Quizzes and Exams
• Please write in non-erasable blue or black pen, no pencil!
(Will lose 1 point for pencil or erasable ink)
• I will put periodic table on backs of quizzes, if you need
them.
• BOX answers! If it’s not obvious where (or which) the
answer is that I’m supposed to grade, it will get no credit.
• Remember to bring your non-programmable calculators to
class on Wednesdays.
• Make sure your answers make sense (can there be 23
grams of zinc in a 4.75 g sample of zinc oxide?).
• You may only use approved calculators on quizzes; others
will be confiscated for the duration of the quiz or exam.
Chapter 2: Atoms and Elements
1. Dalton’s Atomic Theory is based on empirical observations, formulated
as Laws of
Conservation of Mass
No detectable gain or loss of mass occurs in chemical reactions.
Definite Proportions
In a compound, elements are always combined in the same proportion by mass.
Multiple Proportions
Whenever two elements form more than one compound, the different masses of
one element that combine with the same mass of the other element are in the ratio
of small whole numbers.
2. Summary of Dalton’s Theory
--Matter consists of tiny particles called atoms.
--Atoms are indestructible. In chemical reactions, the atoms rearrange but they do not
themselves break apart.
--In any sample of a pure element, all the atoms are identical in mass and other properties.
--The atoms of different elements differ in mass and other properties.
--In a given compound the constituent atoms are always present in the same fixed
numerical ratio.
Sample Problems
1. A sample of caffeine contains 96.1 g of carbon for every 10.1 g of
hydrogen. If another sample of caffeine contains 30.0 g of carbon, how
many g of hydrogen does it contain?
2.* Phosphorus forms two compounds with chlorine. In 5.5 g of one of
these compounds, there were 1.24 g of P. In 5.5 g of the other compound,
there were 0.818 g of P. Explain how these compounds exhibit the law of
multiple proportions.
Sample Problems
1. A sample of caffeine contains 96.1 g of carbon for every 10.1 g of
hydrogen. If another sample of caffeine contains 30.0 g of carbon, how
many g of hydrogen does it contain?
Answer:
30.0
96.1
=
10.1
x
x = 3.15 g
2.* Phosphorus forms two compounds with chlorine. In 5.5 g of one of
these compounds, there were 1.24 g of P. In 5.5 g of the other compound,
there were 0.818 g of P. Explain how these compounds exhibit the law of
multiple proportions.
Answer: According to the law of multiple proportions, if you compare the same
amounts of phosphorus, then the chlorine masses should be in a ratio of small
whole numbers. Since 1.24/0.818 = 1.516, then you should multiply the
compound B quantities by 1.516. In compound A, there is (5.5 - 1.24 =) 4.26 g Cl.
In compound B there is (5.5 - 0.818 =) 4.682 g Cl. Multiply that by 1.516 to get
7.10 g Cl for every 1.24 g P in compound B. Then you can check the Cl ratio:
4.26 g / 7.10 g = 0.6 = 3/5, a ratio of small whole numbers.
“Modern” View of an Atom
electrons(-) form a diffuse
cloud around the nucleus
~Where is most of the
mass of an atom?
Matter is neutral, so # of
electrons = # of protons
nucleus has protons(+) and neutrons
Mass
~If there is a different number of protons, it is a different ___________?
~If there is a different number of neutrons, it is a different __________?
proton  neutron  1 amu (amu = 1.66 x 10-24 g = 1/12 of mass of 12C)
electron  10-4 amu
Atomic number, Z = # of protons
Mass number, A = # of protons + number of neutrons
A
Isotopes = atoms of the same element with different mass numbers ZE
Sample Problems
If a beryllium atom has 4 protons, then it should weigh 4 amu; but it
actually weighs 9 amu! Where is the extra mass coming from?
How many protons, electrons, and neutrons are in an atom of
52Cr?
24
Sample Problems
If a beryllium atom has 4 protons, then it should weigh 4 amu; but it
actually weighs 9 amu! Where is the extra mass coming from?
Answer: neutrons
How many protons, electrons, and neutrons are in an atom
of
24
Answer:Atomic number = 24, so 24 protons
Neutral, so 24 electrons
52 – 24 = 27, so 27 neutrons
52Cr?
Some Notes on Charge
•
Two kinds of charge called + and –
•
Opposite charges attract
–
+ attracted to –
•
Like charges repel
–
+ repels +
–
– repels –
•
To be neutral, something must
have no charge or equal amounts
of opposite charges
The Periodic Table
Decides ordering of elements
Average among
all isotopes
Transition metals
Inner transition elements
Types of Elements
Metals:
Shiny, malleable, ductile solids with
high mp and bp
Good electrical conductors
Nonmetals:
Gases, liquids, or low-melting solids
Non-conductors of electricity
Diatomic elements: H2, O2, N2, F2, Cl2, Br2, I2
Metalloids:
Intermediate properties, often semiconductors
Predicting Ionic Charges
Noble gases have very favorable electron configurations. The other
elements are jealous…
If an ion forms, it will commonly match the nearest noble gas.
1A
2A
3A
-3 O-2
N?
N
Li+1
Na+1 Mg+2
K+1 Ca+2
+1 Sr+2
Rb?
Rb
Cs+1 Ba+2
5A 6A 7A He
Al+3
S-2
-1 Ne
F?
F
Cl-1 Ar
Se-2 Br-1 Kr
Te-2 I-1
Xe
Rn
Calculating Atomic Mass (Weighted
Avg)
Atomic Mass = ∑ (fraction of isotope n) x (mass of isotope n)
n
= (fraction1∙mass1) + (fraction2 ∙ mass2) + (fraction3 ∙ mass3) + …
Example
63Cu
has mass = 62.9396, natural abundance 69.17%,
and 65Cu has mass = 64.9278 and natural abundance 30.83%. What is the
atomic mass?
Answer: Atomic mass = (62.9396)(0.6917) + (64.9273)(0.3083)
= 63.55 amu (4 sig fig!)
Sample Problems
1. In a class A is worth 4 points and B is worth 3. Out of 20 grades, if you
have 4 A’s (20%) and 16 B’s (80%), what is your average?
If an element has two isotopes, A (Z = 4 amu) and B (Z = 3 amu), and their
natural abundance is 20% A and 80% B, what is the average molecular
weight?
2. Naturally occurring boron (Z = 10.8110 amu) is composed of two isotopes,
10B and 11B. Atoms of 10B have a mass of 10.0129 amu and those of
11B have a mass of 11.0093 amu. Calculate the percentages by mass
of the individual isotopes, 10B and 11B.
Sample Problems
1. In a class A is worth 4 points and B is worth 3. Out of 20 grades, if you
have 4 A’s (20%) and 16 B’s (80%), what is your average?
If an element has two isotopes, A (Z = 4 amu) and B (Z = 3 amu), and their
natural abundance is 20% A and 80% B, what is the average molecular
weight?
Answer: Grade average = 3.2 points
Average molecular weight = 3.2 amu
2. Naturally occurring boron (Z = 10.8110 amu) is composed of two isotopes,
10B and 11B. Atoms of 10B have a mass of 10.0129 amu and those of
11B have a mass of 11.0093 amu. Calculate the percentages by mass
of the individual isotopes, 10B and 11B.
Answer:
Mass percent of
Mass percent of
10B
= 19.90%
11B = 80.10%
The Mole; How Chemists “Count”
1. Avogadro’s Number -- The Chemists’ “Dozen”
N0 = number of atoms in exactly 12 g of carbon-12
= 6.022 x 1023 “things” ( a very large number!)
This is a conversion factor, just like 12 things per dozen, e.g.
Mass of one atom of carbon-12 = (12.0 g)/6.022 x 1023 atoms)
= 1.99 x 10-23 g/atom
2. The Mole
One Mole of a substance contains an Avogadro’s
number of formula units.
e.g. 1 mole of H2O = 6.022 x 1023 H2O
molecules
Sample Problems
• How many atoms of gallium are there in 0.2398 moles of
gallium?
• How many molecules of sugar are there in 0.2398 moles of
sugar?
• How many marbles are there in 0.2398 moles of marbles?
Sample Problems
• How many atoms of gallium are there in 0.2398 moles of
gallium?
• Answer: 1.444 x 1023 atoms
• How many molecules of sugar are there in 0.2398 moles of
sugar?
• Answer: 1.444 x 1023 molecules
• How many marbles are there in 0.2398 moles of marbles?
• Answer: 1.444 x 1023 marbles