Online Introductory Chemistry
Dr. Walt Volland,
Answers to Self Test on Balancing Equations and Mass Calculations


Short essay questions


What are the sources of repulsions and attractions in the hydrogen molecule. Give answer in

terms of nuclei and valence electrons.

The illustration shows a "freeze frame" picture of the electrons and protons in a H2 molecule

at a random instant. The positions are changing.

The red lines indicate repulsions. The blue lines indicate attractions

Repulsions result from interaction of one electron with the other electron and the interaction of one nucleus with the other nucleus.

Attractions result from the interaction of the electrons with their own nucleus and with the nucleus in the other atom.


What is the origin of the octet rule? Is it universally followed?

If any exceptions exist, what causes them?

The octet rule arises from the fact that the ns and np subshells can hold only a maximum of

eight electrons.

The row 2 elements cannot have more than 8 electrons in the "2s2p" subshells. The octet

rule is usually followed by C, N, O , and F in row 2. They can form a maximum of 4 bonds.

The elements in the other rows do not always follow the octet rule.

The exceptions to the octet rule result when atoms can use "d" subshells to form bonds.

This means an atom can form 5, 6 or 7 bonds using (n-1)d ns np subshells.


Give the names for the following compounds.

HCl, PCl5, NO2, Cl2O5, CO, CO2, N2O4, SO3

HCl, hydrogen chloride. This is the pure compound and should not be confused with theaqueous solution that is HCl(aq) hydrochloric acid.

PCl5, phosphorus pentachloride

NO2, nitrogen dioxide

Cl2O5, dichlorine pentoxide

CO, carbon monoxide

CO2, carbon dioxide

N2O4 , dinitrogen tetroxide

SO3, sulfur trioxide

What does the term binary compound mean?

A binary compound is any compound made up of two elements like HBr, Cl2O5, N2O4


Draw the Lewis dot formula for HNO3. Identify the central atom and any multiple bonds.

What is the shape around the central atom in HNO3? give bond angles

The shape around the nitrogen central atom is planar triangular or trigonal planar. These are the same. The bond angles are close to 120o.

Indicate the polarity of bonds using +---> arrows . The electrons shift toward the oxygen end of the N-O and H-O bonds. This is true in all bonds involving oxygen. (Exception O-F )

Multiple choice


Which of the following pairs of atoms will form a single covalent bond.

a. Na and Cl This pair forms an ionic bond

b. He and He These two rare gas forms no bond

c. Ne and He This pair of rare gases forms no bondsforms no bond

d. H and O hydrogen can only form a single bond


Which of the following molecules contains exactly 24 valence electrons?

a. NO

b. NO2

c. CO2

d. SO3 Sulfur has 6 valence, each oxygen has 6 valence electrons. 1 x 6 + 3 x 6 = 24


Which of the following pairs of atoms can form a double bond?

a. He and He

b. H and H

c. N and H

d. N and O


Which of the following atoms can form a double bond?

a. C

b. O

c. N

d. all of these


Which of the following pairs of names and symbols is incorrect?

a. silicon tetrafluoride, SiF

b. nitrogen dioxide, NO2

c. phosphorus trichloride, PCl3

d. phosphorus pentachloride, PCl5


Which of the following sets of elements normally exists as diatomic molecules?

a. nitrogen, neon, helium, oxygen

b. nitrogen, oxygen, chlorine, fluorine

c. helium, neon, argon

d. sodium, calcium, zinc


Which of the following molecules is the most polar?

a. hydrogen chloride, HCl

b. hydrogen iodide, HI

c. hydrogen, H2

d. chlorine, Cl2


Which of the following molecules contains a multiple bond?

a. methane, CH4

b. nitrogen, N2

c. hydrogen, H2

d. chlorine, Cl2


The smallest possible unit of a covalent compound is usually considered to be a(an)

a. atom

b. cation

c. molecule

d. polyatomic ion


The systematic name for ICl3

a. iodine trichloride

b. iodine trichlorine

c. iridium trichloride

d. triiodinechloride


A double bond exists when two atoms share ______ electrons.

a. two

b. three

c. four

d. six


Carbon dioxide is a linear molecule. The electronegativity of carbon is 2.5. The electronegativity

of oxygen is 3.5. The bond between C and O is considered to be

a. nonpolar

b. polar

c. ionic

d. weak


Water molecules are polar. This is only possible if the molecule is_________________

a. linear

b. tetrahedral

c. bent

d. pyramidal


What is the predicted shape the carbon tetrachloride molecule CCl4

a. linear

b. trigonal planar

c. pyramidal

d. tetrahedral


Which of the following does not obey the octet rule?

a. phosphorus pentachloride

b. nitrogen monoxide

c. sulfur hexafluoride

d. all of these


Carbon in the Lewis dot formula for carbonate ion has _____ single bond(s)

and _____multiple bond(s).

a. two ...... one

b. two ...... two

c. one ...... none

d. four ...... two