Unit 3 Chemistry Test Review
(For District Interim Test #2 to be given on October 27-28, 2016)
1. What is an element? An element is a pure substance that cannot be broken down into any simpler
substances. Elements are each made from one kind of atom with a particular number of protons.
2. What is an atom? An atom is the smallest piece that can exist of an element, that still has the properties of
that element. Atoms are made of three basic “sub-atomic” particles: protons, neutrons and electrons.
3. What is a compound? A compound is a substance made of two or more elements chemically bonded
together in a set ratio. For example, molecules of the compound carbon dioxide (CO2) always contain one
atom of the element carbon and two atoms of the element oxygen. Compounds do not have the same
physical and chemical properties of the elements from which they are made.
4. What is a molecule? A molecule is a particle with two or more atoms chemically bonded together. A
molecule can contain atoms from the same element, as in O2 (a molecule of an element), or atoms of
different elements, as in H2O (a molecule of a compound).
5. Describe the 3 different types of sub-atomic particles, including their charge, location and mass.
Name
Charge
Location
Mass (in amu’s)
Proton
positive (+)
Nucleus
1 amu
Neutron
neutral (0)
Nucleus
1 amu
Electron
negative (-)
Electron Cloud
~ 0 amu
6. What is an atomic number? What does it tell about an element? In each element square on the periodic
table, the numerically smaller, whole number usually shown at the top of the element square is the atomic
number. The atomic number of an element is the number of protons that each atom of that element has in
its nucleus. Each element has a unique atomic number. In an electrically neutral atom, the number of
protons will equal the number of electrons.
7. Define “atomic mass” as used and listed on the periodic table. What does it tell about an element?
In each element square on the periodic table, the numerically larger, non-whole number, usually shown at
the bottom of the square, is the atomic mass. When the atomic mass is rounded up or down to the nearest
whole number, it indicates the sum of the protons and neutrons in one atom of the most common isotope
of that element.
8. What is the mass number of a potassium atom that has 20 neutrons? Potassium atoms have 19 protons,
because its atomic number is 19. The mass number of an atom is equal to the sum of the protons and
neutrons. Therefore, the mass number is 19 + 20 = 39.
9. What is a group or family on the periodic table? A vertical (up and down) column. What does the group or
family number indicate about the atomic structure of an atom for any element within that group? For the
representative elements in groups 1, 2 & 13-18, the one’s digit of the group number indicates the number of
valence electrons (the number of electrons in the outer shell) in atoms of elements within that group. (The
exception is helium, which only has 2 valence electrons, even though it is in group 18.)
10. What is true about the properties of all elements within a group or family? All of the elements within a
group have similar chemical properties, because they each have the same number of valence electrons.
11. Give an example of three elements that all have the same chemical properties. Any three elements in the
same group, such as calcium (Ca), Magnesium (Mg) and Barium (Ba), because they all will have the same
number of valence electrons.
12. What is a period on the periodic table? A horizontal (left to right) row. What does the period number
indicate about the atomic structure of an atom for any element with that period? The number of electron
shells (energy levels or orbitals) in each atom’s electron cloud. What is true about the properties of all
elements within a period? The chemical and physical properties of elements vary (change) across the
period.
13. How can the number of protons (and electrons) be determined for a particular element, using the periodic
table? The number of protons (and electrons) in one atom of an element is indicated by the atomic
number of that element.
14. How can the number of neutrons for the most common isotope of an element be determined?
When the atomic mass shown on the periodic table is rounded up or down to the nearest whole number, it
(usually) indicates the sum of the protons and neutrons in one atom of the most common isotope of that
element. To find the number of neutrons, subtract the atomic number of that element (which is the
number of protons) from the atomic mass. For example, for Krypton with an atomic number of 36 and a
mass number of 84, the number of neutrons is 48 (84 – 36 = 48).
15. What are valence electrons? Valence electrons are those electrons that are found in the outer electron shell
(energy level or orbital) of an atom. It is the valence electrons that react with other atoms in a chemical
reaction, and that determine how reactive an element will be.
16. How can the number of valence electrons for elements in groups 1-2 & 13-18 be determined? For the
representative elements (the elements in groups 1, 2, & 13-18), the one’s digit of the group number is
equal to the number of valence electrons in an electrically neutral atom of that element. (The number of
valence electrons for most of the transitional elements (groups 3-12) is usually two, but sometimes is one
or three.)
17. How can the number of electrons shells (also called energy levels or orbitals) for an element be determined?
The period number of an element indicates the number of electron shells that atoms of those elements
have. For example, the element potassium is in period 4, and thus has 4 electron shells.
18. What group or family has the most reactive elements? Why? The most reactive elements are found in
groups 1 and 17, because they only need to lose (group 1) or gain (group 17) one electron from their outer
(valence) electron shell. Elements in groups 2 and 16 are somewhat less reactive, because two electrons
must be lost or gained. Elements in groups 3 and 15 are still less reactive (gaining or losing 3 electrons),
and group 14 elements are even less reactive (gaining or losing 4 electrons).
19. What element has 11 protons and 1 valence electron? Sodium. How reactive will that element be? Very
reactive, because it only has one valence electron.
20. Which of the following elements is most reactive: carbon, sodium, magnesium, boron? Why? Sodium is most
reactive, because it only needs to lose one electron to react. Magnesium must lose two electrons, while
boron must lose three and carbon must either lose or gain four electrons to react.
21. Why is carbon less reactive than sodium? Sodium has fewer valence electrons than carbon.
22. What group or family has the least reactive elements? Why? Group 18 elements, which are called the noble
gases, are the least reactive elements because their outer (valence) electron shells are full, and thus they
cannot react with other atoms. Helium’s first and only shell is full with 2 electrons, while the other noble
gases have 8 electrons in their full outer shell.
23. Which group contains the “noble gases”? Group 18. Why are they called noble gases? All of the noble gases
are colorless gases that do not react with other elements. Because they do not react with the other
“common” elements of the periodic table, they are called “noble”.
24. What is a physical change? Give three examples. A physical change changes the size, shape or state of
matter of a substance without changing its chemical composition. For example, boiling liquid water to
form water vapor, or cutting paper into smaller pieces of paper, or bending metal into a different shape are
physical changes that do not change the original substance into a different substance. Dissolving one
substance into another is also a physical change. When substances sink or float in another substance, this
is due to the physical property of density.
25. What is a chemical change? Give three examples. A chemical change (also called a chemical reaction) occurs
when the atoms in substances are rearranged into different substances, by breaking existing chemical
bonds and forming new chemical bonds. Chemical changes always result in new chemical substances being
formed. Burning wood, cooking food, rusting iron, tarnishing metal and spoiling milk are all examples of
chemical changes.
26. Give five different kinds of evidence that would suggest that a chemical change (reaction) has taken place.
The single most important evidence of a chemical change is the creation of a new substance. Other
evidence that may indicate a chemical change include a change of temperature, change in color, creation of
a gas, and the emission of light.
27. In the yeast-sugar chemical reaction you witnessed, what was the evidence that a chemical change did take
place? Creation of a gas (carbon dioxide – CO2) which filled the balloon.
28. Describe the kind (element name) and number of atoms for each element in one molecule of Na2SO4. There
are 2 atoms of sodium, 1 atom of sulfur, and 4 atoms of oxygen.
29. In 2CuSO4, there are two molecules, each with one atom of copper, one atom of sulfur, and four atoms of
oxygen.
30. How many atoms are in one molecule of C10H12N2O? 25
31. How many molecules are in 4NaHCO3? 4
32. Which three of the following compounds have two nitrogen atoms: (A) Co(NO3)2 (B) (NH4)2CO3
(C) (NH4)3PO4 (D) Al(NO3)3 (E) NH4NO3 Compounds A, B and E each have two nitrogen atoms/C & D
have 3 each.
33. What is the law of conservation of mass? In a chemical reaction, atoms are neither created nor destroyed—
they are simply rearranged to form different compounds. For this reason, the number and kinds of atoms
on each side of a chemical equation must be balanced.
34. In a balanced chemical reaction, the mass of the reactants equals the mass of the products.
35. In the chemical equation 6H2O + 6CO2 C6H12O6 + 6O2, what are the reactants? Water (H2O) and carbon
dioxide (CO2), on the left side of the equation, are the reactants. What are the products? Sugar-glucose
(C6H12O6) and oxygen (O2), on the right side of the equation, are the products.
36. What is the common name for each of these chemical formulas? H2O: water; CO2: carbon dioxide;
O2 : oxygen; C6H12O6 : sugar (glucose).
37. Is each of the following chemical equations balanced? (Hint: An equation must be balanced because of the
law of conservation of mass.):
2H2O
2H2 + O2
6HCl + 2Al
C6H12O6 + O2
balanced
2AlCl3 + 3H2 balanced
CO2 + H2O not balanced
Al + CuCl2
not balanced
AlCl3 + Cu
4Fe + 3O2
2Fe2O3
2HCl + NaOH
H2 + O2
H2O
balanced
4NaCl + H2O not balanced
not balanced
38. For the following Bohr models of atoms, what is the element name, number of electron orbits and number of
valence electrons for each. (Hint: count the number of electrons to find the element name (atomic number =
number of protons (= # electrons)).
Element Name
Sulfur
Aluminum
Neon
Oxygen
# of orbits
3
3
2
2
# valence elec.
6
3
8
6
Which of the above elements have the same group number (and thus the same chemical properties)? Sulfur
and Oxygen in group 16, because they both have 6 valence electrons.
39. Label each of the following diagrams, as one of the following: nucleus, atom, element, or compound.
Element
Compound
Atom
Nucleus
40. In the
food web above,
which two organisms have a producer/consumer relationship? Microscopic algae and white suckers
(anything from algae following an arrow; anything from Detritus or anything following phytoplankton.
41. When classifying organisms, what are the main characteristics of animals? Animals are always multicellular
(made of many cells), eukaryotic (having cells with nuclei and organelles) and heterotrophic (eating other
organisms for food).
42. How does biodiversity contribute to the sustainability of an ecosystem? There are three main reasons: First,
the presence of more species with different adaptations makes it more likely that some organisms will
survive an ecological disaster. Second, greater genetic variation within species makes it more likely that
some individuals will survive a disease outbreak. Third, the presence of a variety of herbivore species that
can feed on a large number of producer species helps ensure abundant prey for predators in the
ecosystem.