polar without being considered ionic?Mg–O C–O O–O Si–O N–O

The correct answer is Si-O. To not be considered ionic, generally the bond needs to be between two nonmetals. The most polar bond between the nonmetals occurs with the bond that has the greatest difference in electronegativity.

Bond Polarity and Dipole Moments

In polar bonds, electrons are not equally shared between atoms, creating a dipole moment. Dipole moment arises from differences in the electronegativities of bonded atoms. Molecules with dipole moment have a charge distribution that can be represented by a center of a positive charge and a center of a negative charge. An arrow is used to represent a dipole moment. The arrow points to the negative center with its tail starting from the positive center.

Why do atoms form bonds?

Stable Compounds:

Atoms form bonds to have a full valence shell of eight electrons (the octet rule). As atoms have a full valence shell, they become stable and have an electron configuration that resembles the nearest noble gas.

When two nonmetals react to form a covalent bond, they share electrons in a way that completes the valence electron configurations of both atoms.
When a nonmetal and a representative-group metal react to form a binary ionic compound, the ions form so that the valence electron configuration of the nonmetal achieves the electron configuration of the next noble gas atom. The valence orbitals of the metal are emptied.

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