HOMEWORK for UNIT 2. Atoms
Atomic theory
1. Define the terms element, compound, and mixture. Illustrate your definitions with simple circle pictures.
2. Identify each of these circle pictures as pure element, pure compound, or mixture. a b c d e
A B
3. Using the symbols shown at right, draw circle pictures that depict a. 4 atoms of pure B b. A mixture of 3 A atoms and 2 B atoms c. 3 molecules of pure AB d. 2 molecules of pure A
2
B e. a mixture of 1 molecule of A
2
, 2 molecules of B
2
, and 2 molecules of AB
2
4. The drawing at right represents one molecule of CD. Draw circle pictures that depict 4 molecules of CD a. turning from liquid CD to gaseous CD b. decomposing into C atoms and D atoms
Nuclear atom
5. a. How does the mass of a neutron compare to the mass of a proton? b. How does the mass of an electron compare to the mass of a proton? c. According to the nuclear model of the atom, how is mass distributed in the atom? d. Compare the size of the nucleus to the size of the atom.
6. Complete this table of atoms. The symbol must include the mass number.
Symbol Protons Neutrons Electrons Atomic number Mass number
34
S
31 40
60
31
47
74 182
26 26 56
7. Argon has three naturally-occurring isotopes, with 18, 20, and 22 neutrons. Write the symbols for isotopes of argon.
8. List the number of protons & neutrons in the four naturally-occurring isotopes of chromium:
50
Cr,
52
Cr,
53
Cr, and
54
Cr.

Atomic mass
9. One atom of
12
C has been assigned a mass of exactly 12 amu. What is the mass of one proton in amu?
What is the mass of one neutron in amu?
10. Using an instrument called a mass spectrometer, it was found that atoms of element X were 2.25 times as heavy as atoms of
12
C. What is the atomic mass of element X?
11. Copper has two naturally-occurring isotopes. 69.17% of naturally occurring copper is 63 Cu (mass
62.9396 amu), and 30.83% is
65
Cu (mass 64.9278 amu). What is the average atomic mass of Cu? isotope mass (amu) abundance
12. Silicon has three naturally-occurring isotopes, whose masses and abundances are shown in the table at right. Calculate the average atomic mass of Si.
Si-28 27.9769
Si-29 28.9765
Si-30 29.9737
92.2%
4.67%
3.10%
Periodic table
13. Write these items into a blank periodic table: a. the staircase that marks the boundary between metals and nonmetals b. the metal, semi-metal (or metalloid), and nonmetal regions c. alkali metals, alkaline earths, transition metals, halogens, and noble gases d. the trends in atomic size across a period and down a group e. period numbers and group numbers
14. Write the symbol for each element (some may have more than one possible answer): a. period 3, group 4A b. period 2, group 6A c. period 1 noble gas d. period 6 alkali metal e. period 5 halogen f. the fifth transition metal in period 4 g. period 4 semi-metal (metalloid) h. period 3 alkaline earth i. the element that does
not belong to any family
15. Which is larger: a. P or Cl b. Kr or Xe c. O or C d. Rb or Na