Download 4.1 Studying the structure of ______ is a little like studying wind

4.1
Studying the structure of __________ is a little like studying wind. Because you cannot see air, you must
use indirect evidence to tell the direction of the wind. Atoms pose a similar problem because they are
extremely __________. Even with a microscope, scientists cannot see the structure of an atom.
Ancient Greek Models of Atoms
If you cut a piece of aluminum foil in half, you have two smaller pieces of the same shiny, flexible
substance. You could cut the pieces again and again. Can you keep dividing the aluminum into smaller
pieces? Greek philosophers debated a similar question about 2500 years ago.
The philosopher Democritus believed that all matter consisted of _____________________________
that could not be divided. He called these particles ___________ from the Greek word atomos, which
means “uncut” or “indivisible.”
Aristotle thought that all substances were made of only four elements—earth, air, fire, and water. He
did not think there was a limit to the division of matter.
For many centuries, most people accepted Aristotle’s views on the structure of matter. By the 1800s,
scientists had enough experimental data to support an atomic model.
Dalton’s Atomic Theory
Dalton proposed the theory that all matter is made up of individual particles called atoms, which
______________be divided.
Evidence for Atoms
John Dalton studied the behavior of gases in air. Based on the way gases exert _______________,
Dalton correctly concluded that a gas consists of individual _________________.
Dalton measured masses of elements that combine when compounds form. The ratio of the masses of
the elements in each compound was always the same. In other words, compounds have a
______________________________.
When magnesium burns, it combines with oxygen. In magnesium oxide, the ratio of the mass of
magnesium to the mass of oxygen is always about 3 : 2. Magnesium dioxide has a fixed composition.
Dalton’s Theory

All elements are composed of _______________.

All atoms of the same element have the same ___________, and atoms of different elements
have different _________.

_____________________ contain atoms of more than one element.

In a particular compound, atoms of different elements always combine
________________________________________________.
Dalton made these wooden spheres as a model to represent the atoms of different elements. A tiny,
solid sphere with a different mass represents each type of atom.
A theory must explain the data from many experiments. Because Dalton’s atomic theory met that goal,
the theory became widely accepted.
Over time, scientists found that not all of Dalton’s ideas about atoms were completely correct. They
revised the theory to take into account new discoveries.
Thomson’s Model of the Atom
Thomson’s experiments provided the first evidence that atoms are made of even smaller
particles.
When some materials are rubbed, they gain the ability to attract or repel other materials. Such materials
are said to have either a positive or a negative electric charge.
•
Objects with ____________ charges repel, or push apart.
•
Objects with ______________________ charges attract, or pull together.
Amber is the hardened form of a sticky, viscous liquid that protects trees from insects and disease. If
amber is rubbed with wool, it becomes charged and can attract a feather.
Thomson’s Experiments
In his experiments, Joseph John Thomson used a sealed tube containing a very small amount of
_______________________.
When the current was turned on, the disks became charged, and a glowing beam appeared in the tube.
•
Thomson hypothesized that the beam was a stream of ________________ that
interacted with the air in the tube and caused the air to glow.
•
Thomson observed that the beam was _____________ by the negatively charged plate
and attracted by the _______________ charged plate.
Evidence for Subatomic Particles
Thomson concluded that the particles in the beam had a negative charge because they were attracted
to the positive plate. He hypothesized that the particles came from inside atoms because
•
no matter what ___________ Thomson used for the disk, the particles produced were
identical.
•
the particles had about 1/2000 the ___________ of a hydrogen atom, the lightest atom.
Thomson’s Model
Thomson revised Dalton’s model to account for these subatomic particles.
•
The atom has neither a positive nor a negative charge, but there must always be some
_____________ charge in the atom.
•
The atom is filled with a positively charged mass of matter that has negative charges
evenly ____________________ throughout it.
Rutherford’s Atomic Theory
Rutherford’s Hypothesis
Ernest Rutherford designed an experiment to find out what happens to alpha particles when they pass
through a thin sheet of gold. Alpha particles are fast-moving, _____________ charged particles.
•
Based on Thomson’s model, Rutherford hypothesized that the mass and charge at any
location in the gold would be too small to change the path of an alpha particle.
•
He predicted that most particles would travel in a _____________________ from their
source to a screen that lit up when struck.
The alpha particles whose paths were deflected must have come close to another charged object. The
closer they came, the greater the _________________.
However, many _________________ passed through the gold without being deflected. These particles
did not pass close to a charged object.
Thomson’s model did not explain all of the evidence from Rutherford's experiment. Rutherford
proposed a new model.
•
The positive charge of an atom is not evenly spread throughout the atom.
•
_________________ charge is concentrated in a very small, central area.
•
The __________________ of the atom is a dense, positively charged mass located in the
center of the atom.
Assessment Questions
1. Dalton’s theory did not include which of the following points?
a. All elements are composed of atoms.
b. Most of an atom’s mass is in its nucleus.
c. Compounds contain atoms of more than one element.
d. In a specific compound, atoms of different elements always combine in the same way.
2. J. J. Thomson’s experiments provided the first evidence of
a. atoms.
b. a nucleus.
c. subatomic particles.
D. elements.
1. The concept of an atom as a small particle of matter that cannot be divided was proposed by the
ancient Greek philosopher, Democritus.
True
False
4.2 Subatomic Particles
Protons, electrons, and neutrons are subatomic particles.
Protons
Based on experiments with elements other than gold, Rutherford concluded that the amount of positive
charge varies among ________________.
A ___________ is a positively charged subatomic particle that is found in the nucleus of an atom. Each
proton is assigned a charge of 1+. Each nucleus must contain at least ___________ proton.
Electrons
The particles that Thomson detected were later named ________________.
An electron is a negatively charged subatomic particle that is found in the space outside the nucleus.
Each electron has a charge of 1.
Neutrons
In 1932, the English physicist James Chadwick carried out an experiment to show that neutrons exist.
Chadwick concluded that the particles he produced were ______________ because a charged object did
not deflect their paths.
A ___________________ is a neutral subatomic particle that is found in the nucleus of an atom. It has a
mass almost exactly _______________ to that of a proton.
Comparing Subatomic Particles
Protons, electrons, and neutrons can be distinguished by mass, charge, and
__________________ in an atom.
Atomic Number

The ____________________ of an element is the number of protons in an atom of that
element.

All atoms of any given _____________ have the same atomic number. Each hydrogen atom has
one proton in its nucleus. Hydrogen is assigned the atomic number 1.
Each element has a unique atomic number
Each element has a different atomic number. A The atomic number of sulfur (S) is 16.
B The atomic number of iron (Fe) is 26.
C The atomic number of silver (Ag) is 47.
Atoms are _________________, so each positive charge in an atom is balanced by _________________.
That means the atomic number of an element also equals the number of electrons in an atom of that
element.
Hydrogen has an atomic number of 1, so a hydrogen atom has 1 electron.
•
Sulfur has an atomic number of 16, so a sulfur atom has 16 electrons.
Mass Number
The mass number of an atom is the _________________________ in the nucleus of that atom. To find
the number of neutrons in an atom, you need the mass number of the atom and its atomic number.
The atomic number of aluminum is 13. An atom of aluminum that has a mass number of 27 has 13
protons and 14 neutrons.
Isotopes
______________ of an element have the same atomic number but different mass numbers
because they have different numbers of _______________.
Isotopes are atoms of the same element that have different numbers of neutrons and different mass
numbers.
To distinguish one isotope from another, the isotopes are referred by their _____________ numbers.
For example, oxygen has 3 isotopes: oxygen-16, oxygen-17, and oxygen-18.
All three oxygen isotopes can react with hydrogen to form water or combine with iron to form rust.
With most ____________, it is hard to notice any differences in the physical or chemical properties of
their isotopes. Hydrogen is an exception.
Hydrogen-1 has no neutrons. (Almost all hydrogen is hydrogen-1.) Hydrogen-2 has __________ neutron,
and hydrogen-3 has _______________ neutrons. Because a hydrogen-1 atom has only one proton,
adding a neutron doubles its mass.
Water that contains hydrogen-2 atoms in place of hydrogen-1 atoms is called _________________.
Hydrogen-2 atoms have twice the mass of hydrogen-1 atoms, so the properties of heavy water are
different from the properties of ordinary water.
Assessment Questions
1. In which way do isotopes of an element differ?
a. number of electrons in the atom
b. number of protons in the atom
c. number of neutrons in the atom
d. net charge of the atom
2. Of the three subatomic particles that form the atom, the one with the smallest mass is the
neutron.
True
False
4.3 Compounds
Have you ever wondered what produces the different colors in a fireworks display?
Certain ____________________ will produce certain colors of light when they are heated. Compounds
containing the element strontium produce red light. Compounds containing barium produce green light.
What can happen to electrons when atoms gain or lose energy?
Bohr’s Model of the Atom
As did Rutherford's atomic model, Bohr’s atomic model had a _____________ surrounded by a large
volume of space. But Bohr’s model focused on the __________________________________________.
In Bohr’s model, electrons move with constant speed in fixed orbits around the nucleus, like planets
around a sun. Each electron in an atom has a specific amount of ___________________.
Energy Levels
When an atom gains or loses energy, the energy of an electron can change.
•
The possible energies that electrons in an atom can have are called
________________________________. An electron cannot exist between energy levels.
An electron in an atom can move from one energy level to another when the atom gains or losses
energy.
An analogy for energy levels of electrons is a staircase.
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The landing at the bottom of the staircase is the lowest level.
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Each step up represents a higher energy level.
•
The step
________________________________________________________________________
_______.
•
You can only move in whole numbers of stairs.
An electron may move up or down two or more energy levels if it gains or loses the right amount of
energy.
The size of the ________________ between energy levels determines the amount of energy gained or
lost.
_____________________________________________________________________________________
__________________________________________________________________________________.
Evidence for Energy Levels
Scientists can measure the _______________________ when electrons absorb energy and move to a
higher energy level or measure the _________________________ when the electron moves to a lower
energy level.
_____________________________________________________________________________________
____________________________________________________________________________________.
The movement of electrons between energy levels explains the light you see when fireworks explode.
•
______________causes some electrons to move to higher energy levels.
•
When those electrons move back to lower energy levels, they release energy. Some of
that energy is __________________________________________________________.
•
Different elements emit different colors of light because no two elements have the
same set of energy levels.
What model do scientists use to describe how electrons behave in atoms?
An _______________________________________ is a visual model of the most likely locations for
electrons in an atom.
Scientists use the electron cloud model to describe the possible locations of electrons around the
nucleus.
Electron Cloud Model
Bohr’s model was improved as scientists made further discoveries. Bohr correctly assigned
_________________________ to electrons, but electrons do not move like planets in a solar system.
Today, scientists use probability when trying to predict the locations and motions of electrons in atoms.
An electron cloud is
_____________________________________________________________________________________
___________________________________________________________________________________.
What model do scientists use to describe how electrons behave in atoms?
An ___________________ is a region of space around the nucleus where an electron is likely to be
found. The electron cloud represents all the orbitals in an _________________.
An electron cloud is a good approximation of how electrons behave in their orbitals.
For an analogy to the concept of an orbital, imagine a map of your school. Mark your exact location with
a dot once every 10 minutes over a period of one week. The dots on your map are a model of your
“orbital.” They describe your most likely locations.
•
The places you visit the most would have the highest concentration of dots.
•
The places you visit the least would have the lowest concentration of dots.
The level in which an electron has the least energy—the lowest energy level—has only one
orbital. Higher energy levels have _____________________________________________.
Electron Configurations
What is the most stable configuration of electrons in an atom?
An __________________________________ is the arrangement of electrons in the orbitals of an atom.
When all the electrons in an atom have the ______________________________________, the atom is
said to be in its ground state.
The ground state of a person is on the floor. A gymnast on a balance beam is like an atom in an excited
state—not very stable.
When she dismounts, the gymnast will return to a lower, more ______________________________.
Assessment Questions
1. According to Bohr’s model of the atom, which of the following can happen when an atom gains
energy?
a. An atom returns to its ground state.
b. A neutron can be changed into a proton.
c. A proton can move to a higher energy level.
d. An electron can move to a higher energy level.
2. How does the modern atomic theory describe the location of electrons in an atom?
a. Electrons move randomly in space around the nucleus.
b. Electrons can be described as a cloud based on probable locations.
c. Electrons orbit the nucleus in the same way that planets orbit the sun.
d. Electrons move in a spiral pattern if increasing distance from the nucleus.
3.
What is meant when an atom is said to be in its ground state?
a. There is no net charge on the atom.
b. The number of protons equals the number of neutrons.
c. The atom’s electrons all have the lowest possible energies.
d. It is the isotope with the least number of neutrons.