Chapter U2.17, Problem 5E

Interpretation Introduction

Interpretation : Whether hydrogen is always partially positive when bonded to another atom has to be explained.

Concept Introduction :

In a polar covalent bond, atom with higher electronegativity value attracts more electrons than atom with low electronegativity value. Atoms that are less electronegative will have positive charge. Atomwith higher electronegativity value will have partial negative charge.

Answer to Problem 5E

No, hydrogen will not always have partial positive charge when bonded to other atoms. When it bonds with atoms having low electronegativity value such as boron, it will have partial negative charge.

Explanation of Solution

Given information:

Hydrogen is partially positive when bonded to some atoms.

Atoms that are more electronegative have partial negative charge and atoms that are less electronegative have partial positive charge.

For example in compound HCl hydrogen and chlorine atomsform a covalent bond. The difference between electronegativity value of hydrogen (2.10) and of chlorine (3.16) is 1.06. Here hydrogen has less electronegativityvalue than chlorine so it will have partial positive charge and chlorine will have partial negative charge.

  ${\text{H}}^{\delta +}-{\text{Cl}}^{\delta -}$

Another example is hydrogen bonding with boron forming HF-From the electronegativity scale, electronegativity value of hydrogen is 2.10 and boron is 2.04. Hence, hydrogen will attract the bonded electrons more strongly than boron. So here hydrogen has a partial negative charge and boron has a partial positive charge.

  ${\text{H}}^{\delta -}-{\text{B}}^{\delta +}$

Conclusion

In case of compounds where hydrogen bonds with atoms having less electronegativity value than 2.01, it will have a partial negative charge. In all other cases it will have a partial positive charge.