Exam 1 Study Guide Answers
Use your periodic table to determine the following information about the element
Chlorine (Cl).
Atomic #: 17
Average Atomic Mass: 35.453
Mass #: 35 (round the average atomic mass to a whole #)
# protons: 17 (equal to the atomic #)
# neutrons: 18 (equal to the mass # - atomic # or protons: 35 – 17 = 18)
# electrons: 17 (equal to the atomic # as long as there isn’t a charge)
What two things does the atomic # always tell you?
a. # protons
b. identity of the element
The Mass # is equal to the number of protons + the number of neutrons.
In the element symbol, 8035Br:
a. The top # is the mass number (# protons + # neutrons)
b. The bottom # is the atomic number (# protons, #electrons if no charge
present)
In the element symbol Carbon-14, the number 14 is the mass number
An isotope has 15 protons, 16 neutrons, and 15 electrons.
a. What is the atomic # of this isotope? 15 (atomic # = # protons)
b. What is the identity of the isotope? P (P has atomic # 15)
c. What is the mass # of this isotope? 31 (find the average atomic mass for
for P in the periodic table and round it to the nearest whole #)
d. What is the average atomic mass of all of the isotopes of this element?
30.974 (this is the decimal # in the periodic table)
What part of Dalton’s atomic theory was disproved by Thomson’s experiment?
Dalton thought that the atoms of an element were indivisible. When Thomson’s
experiment led to the discovery of the electron, it showed that the atom could be divided
into smaller particles.
Dalton
Thomson
Rutherford
Name of Atomic Model
Billiard Ball
Plum Pudding
Nuclear
Thomson
Rutherford
Chadwick
Discovered
Electron
Positively charged nucleus (protons)
Neutrons
Thomson
Rutherford
Experiment
Cathode Ray Tube
Gold Foil
Describe Thomson’s experiment:
Scientists knew about cathode rays, but didn’t know what was in them. Thomson put a
positively charged plate on one side of the cathode ray and a negatively charged plate
on the other side of the cathode ray. The ray bent toward the positively charged plate
and away from the negatively charged plate. This meant that the cathode ray must
contain negatively charged particles that were attracted by the positively charged plate
and repelled by the negatively charged plate. The negatively charged particles were
called electrons.
Describe Rutherford’s experiment:
Rutherford shot alpha particles through a thin piece of gold foil. According to the current
model of the atom at the time (Thomson’s Plum Pudding Model), Rutherford expected
the alpha particles to go through the foil and come out on the other side. Most of the
alpha particles behaved as expected, but a small number of them hit something in the
atom and bounced back. This led to the conclusion that atoms must contain a small,
dense, positively charged nucleus in the center of the atom. A new model of the atom
was needed to incorporate this information (Rutherford’s Nuclear Model).
Isotopes have the same number of protons, different numbers of neutrons and different
mass numbers
Ions have gained or lost one or more electrons.
Cations have lost electrons and have a positive charge.
Anions have gained electrons and have a negative charge.
16 O
8
16 O28
17 O
8
23 Na1+
11
23 Na
11
Which of the above symbols are isotopes of an element?
16 O and 17 O (same element,O; same atomic # (protons) – the bottom #, 8; different
8
8
mass #’s, 16 and 17 – different because they have different numbers of neutrons
making causing one to have more mass than the other)
Which of the above symbols is/are ions?
23 Na1+, 16 O2- (because they have charges indicating that they have gained or lost one
11
8
or more electrons)
Which of the above symbols is/are cations?
23 Na1+ (because cations have lost one or more electrons causing the atom to have
11
more positively charged protons than negatively charged electrons – this gives the
overall atom a positive charge)
Which of the above symbols is/are anions?
16 O2- (because anions have gained one or more electrons causing the atom to have
8
more negatively charged electrons than positively charged protons – this gives the
overall atom a negative charge)
How many electrons are indicated by the following symbols?
Mg2+ 10 (normally Mg has 12 electrons (equal to the atomic #) – the positive
charge indicates that the atom has lost electrons – the small “2” indicates
that the atom has lost 2 electrons. So, 12 – 2 = 10 electrons)
P3- 18 (normally P has 15 electrons (equal to the atomic #) – the negative
charge indicates that the atom has gained electrons – the small “3”
indicates that the atom has gained 3 electrons. So, 15 + 3 = 18 electrons)
Draw the following atoms – the circles represent the nucleus. (The numbers indicate the
number of dots you would draw for the atom and where you would draw them. p =
protons, n = neutrons, e = electrons)
39 K
19
14 N37
19 F19
19e
10e
10e
Here’s another one:
40 Ca2+
20
18e
19p,
20n
7p,
7n
9p,
10n
20p,
20n
# protons = atomic # = 19 (bottom # in the symbol)
If no charge, # electrons = atomic # = 19 (bottom # in the symbol)
# neutrons = mass # (top # in the symbol) – atomic # (bottom # in
the symbol): 39 – 19 = 20
# protons = atomic # = 7 (bottom # in the symbol)
If neg. charge, # electrons = atomic # (bottom # in the symbol) plus
the # in the charge: 7 + 3 = 10
# neutrons = mass # (top # in the symbol) – atomic # (bottom # in
the symbol): 14 – 7 = 7
# protons = atomic # = 9 (bottom # in the symbol)
If neg. charge, # electrons = atomic # (bottom # in the symbol) plus
the # in the charge: 9 + 1 = 10
# neutrons = mass # (top # in the symbol) – atomic # (bottom # in
the symbol): 19 – 9 = 10
# protons = atomic # = 20 (bottom # in the symbol)
If pos. charge, # electrons = atomic # (bottom # in the symbol) minus
the # in the charge: 20 - 2 = 18
# neutrons = mass # (top # in the symbol) – atomic # (bottom # in
the symbol): 40 – 20 = 20
What is the average atomic mass of Mg if 78.99% of Mg atoms are isotopes with a
mass of 23.9850 amu, 10.00% of Mg atoms are isotopes with a mass of 24.9858 amu,
and 11.01% of Mg atoms are isotopes with a mass of 25.9825 amu?
Create a table to make this problem easier to work with.
Isotope
1
2
3
% abundance
78.99 %
10.00 %
11.01 %
mass (amu)
23.9850
24.9858
25.9825
Step 1: Change the %’s to decimals by moving the decimal 2 places to the left!!!!
Isotope
1
2
3
% abundance
.7899
.1000
.1101
mass (amu)
23.9850
24.9858
25.9825
Remember! You must move the decimal two places to the left – if there isn’t a
number there – fill in with 0’s.
Ex. 5.32% → .0532
0.72% → .0072
Step 2: Multiply the % abundance (in decimal form!) x mass for each isotope.
.7899 x 23.9850 = 18.9
.1000 x 24.9858 = 2.5
.1101 x 25.9825 = 2.9
Step 3: Add together all the answers from step 2.
18.9 + 2.5 + 2.9 = 24.3
The answer is 24.3 amu
What is the difference between a hypothesis and a theory?
Hypothesis: A testable question
Theory: Consistent evidence from many experiments