SPECTRA OF SCIENCE
BLIZZARD BAG #3:
Chemistry Review: Changes in Matter
Instructions: Read the attached passage titled Changes in Matter.
Answer Question Sets 3 on the answer sheet located on the last page of
this packet. Bring this answer sheet back to class to be collected and
graded for accuracy. You may keep the reading passage for your notes.
Due on April 24, 2015
CHANGES IN MATTER
COMPOUNDS
Scientists know of millions of different substances. How is this possible if there are only
115 elements? Elements can combine to form new substances. A substance that is formed
when two or more different elements combine is called a compound. Since many different
combinations of elements are possible, many different compounds can exist. Water, for
example, is a compound formed when the elements hydrogen and oxygen combine.
A compound is represented by a chemical formula that indicates which elements have
combined, and in what proportions. The chemical formula for water, H20, indicates that water
contains two atoms of hydrogen to every atom of oxygen. Table 2-6 lists some common
compounds and their chemical formulas. The small numbers in a formula that follow element
symbols are called subscripts. The subscript indicates the number of atoms of the element in
each particle of that compound. The number "one" is never written; it is understood that if a
symbol appears without a subscript, its subscript is one.
The smallest particle of a compound is called a molecule. A water molecule is composed
of one oxygen atom and two hydrogen atoms, as shown in Figure 2-7. Atoms of the same
element can also combine to form molecules. For example, two oxygen atoms combine to form
a molecule of oxygen gas, 02.
Atoms and molecules are extremely small. To get an idea of just how small, consider
that one teaspoonful of water contains about 175 sextillion water molecules.
(That would be written as 1.75 X 1023, or 175 followed by 21 zeros!)
CHEMICAL BONDS
Atoms in a molecule are joined together by a special link called a chemical bond (see
Figure 2-7). These bonds store chemical energy. Sometimes this energy can be released by a
chemical reaction. Burning is one type of chemical reaction that releases energy. When wood is
burned, energy stored in the chemical bonds within the wood is released as heat and light.
Respiration is another chemical reaction that releases energy from chemical bonds (in food).
Reactions that release heat energy are called exothermic reactions. Bond formation is always
exothermic. On the other hand, energy is required to break chemical bonds. Reactions that
absorb heat, using that energy to break bonds, are called endothermic reactions. All chemical
reactions involve breaking or forming bonds, or both. All chemical reactions either absorb or
release energy.
An exothermic reaction occurs in the hand warmers used by winter hikers and skiers.
When the chemicals in the hand warmer are mixed, the chemical change that takes place
releases heat. An endothermic reaction occurs in a cold pack. When the two chemicals in the
cold pack are mixed, a change takes place that absorbs heat. The cold pack feels cold because it
is absorbing thermal energy from its surroundings.
IONIC BONDS
Metals are elements that have a weak attraction for electrons. Nonmetals have a much
stronger attraction for electrons. When a metal reacts with a nonmetal, valence shell electrons
are transferred from the metal atoms to the nonmetal atoms. When this occurs, the number of
electrons in each atom no longer equals the number of protons in that atom. The resulting
charged particles are called ions. The metal atoms become positive ions while the nonmetals
become negative ions. These oppositely charged ions attract each other and form a chemical
bond called an ionic bond.
The strong attractions between oppositely charged ions result in the formation of
compounds called ionic solids. For example, sodium metal reacts with the nonmetal chlorine to
form sodium chloride, an ionic substance. When the sodium atom loses one electron, its
electron configuration changes from 2-8-1 to 2-8. When the chlorine atom gains one electron,
its configuration changes from 2-8-7 to 2-8-8. By forming these ions, both atoms have achieved
a noble gas electron configuration, that is, a complete outer shell. Recall that a noble gas
configuration has extra stability. Elements form compounds to gain stability. Figure 2-8 shows
how dot structures can be used to illustrate the formation of a molecule by an ionic bond.
In the compound above, the positive (+) charge of the sodium ion is exactly balanced by
the negative (-) charge of the chloride ion. (Negative ions are indicated with the ending "ide.")
The resulting compound is electrically neutral, as all compounds must be. What would happen
if chlorine combined instead with calcium? Calcium loses two valence electrons to form an ion
with a 2+ charge. The chloride ion, with its 1- charge, is formed when a chorine atom gains
one electron. If calcium loses two electrons, while chlorine gains only one, where
does the other electron go? In this case, two chlorine atoms are needed to accept the two
electrons lost by one calcium atom. The resulting compound, calcium chloride, has the formula
CaCI2. Note that the total positive charge is 2 + , the charge of the calcium ion. The total
negative charge is 2-, the charge of two chloride ions. The amounts of positive charge and
negative charge are exactly
equal. The resulting compound is therefore electrically neutral.
By knowing the charges of the metal and nonmetal ions, we can predict the formula of a
compound of those ions. For example, aluminum ions are 3 +, while fluoride ions are 1-. These
ions combine to produce a neutral compound with the
formula AIF 3.
COVALENT BONDS
Carbon dioxide is a compound formed from two nonmetals. In this case, electrons are
not transferred from one atom to another. Instead, two or more electrons are shared between
the two bonding atoms. A chemical bond in which electrons are shared is called a covalent
bond.
Electrons are always shared in pairs. Bonding atoms may share one, two, or three pairs of
electrons. Figure 2-9 shows how we can represent a compound in which two atoms share an
electron pair. The molecules formed by covalent bonds can also be represented by dot
structures. In the dot structure of such a molecule, the shared electrons are shown between
the symbols of the two bonding elements.
Table 2-7 shows the dot structures of some common molecules. As in ionic bonding, the
elements form noble gas electron configurations during covalent bonding. Count the electrons
around both atoms in the dot structure for HCl (in Table 2-7). The shared electrons should be
included in the total for each atom sharing those electrons. Notice that there are eight
electrons surrounding chlorine and two electrons surrounding hydrogen. (Nate: Hydrogen has
only one shell, which is filled by two electrons-the configuration of the noble gas helium.)
Although the element hydrogen is found on the left side of the Periodic Table, it is not
considered a metal. When hydrogen reacts with nonmetals to form compounds such as water,
ammonia, and sugar, it always forms covalent bonds.
When atoms share electrons, the electrons are attracted to the nuclei of both atoms. If
the atoms sharing the electrons are of the same element, as in N 2 or C12, then the attractions
are identical. In such cases, the two atoms share the electrons equally. However, when atoms
of different elements share electrons, as in the case of HCI, the electrons are generally
attracted more strongly to one of the atoms than to the other. Chlorine, for example, attracts
electrons more strongly than does hydrogen. Therefore, the shared electrons in HCI are shared
unequally, giving a more negative charge to the chlorine atom than to the hydrogen.
The unequal sharing of electrons may form a molecule that has an unbalanced
distribution of electrical charge. Such molecules are called polar molecules. The properties of
these molecules are greatly influenced by the unbalanced charge distribution. For example,
water, a polar molecule, is a liquid at room temperature. If it were not for the polarity of water,
it would be a gas at room temperature. The attractions between polar molecules tend to be
stronger than those between nonpolar molecules, but these attractions are much weaker than
the attractions between oppositely charged ions, as in NaCl.
The forces that hold all solids and liquids together result from the electrical attractions
between positive nuclei and negative electrons in neighboring particles.
QUESTION SET #3
Multiple Choice
Write the letter of the correct answer on your answer sheet. You may need to refer to a
Periodic Table of Elements to answer some of the following questions.
1. Atoms within a molecule are joined together by
a. Chemical bonds
b. electrical
c. magnetism
d. gravity
2. Which of the following formula represents a compound?
a. Na
b.O2
c. Ca
d. CO2
3. Which pair of atoms would most likely form an ionic bond?
a. Sodium and calcium
c. sodium and oxygen
b. Oxygen and chlorine
d. calcium and helium
4. When an atom of chlorine reacts chemically, it obtains the noble gas configuration of
a. He
b. Ne
c. Ar
d. Kr
5. Study the dot structure show. A noble gas configuration is achieved by
a. Nitrogen only
c. neither hydrogen nor nitrogen
b. Hydrogen only
d. both hydrogen and nitrogen
6. Which would most likely occur during the formation of an ionic bond between
potassium (K) and bromine (Br)?
a. The potassium would lose electrons to the bromine
b. The bromine would lose electrons to the potassium
c. The potassium and the bromine would share electrons equally
d. Both the potassium and bromine would lose electrons
7. Which diagram illustrates an equal sharing of electrons?
a.
c.
b.
d.
8. A molecule of carbon dioxide (CO2) is made up of one atom of carbon and two atoms of
oxygen. Carbon dioxide is best described as
a. An element because it is made up of one type of atoms
b. An element because it is made up of two types of atoms
c. A compound because it is made up of one type of atom
d. A compound because it is made up of two types of atoms
9. Which subatomic particles are found in the nucleus of an atom?
a. Positively charged protons and neutral neutrons
b. Positively charge electrons and neutral neutrons
c. Negatively charge neutron and positively charge protons
d. Negatively charged electrons and positively charged protons
10. Aluminum ions have a charge of 3+ while oxide ions have a charge of 2-. Which is the
correct formula for the compound called aluminum oxide?
a. AlO3
b. Al2O3
c. Al3O2
d. AlO
11. Food is often cooked over burning charcoal. What type of reactions is the burning of
coal?
a. An endothermic reaction that absorbs heat
b. An exothermic reactions that absorbs heat
c. An exothermic reaction that releases heat
d. An endothermic reaction that releases heat
Extended Response
12. What happens to the electrons in the formation of a covalent bond?
13. Explain how a chemical bond forms between potassium and chlorine.
14. How are exothermic chemical reactions different from endothermic chemical reactions?
Blizzard Bag #3 Answer Sheet
*Be sure to bring this paper back to class with you on the due date to be collected and graded.*
QUESTION SET #3:
Multiple Choice
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Extended Response
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