CHAPTER 2
Atoms and Molecules
 SECTION 2.1 Symbols and Formulas |
1. The elemental symbol is based on the element’s name and is made up of a single capital letter or a capital letter followed by a lowercase letter.
Give the symbols for the following elements (see Table 2.1).
a) lithium
b) helium
c) sodium
2. A compound formula is used to symbolize a molecular compound. It consists of the elemental symbols for each type of atom within the compound. Also, if there is more than one atom of a certain element, a subscript number is written next to that elemental symbol to indicate how many atoms there are.
Write a formula for the following compound: calcium carbonate (one calcium atom, one carbon atom, and three oxygen atoms).
3. Write formulas for the following molecules using elemental symbols and subscripts:
a) aluminum phosphate (one aluminum atom, one phosphorus atom, and four oxygen atoms)
b) hydrogen sulfide (two hydrogen atoms and one sulfur atom)
c) rust (two iron atoms and three oxygen atoms)
d) vinegar (two carbon atoms, four hydrogen atoms and two oxygen atoms)
4. Give the name and number of each type of atom in the following formulas:
a) sand (SiO2)
b) carbon tetrachloride (CCl4)
c) baking soda (NaHCO3)
d) strontium bromide (SrBr2)
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SECTION 2.2 Inside the Atom |
5. According to current theories, there are three key particles contained within atoms. They are protons, neutrons and electrons and are known as subatomic particles.
The fundamental subatomic particles are: protons, electrons and ________.
6. Some characteristics of these subatomic particles, such as their mass, electrical charge and location within the atom, help to explain the behavior of matter (see Table 2.3 and Figure 2.2).
Three characteristics of subatomic particles that help explain the behavior of matter are the location within the atom, mass and ________ charge.
7. Protons, along with neutrons, make up the central portion of an atom known as the nucleus. Protons each have a charge of +1.
Where are protons located within the atom?
8. Because protons each have a +1 charge and neutrons have no charge, the nucleus of an atom has a positive charge that is equal to the number of protons it contains.
A nucleus contains four protons. What is its charge?
9. Because most of the mass of an atom comes from the protons and neutrons it contains, the mass of an atom in atomic mass units (u) is equal to the number of protons plus the number of neutrons (p + n).
Determine the charge and mass (in u) of nuclei made up of the following particles:
a) 4 protons and 5 neutrons
b) 19 protons and 20 neutrons
c) 25 protons and 30 neutrons
10. Electrons are located outside the nucleus and carry a –1 charge. In a neutral atom, the number of electrons is the same as the number of protons, giving the overall atom no electrical charge.
a) The electrical charge of an electron is ________. In an electrically neutral atom with 4 protons there would be ________ electrons.
b) Complete the following table:
| mass in amu | charge | location | |
| proton | |||
| neutron | |||
| electron |
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SECTION 2.3 Isotopes |
11. The atomic number of an atom refers to the number of protons within the nucleus of that atom. Therefore, all atoms of a specific element have the same atomic number. The atomic number is symbolized as “Z” and is the smaller number contained in an element’s box on the periodic table (see the Periodic Table inside the text cover).
Which element has 26 as its atomic number?
12. The number of protons plus the number of neutrons within an atom make up its mass number and is represented by the symbol A (see Example 2.2).
The element copper (Cu) has an atomic number of 29 and a mass number of 64. How many neutrons does one atom of Cu contain?
13. Isotopes are atoms that have the same atomic number but different mass numbers.
According to the statement above, atoms that are isotopes must have the same number of protons but different numbers of ________.
14. The following
notation is used to identify isotopes: 
. Where E is
the symbol for the element, A is the mass number, and Z is the atomic number.
Determine the number of protons, number of neutrons and the number of electrons in atoms of the following isotopes. Remember, the number of electrons equals the number of protons in electrically neutral atoms.
a)
b)
c)
15. Another method of denoting isotopes is to give the element name followed by the mass number. For example: carbon -13 implies that the atom has 6 protons (remember, the number of protons determines which element it is) and 7 neutrons (13(p+n) - 6(p)=7(n)).
State how many protons, neutrons and electrons each of the following isotopes have.
a) carbon -14
b) phosphorus -32
c) iodine -129
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SECTION 2.4 Relative Masses of Atoms
and Molecules |
16. The atomic
mass unit (u) is used to express the relative mass of atoms. One u is equal to
1/12 the mass of an atom of 
.
1/12 the mass of a
atom is equal to the mass of one ________.
17. The relative masses of atoms are expressed in atomic mass units (u). They appear with the elements on the periodic table and are more commonly referred to as atomic weights. The atomic weight is the larger of the two numbers appearing with an element in the periodic table.
The atomic weights of elements are expressed as ________ on the periodic table.
18. Molecular weight is the relative mass of a molecule found by adding together the atomic weights of all the atoms within the molecule.
By adding the atomic weights of all the atoms within a molecule, one can determine the ________ weight.
19. After reviewing Example 2.4, determine the molecular weights for the following compounds. Before adding atomic weights, round off the values to the hundredths.
a) water H2O
b) copper sulfate CuSO4
c) barium chloride BaCl2
d) acetone C3H6O
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SECTION 2.6 Avogadro’s Number: The
Mole |
20. We often use words to describe larger quantities of objects. For example, 1 dozen is 12 of something, 1 gross is 144 of something, and 1 gallon is 128 ounces of something. Now here's another term to describe a very large number: 1 mole (or Avogadro's number) equals 6.02 x 1023 of that something. Now this is a very larger number. If we had Avogadro's number of giraffes, the earth would be entirely covered by giraffes 100's of miles deep!
a) There would be ________ sodium atoms in 1 gross of sodium atoms.
b) There would be ________ sodium atoms in 1 mole (abbreviated mol) of sodium atoms.
c) If you had Avogadro's number of sodium atoms you would have _________ sodium atoms and _________ mole of sodium atoms.
21. If we had 1 gross of baseballs, it may have a weight of 70 lbs. If we had a gross of basketballs, it may weigh 105 lbs. A gross of different types of balls will have different weights. The same is true of different atoms and molecules. One mole of sodium atoms will weigh less than 1 mole of sodium chloride molecules. Now it just so happens that 1 mol of something in the atomic world will have the same mass in grams equal to the atomic or molecular weight. Remember, atomic weights for elements are gotten directly from the periodic table (the larger number in the square for the element) and that molecular weights are calculated by adding together the atomic weights for all the individual atoms in a molecule.
a) 1 mol of Na atoms would have a mass of ________ grams. Avogadro's number of Na atoms would have a mass of ________ grams. 6.02 x 1023 atoms of Na would have a mass of ________ grams.
b) 1 mol of NaCl molecules would have a mass of ________ grams. Avogadro's number of NaCl molecules would have a mass of ________ grams. 6.02 x 1023 NaCl molecules would have a mass of ________ grams. (Review question #19 to calculate molecular weights)
22. Now comes the fun part. From these relationships we can create 3-way equations, such as:
1 gross baseballs = 144 baseballs = 70lbs. of baseballs and
1 gross basketballs = 144 basketballs = 105 lbs. of basketballs and,
1 mol Na atoms = 6.02 x 1023 Na atoms = 22.99 grams Na
You can complete:
1 mole NaCl molecules = ________ NaCl molecules = ________ grams NaCl
23. From each of these 3-way equation relationships we can create factors that can be used to solve problems (please review using factors in Chapter 1, Section 9). For example, we could make the following factors based on baseballs:
|
144 baseballs
|
or |
70 lbs baseballs
|
| 70 lbs baseball | 144 baseballs |
By using these factors we can convert between units to change the values into something we want. For example, we could convert 567 lbs of baseballs into the number of baseballs present:
|
567 lbs baseballs x |
144 baseballs | = 1,166 baseballs |
| 70 lbs baseballs |
To make the factor that was needed to solve the problem all we did was use numbers and units from the 3-way equation. First, look at the number and units that are given to you in the problem (567 lbs baseballs) and the units of the value you are converting to (number of baseballs). The factor you need to make should have the value from the 3-way equation that has the units given to you in the problem on the bottom (567 lbs given in problem, so choose 70 lbs baseballs from 3-way equation), and the desired units on the top (want number of baseballs, so choose 144 baseballs).
You complete the following:
1.65 gross baseballs x (give factor) = (calculate) lbs baseballs
24. Now let's just change from calculating about baseballs to calculating about atoms (basically just balls of different sizes). So far we have the 3-way equation:
1 mol Na atoms = 6.02 x 1023 Na atoms = 22.99 grams Na
Now let's use it to solve some problems. First, just write down the number and units that are given to you in the problem ("56.7 grams Na" from problem (a) below), then put in a multiplication sign (x), followed by the desired units you wish to convert to ("Na atoms" from problem (a) below):
a) Suppose you have 56.7 grams of sodium, how many sodium atoms would this be?
56.7 grams Na x (give factor) = (calculate) Na atoms
b) How many moles of Na atoms are there in 3.4 x 1025 atoms of Na?
(given number and units) x (factor) = (calculate) (desired unit)
25. Everything above also holds true for dealing with compounds (generally, which are bigger and heavier than atoms, just as basketballs are bigger and heavier than baseballs).
From above, you can complete:
a) 1 mol NaCl molecules = ________ NaCl molecules = ________ grams NaCl
Now use this 3-way equation to solve some problems:
b) Find the number of NaCl molecules in 4.55 mole of NaCl.
4.55 mol NaCl x (give factor) = (calculate) NaCl molecules
c. How many NaCl molecules are in 29.4 grams of NaCl?
(given number and units) x (factor) = (calculate) (desired units)
26. Now you're on your own. Remember to always first calculate the molecular mass of the compound (element) given before making the 3-way equation.
a) How many molecules of CaCO3 are there in 7.90 grams of CaCO3?
1) Molecular mass of CaCO3: ______
2) 3-way equation for CaCO3:
________ mole CaCO3 = ________ CaCO3 molecules = ________ grams of CaCO3
3) Fill in the template below:
(given number and units) x (factor) = (calculate) (desired units)
b) How many grams of BaCl2 do 500 BaCl2 molecules weigh?
1) Molecular mass of BaCl2: ________
2) 3-way equation for BaCl2: _________
3) Show calculations:
c) 21.2 g of P contains how many atoms of P?
1) Molecular mass: _______
2) 3-way equation: ________
3) Show calculations:
d) How many moles of sulfur dioxide are in 8.4 x 1010 molecules of SO2?
1)
2)
3)
27. Self Test:
Do the following calculations for NO2
a) What is the molecular mass of NO2?
b) Give the 3-way equation for NO2.
c) How many NO2 molecules are in 24.0g of NO2?
Do the following calculations for NaBr.
a) What is the molecular weight of NaBr?
b) Give the 3-way equation for NaBr.
c) How many moles are in 4.60g of NaBr?
Do the following calculations for K2O.
a) What is the molecular weight of K2O?
b) Give the 3-way equation for K2O.
c) How many moles of K2O are in 1.0 million (1.0 x 106) K2O molecules?
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Section 2.7 The Mole and Chemical Formulas |
28. The mole concept also allows us to determine the number of moles of each type of atom contained within compounds. For example, in 1 mole of barium sulfate (BaSO4). there are 1 mol Ba atoms, 1 mol S atoms, and 4 mol O atoms (see Example 2.9).
a) How many moles of each type of atom are contained in 1 mole of acetone (C3H6O)?
b) How many atoms of each element are in 1 mole of acetone (C3H6O)?
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