Download What are atoms? - Riverdale Middle School

Chapter Introduction
Lesson 1 Atoms
Lesson 2 Elements
Lesson 3 Compounds
Lesson 4 Chemical Reactions
and Energy
Chapter Wrap-Up
What happens to atoms
and energy during a
chemical reaction?
Atoms
Essential Questions
• What is an atom and how has the
model of the atoms changed over
time?
Atoms
Atom
Neutron
Electron
Electron
cloud
Nucleus
Proton
What are atoms?
An atom is the smallest particle of an
element that still has the same chemical
properties of the element.
• Where are the electrons, neutrons, and
protons located in an atom?
• What are the charges of the parts of the
atom?
• How does the mass of a neutron compare
to the mass of a proton?
• How does the mass of an electron
compare to the mass of a proton or a
neutron?
• The protons and neutrons are in the
nucleus. The electrons are all located
within the electron cloud.
• Electrons are negatively charged, protons
are positively charged, and neutrons are
neutral or no charge.
• The mass of a proton and neutron are
about equal.
• The mass of an electron is 1/1,840 times
smaller than a proton or a neutron.
What are atoms?
Democritus (460–370 BC) believed that
matter is made of small, solid objects
called atomos, from which the English
word atom is derived.
What are atoms? (cont.)
• Aristotle (384–322 BC) did not believe
that empty space exists, but instead
believed that all matter is made of fire,
water, air, and earth.
• Because Aristotle was so influential,
his ideas were accepted and
Democritus’s ideas about atoms were
not studied again for more than 2,000
years.
What are atoms? (cont.)
• In the late 1700’s, John Dalton combined
from his own scientific research with data
from the research of other scientists to
propose a new atomic theory.
– Dalton’s experiments indicated that matter
was made of tiny particles with spaces
between them.
What are atoms? (cont.)
• Not long after Dalton’s findings, another
scientist, J.J. Thomson, made some
important discoveries using a cathode
ray tube.
What are atoms? (cont.)
• Through further experiments, Thomsom
found that the rays were made of
particles. He called these particles
electrons.
• electron is a particle with a negative
electrical charge.
What are atoms? (cont.)
• Thomson believed that atoms also contain a
positive charge that balances the negatively
charged electrons.
• Thomson thought an
atom was a sphere with
a positive charge evenly
spread throughout with
negatively charged
electrons mixed through
the positive charge.
How was Thomson’s atomic
model modified?
• In the early 1900’s, scientist, Ernest
Rutherford set up experiments to test
Thomson’s atomic model and to learn
more about what atoms contain.
• Rutherford and his students fired alpha
particles at gold foil.
– Alpha particles are tiny, positively
charged particles.
How was Thomson’s atomic
model modified? (cont.)
•Rutherford expected the positive alpha
particles to travel straight through the foil
without changing direction.
How was Thomson’s atomic
model modified? (cont.)
• Some alpha particles traveled in a straight
path, as expected. But some changed
direction, and some bounced straight back.
How was Thomson’s atomic
model modified? (cont.)
• Because some of the positively charged
alpha particles bounced off something, the
gold atoms must contain some positively
charged object in the empty space.
How was Thomson’s atomic
model modified? (cont.)
• Rutherford concluded that most of an
atom’s mass and positive charge is
concentrated in a small area in the
center of the atom called the nucleus.
How was Thomson’s atomic
model modified? (cont.)
• Nucleus the center of an atom, which contains
most of the atom’s mass.
• Further research showed that the positive
charge in the nucleus is made up of positively
charged particles called protons.
– Proton a positively charged particle inside an
atom’s nucleus.
• Negatively charged electrons move in the
empty space surrounding the nucleus.
How was Thomson’s atomic model
modified? (cont.)
• Rutherford’s model contains a small, dense,
positive nucleus. Tiny, negatively charged
electrons travel in empty space around the
nucleus.
How was Thomson’s atomic model
modified? (cont.)
• James Chadwick, Rutherford’s coworker,
discovered that, in addition to protons,
the nucleus also contained neutrons.
– Neutron a particle that is found in the
nucleus of an atom and has no electrical
charge.
How was Thomson’s atomic model
modified? (cont.)
• Early in the
twentieth
century, Niels
Bohr proposed
that electrons
move in circular
orbits, or energy
levels, around
the nucleus.
How was Thomson’s atomic model
modified? (cont.)
• Each energy level can hold a certain
number of electrons:
– 1st energy level = 2 electrons (closest to the nucleus)
• When full, electrons begin to fill the second level.
– 2nd energy level = 8 electrons
• When full, electrons go to the next-higher level.
– 3rd energy level = 18 electrons
• When full, electrons go to the next-higher level.
How was Thomson’s atomic model
modified? (cont.)
• The last occupied energy level might or
might not be completely filled.
• Electrons in the outermost level are
called valence electrons.
– These outermost electrons are involved in
chemical bonding.
How was Thomson’s atomic model
modified? (cont.)
• In Bohr’s model of the atom, electrons
move in circular orbits around the atom.
What is the modern atomic
model?
• The mass of
electrons is
much smaller
than the mass
of protons or
neutrons.
• Most of the
mass of an
atom is found
in the nucleus.
What is the modern atomic
model?
• In the modern atomic model, electrons
form an electron cloud.
• An electron cloud is an area around an
atomic nucleus where an electron is
most likely to be.
What is the modern atomic
model?
• The modern atomic model shows the
electrons moving around the nucleus in the
electron cloud. The cloud represents the
area where the electrons are more likely to
be found.
Which term describes a particle
with one negative charge?
A. atom
B. electron
C. nucleus
D. proton
Whose model of the atom
contained a sphere of positive
charge with negatively charged
electrons within it?
A. Dalton
B. Democritus
C. Rutherford
D. Thomson
Which term refers to an area
around an atomic nucleus where
an electron is most likely to be?
A. electron cloud
B. neutron
C. nucleus
D. proton
Elements
Essential Question
• What are the properties of
elements?
Elements
Element
Atomic
number
Isotope
Average
atomic mass
Periodic table
What are elements?
• Everything around you is made of elements.
– Elements a pure substance that cannot be broken
down into a simpler substance.
– Ex. The human body is
made mostly of water
(H2O). Water is not an
element because it can be
broken into simpler
substances of Hydrogen
(H) and Oxygen (O).
Hydrogen and oxygen are
elements.
What are elements? (cont.)
• Every element has a symbol that is one or two
letters long.
– Ex. Hydrogen’s symbol is H
Oxygen’s symbol is O
Copper’s symbol is Cu
• The first letter is always capitalized.
• The symbols are used in chemical formulas.
What are elements? (cont.)
• Each element has atoms consisting of a
specific number of protons, neutrons and
electrons.
• An element is identified by the number of
protons contained in each of its atoms.
– Ex. Oxygen atoms contain 8 protons
Nitrogen atoms contain 7 protons
• Atomic number the number of protons in an
atom of an element.
What are elements? (cont.)
• Different elements have different atomic
numbers.
• Atoms normally have the same number of
electrons as protons.
What are elements? (cont.)
• Atoms of the same element can have
different numbers of neutrons.
• Isotopes are atoms of the same element
that have different numbers of neutrons.
• Most elements have several isotopes.
What are elements? (cont.)
• The mass number of an atom is the
sum of the number of protons and
neutrons in an atom.
Mass number = number of protons + number of neutrons
• An isotope is often written with the
element name followed by the mass
number.
What are elements? (cont.)
• Average atomic
mass is the average
mass of the atoms of
an element.
• The average mass is
weighted toward, or
favors, the isotope
that is most abundant.
Atomic
number
What is the periodic table?
• Periodic table a chart that shows the
elements in order of increasing atomic
number.
– The atomic number increases from left to
right as you move across a period.
• Elements on the periodic table are
organized in periods (rows) and groups
(columns) according to their physical and
chemical properties.
What is the periodic table? (cont.)
• Each box in the periodic table shows
information about a different element.
– Name
– Chemical symbol
– Atomic number
– Average atomic mass
– State of matter at room temperature
What are metals and nonmetals?
• Scientists divide the elements into three
groups based on similar properties.
What are metals and nonmetals? (cont.)
• Electrical resistance is a measure of
how difficult it is for an electric charge to
flow through an object.
• Conductors (like metals) have low
electrical resistance and allow electricity
to flow easily because their atoms’
valence electrons can easily move from
atom to atom.
– Ex. Electric wires are usually made of
copper.
What are metals and nonmetals? (cont.)
• A good insulator has high electrical
resistance.
• Atoms of an insulator hold electrons
tightly and prevent electricity from
moving easily through the material.
– Ex. Electrical wiring covered with plastic.
What are metalloids?
• Elements get more metallic to the left
across any row in the periodic table.
• Elements get more metallic toward the
bottom of any column.
• In the middle, the properties switch from
metallic to nonmetallic. These elements
are called metalloids.
What are metalloids? (cont.)
• Metalloids have some properties of
metals and some of nonmetals.
Use the periodic table. What is
the average atomic mass of
magnesium?
A. 12
B. 25
C. 24.31
D. 54.94
Which term refers to the sum
of the number of protons and
neutrons in an atom?
A. atomic number
B. average atomic mass
C. isotope
D. mass number
Ruthenium has an atomic number
of 44 and a mass number of 101.
How many neutrons does
ruthenium have?
A. 44
B. 88
C. 101
D. 57
Compounds
Essential Question
• How do atoms combine to form
all types of matter?
Compounds
Compound
Ion
Ionic bond
Covalent bond
What are compounds?
• Compound is a substance formed by
the chemical combination of two or more
elements.
– Ex. Citric acid – found in oranges and
lemons – is a compound that is made of
the elements carbon (C), hydrogen (H) and
oxygen (O)
citric acid is written as = C6H8O7
What are compounds? (cont.)
• The properties of a compound are
usually different from the properties of
the elements from which it is made.
– Ex. Table salt (sodium chloride)
• Sodium (Na) = soft, silvery and highly
reactive metal
• Chlorine (Cl) = poisonous green gas.
• Table salt (NaCl) = white, brittle, crystalline
solid
How do atoms bond?
• Atoms form bonds with other atoms by
losing, gaining or sharing their valence
electrons.
– Ionic bonds – When a metal and a
nonmetal bond, one or more valence
electrons transfer from the metal atom to
the nonmetal atom.
• Ion is an atom that is positively or negatively
charged because it has gained or lost electrons.
How do atoms bond? (cont.)
– Ionic bonds (cont.)
• Atom loses electron(s) = positively charged
• Atom gains electron(s) = negatively charged
• Two ions have opposite charge and are
strongly attracted to each other.
• Ionic bond an attraction that holds oppositely
charged ions close together.
How do atoms bond? (cont)
– Covalent bonds– When nonmetal atoms bond
together, they bond by sharing valence electrons.
• Covalent bond is a chemical bond that forms when
atoms share electrons.
• Shared electrons move back and forth between the outer
energy levels of each atom in the covalent bond.
• Form a neutral particle called a molecule.
What compounds make up
common substances?
• Compounds that
contain carbon are
the building blocks
of living things and
are called organic
compounds.
• Compounds that do
not contain carbon
as well as carbon
monoxide and
carbon dioxide are
called inorganic
compounds.
Which part of the atom is gained
or lost when forming a chemical
bond?
A. neutron
B. nucleus
C. proton
D. electron
Which term refers to chemical
combinations of different types
of atoms?
A. covalent bond
B. chemical formula
C. compound
D. polar molecule
Which substance is a compound?
A. chlorine
B. calcium
C. table sugar
D. sodium
Chemical Reactions and
Energy
Essential Question
• Why do chemical reactions
always involve a change in
energy?
Chemical Reactions and
Energy
Exothermic
reaction
Rate of
reaction
Endothermic
reaction
Catalyst
What are exothermic reactions?
• Chemical bonds contain a form of energy called
chemical energy.
– When a chemical bond breaks, the bond releases
chemical energy.
– When a chemical bond forms, chemical energy is
absorbed.
• You might feel this energy change as a change
in the temperature of the surroundings.
What are exothermic reactions? (cont.)
• Some chemical reactions release more
energy than they absorb.
• Some chemical reactions absorb more
energy than they release.
• Energy is conserved in all chemical
reactions.
What are exothermic reactions? (cont.)
• Exothermic reaction is a chemical reaction
that releases energy.
• In an exothermic
reaction, not all the
chemical energy
released
from breaking the
reactant(s) bonds is
used
in the formation of the
product bonds.
What are exothermic reactions? (cont.)
• Chemical reactions can release several kinds
of energy: electrical, light, sound and thermal.
– Ex. Welding torch is an exothermic reaction
between hydrogen and oxygen to make water that
releases light energy and thermal energy.
– Ex. Energy released during digestion of food is
transferred to your cells to help you grow, move
and think.
• In an exothermic reaction, the energy released
comes from the bonds of the reactants.
What are endothermic reactions? (cont)
• Endothermic reaction is a chemical
reaction that absorbs energy.
• In an endothermic
reaction, more
energy is required
to break the
bonds of the
reactants than
is released when
the products form.
What are endothermic reactions?(cont.)
• Chemical reactions can absorb several types
of energy: electrical, light and thermal.
– Ex. Photosynthesis – plants absorb light energy
from the Sun.
– Ex. Decomposition of water – electrical energy is
needed to break water molecules into hydrogen
and oxygen gas.
• In an endothermic reaction, the energy
absorbed comes from the environment
surrounding the reactants.
What can affect the rate of a
chemical reaction?
• Rate of reaction the measure of how
fast a chemical reaction occurs.
• Chemical reactions occur faster if
particles collide more often or move
faster when they collide.
What can affect the rate of a
chemical reaction? (cont)
• Surface area - Increased surface area
increases reaction rate because more
particles on the surface of a solid come
into contact with the particles of another
substance.
• Temperature - At higher temperatures,
the average speed of particles is greater,
particles collide more often, and collisions
with more energy are more likely to break
chemical bonds.
What can affect the rate of a
chemical reaction? (cont)
• Concentration - Increasing the
concentration of one or more reactants
increases collisions between particles,
resulting in a fast reaction rate.
• Pressure - In gases, an increase in
pressure pushes gas particles closer
together, resulting in more collisions.
What are catalysts?
• Catalyst is a substance that speeds up a
chemical reaction but is not used up
itself or permanently changed.
– Ex. Enzymes – Are used by your body to
speed up chemical reactions that convert
food to fuel, store extra energy in fat and
produce other enzymes.
In which type of reaction is more
energy released when the
products form than is required to
break the bonds in the reactants?
A. endothermic
B. exothermic
C. single-replacement
D. decomposition
Which reaction is endothermic?
A. using an athletic cold pack
B. welding
C. iron rusting
D. burning wood
Which explains why reaction
rates increase when the reactants
are crushed?
A. The substances melt faster.
B. More energy is needed to break the
chemical bonds.
C. The bonds of the reactants do not
require energy to break.
D. The substances have a greater
surface area.
Visual Summary
Chapter Review
Standardized Test Practice
During a chemical
reaction, bonds are
broken and atoms are
rearranged to form new
substances. Energy
involved in chemical
reactions is either stored
in bonds or released.
Lesson 1: Atoms
• All matter is composed of tiny particles
called atoms.
• The first model of the atoms was a solid
sphere. Today’s model has a central
nucleus with the protons and neutrons
and an electron cloud surrounding it.
Lesson 2: Elements
• An element is a pure substance that cannot
be broken down into any simpler
substances. All atoms of the same element
have the same number of protons.
• An element’s atomic number tells how
many protons its atoms contain and its
average atomic mass tells the average
mass of its atoms.
• Elements are arranged on the periodic
table according to their atomic numbers.
Lesson 3: Compounds
• Atoms of different elements can
combine to form compounds.
• Ionic bonds usually involve the
transfer of electrons from a metal atom
to a nonmetal atom. A chemical bond
that forms when atoms of nonmetals
share electrons is called a covalent
bond.
Lesson 4: Chemical Reactions
and Energy
• In chemical reactions, energy is either
absorbed or released. Energy is
released in exothermic reactions.
Energy is absorbed in endothermic
reactions.
Which term describes a neutral
particle that exists in the nucleus
of an atom?
A. atom
B. electron
C. neutron
D. proton
Who discovered that, in addition
to protons, the nucleus also
contained neutrons?
A. Thomson
B. Rutherford
C. Chadwick
D. Bohr
Which term refers to the
substances produced by
a chemical reaction?
A. reactants
B. products
C. elements
D. atoms
Which of these describes a
chemical bond formed when two
atoms share one or more pairs of
valence electrons?
A. polar molecule
B. covalent bond
C. chemical compound
D. none of these
Which describes an atom with a
positive charge?
A. positive ion
B. negative ion
C. isotope
D. quarks
All of these increase the rate of a
chemical reaction between two
liquids except
A. Increasing the surface area of the
reactants.
B. Increasing the temperature of the liquids.
C. Increasing the concentration of the liquids.
D. Decreasing the temperature of the liquids.
An ionic bond forms between
atoms when
A. An electron in one atoms attracts a
proton in another atom.
B. One atom takes an electron from
another atom.
C. Two atoms share electrons and
protons.
D. A neutron is transferred from one
atom to another.
Which would be best to use in
making protective gear for an
electrician?
A. rubber
B. copper
C. gold
D. aluminum
The atomic number for an
element indicates the number of
A. atoms
B. electron clouds
C. protons
D. neutrons
What elements do
laughing gas and
rubbing alcohol have
in common?
A. Nitrogen and
oxygen
Compound Chemical
formula
Baking
soda
NaHCO3
Table
sugar
C12H22O11
B. Oxygen
Rubbing
alcohol
C3H7OH
C. Carbon
Milk of
Magnesia
Mg(OH)2
Laughing
gas
N 2O
D. Magnesium and
sodium