Video: Identifying the Type of Bond between Atoms in a Water Molecule Given an Electronegativity Difference in a Set of Bond Types

Which of the following is the type of bond between atoms within a water molecule (electronegativity difference = 1.2)? [A] Hydrogen bond [B] Ionic bond [C] Polar covalent bond (D) Nonpolar covalent bond [E] Metallic bond

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Video Transcript

Which of the following is the type of bond between atoms within a water molecule when the electronegativity difference is 1.2? (A) Hydrogen bond, (B) ionic bond, (C) polar covalent bond, (D) nonpolar covalent bond, or (E) metallic bond.

A bond or chemical bond is a strong force of attraction holding atoms or ions together. A hydrogen bond is an example of an intermolecular bond or force. Ionic bonds, polar covalent bonds, nonpolar covalent bonds, and metallic bonds are all examples of intramolecular bonds.

Inter- in the term intermolecular bond means between. So, an intermolecular bonds such as a hydrogen bond is the bond or force of attraction between two separate molecules. Intra- in the term intramolecular bond means within. So, an intramolecular bond is a strong force of attraction within the simplest unit of a compound such as a molecule or formula unit.

The question asks for the type of bond between atoms within a water molecule. So, we know that they are asking about an intramolecular bond. We can immediately rule out option (A) hydrogen bond. The water molecule consists only of the nonmetals hydrogen and oxygen. Since metallic bonds exist only in metals, we can rule out option (E) metallic bond. Let’s investigate the remaining three intramolecular bonds, ionic, polar covalent, and nonpolar covalent, to see which one exists within a water molecule.

First, we need to understand the term electronegativity. Electronegativity is the tendency of an atom to attract a bonding pair of electrons towards itself. When two nonmetal atoms react, they each donate one electron into a bond. The positively charged nuclei attract or pull the negatively charged electrons with an attractive force. Sometimes, there is an equal or almost equal pull on the bonding pair of electrons. And sometimes there is an unequal pull.

To determine whether the pull is equal or unequal, we need to know the relative electronegativity of each atom in the bond. Each element has been assigned an electronegativity value, which is unitless. The higher the value, the stronger the pull on a bonding pair of electrons.

When the difference in the electronegativity values for two bonded atoms is less than 0.4, we know that the pull on the bonding pair of electrons is equal or almost equal. And we say the type of bond is nonpolar covalent. When the difference in electronegativity is between 0.4 and 1.8, we know that the pull on the bonding pair of electrons is not equal. And we call that type of bond a polar covalent bond.

For a water molecule, looking at only one bond and two atoms at a time, the electronegativity values are 3.4 for oxygen and 2.2 for hydrogen. To find the difference, we take the bigger minus the smaller and we get an answer of 1.2. 1.2 fits into the polar covalent range. So, we know that in a water molecule, the type of bond which exists is polar covalent. And the bonding pair of electrons between the oxygen and the hydrogen are unequally shared.

They are pulled more strongly towards the oxygen atom because it has a higher electronegativity. And this results in a 𝛿 minus or slightly negative charge on the oxygen atom because the bonding electrons spend more time in the vicinity of the oxygen atom. And the hydrogen atom is left 𝛿 positive. The two ends of that bond or the two poles of the bond are different in terms of charge. And so, we say the bond is polar.

When a metal and a nonmetal bond together, they bond in the form of ions. The metal is a cation, positively charged, and the nonmetal, an anion, negatively charged. For example, in sodium chloride, NaCl, where sodium is Na+ and chlorine is Cl-. The positive and negative ions attract each other with an electrostatic force. And we call this type of attraction or bond an ionic bond. Ionic bonds exist between metal cations and nonmetal anions where the difference in electronegativity is larger than 1.8. And we can rule out ionic bond as an answer because water does not contain a metal.

Lastly, let’s look at what a hydrogen bond and a metallic bond are. We have seen that, in a water molecule, the bonds are polar covalent and that the oxygen atoms and hydrogen atoms have 𝛿 negative and 𝛿 positive charges, respectively. The force of attraction which exists between separate water molecules due to the 𝛿 positive hydrogens and the 𝛿 negative oxygens is called a hydrogen bond. Hydrogen bonds exist in other similar compounds where a hydrogen is bonded to a more electronegative element such as oxygen, nitrogen, or fluorine.

A metallic bond exists in metals, for example, in the metal of the pot on your stove. The positively charged nuclei of the atoms are held together by a sea of valence electrons between the atoms. Finally, the type of bond between atoms within a water molecule when the electronegativity difference is 1.2 is polar covalent bond.

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