Chapter 3 – Review KEY
1. Explain what the formula of water, H2O tells us in your own words.
For every 1 oxygen atom, 2 hydrogen atoms the molecule, (ratio 2 to 1)
2. What is the difference between 2N and N2?
A big 2 stands for “two” molecules of whatever is after it. A little 2 stands for both
stuck together.
3. Explain Dalton’s “Law of Constant Composition.”
Constant ratio throughout substance
4. What did Dalton’s atom look like?
A round ball (very small)
5. List the 5 main ideas of Dalton’s atomic theory. Which one of these is no longer
accepted?
See book for all five, but all atoms are indivisible now.
6. Pick the true statement, and explain why it is true.
An ion is formed
a. by either adding or subtracting protons from the atom
false, changes element
b. by either adding or subtracting neutrons from the atom
false, changes isotopes
c. by either adding or subtracting electrons from the atom
true, it’s the only item that can be added or subtracted
d. all of the above
e. two of the above
7. A certain ion with 27 electrons has a charge of 2+. What ion is it?
a. Mn2+
b. Co2+
c. Cu2+
Explain your choice.
Cu has 29 e-, Cu +2 (remove 2 e-) has 27e8. An element forms an ionic compound with chlorine having the formula XCl2 (the
charge of X in the compound is a +2). The ion of element X has a mass number of 89
and 36 electrons. Identify the element X and tell how many neutrons it has.
36e- ion  38e- atom must be Sr  p+n = 89 with 38 p, Sr with 51 neutrons
9. Which of the following did Dalton not discuss in his atomic theory? Explain.
a. isotopes
b. ions
c. protons
d. electron
e. neutrons
He didn’t discuss any of them; he thought that all atoms were identical
10. How did JJ Thomson discover the electron?
Atoms emit tiny particles, and when put through an electric field, it is attracted to the
positive side
11. What part of the atom did Rutherford’s Gold Foil experiment discover? How?
Nucleuspositive alpha particles thrown at gold foil ricocheted.
12. Knowing the number of protons of a neutral atom enables you to determine which of
the following (there may be more that one answer)
a. the number of neutrons
b. the number of electrons
c. the name of the atom
What other information would you need to find the remaining items?
 Mass number
1.) Explain how the atoms of one element differ from those of another element.
For sure: different number of protons
In neutral atom: different number of electrons
Could have different number of neutrons
2.) In what way are two isotopes of the same element? Explain why isotopes of the same
element have the same chemical behavior.
Same: Exact same number of protons and electrons. Different: different number of
neutrons
Chemical behavior: chemical behavior depends on electrons
3.) Complete the Table:
Atomic
Mass
Number of
Number of
Number of
Number
Number
Protons
Neutrons
Electrons
8
16
8
8
8
7
14
7
7
7
20
41
20
21
20
11
23
11
12
11
26
56
26
30
26
4.) There are four naturally occurring isotopes pf the element chromium. The relative
abundance of each is: 50Cr = 4.31%, 52 Cr = 83.76%, 53Cr = 9.55%, 54Cr = 2.38%.
Calculate the average atomic number.
Mp =
1.00728 amu
Mn =
1.00866 amu
Me = 5.48 x 10^-4 amu
.0431[24(1.00728) + 26(1.00866) + 24(5.48x10^-4)] + 0.8376[24(1.00728) +
24(5.48x10-4) + 28(1.00866)] + .0955[24(1.00728) + 24 (5.48x10-4) + 29(1.00866)] +
0.0238[24(1.00728) + 24(5.48x10-4) + 30(1.00866)]
= 52.49 amu