Download Atoms - Willmar Public Schools

HIGH SCHOOL SCIENCE
Physical
Science 9:
Atomic
Structure
WILLMAR PUBLIC SCHOOL
2013-2014 EDITION
C HAPTER 9
Atomic
Structure
In this chapter you will:
1. Compare and contrast quarks, leptons, and bosons.
2.Describe the standard model. Identify three
subatomic particles and compare their properties.
3.Distinguish the atomic number of an element
from the mass number of an isotope, and use
these numbers to describe the structure of atoms.
4.Describe ancient Greek models of matter.
5. Describe Dalton’s atomic theory and his evidence for the existence of atoms.
6.Explain Thomson’s, Rutherford’s, and Bohr’s
atomic models as well as the electron cloud
model.
7. Distinguish the ground state from excited states
of an atom based on electron configurations.
S ECTION 9.1
Standard Model
O BJECTIVES :
1. Compare and contrast quarks, leptons, and
bosons.
For decades scientists have known that atoms consist of
electrons and other particles called protons and neutrons.
They created the explosion in the picture in their search for
even smaller particles of atoms. Today, scientists think that
electrons truly are fundamental particles that cannot be
broken down into smaller, simpler particles. They are a type
of fundamental particles called leptons. Protons and neutrons,
on the other hand, are no longer thought to be fundamental
particles. Instead, they are now thought to consist of smaller,
simpler particles of matter called quarks. Quarks and leptons
are classified as fermions. A fermion is the true building
blocks of matter. Scientists theorize that fermions are held
together by yet another type of fundamental particles called
bosons.
2. Describe the standard model.
Vocabulary:
fermion
quark
lepton
boson
standard model
2
A quark is a tiny fundamental particle or fermion that make
up protons and neutrons. There are six types of quarks. In
ordinary matter, virtually all quarks are of the types called up
and down quarks. All quarks have mass, and they have an
electric charge of either +2/3 or -1/3. For example, up quarks
have a charge of +2/3, and down quarks have a charge of -1/3.
Quarks also have a different type of charge, called color
charge, although it has nothing to do with the colors that we
see. Quarks are never found alone but instead always occur in
groups of two or three quarks.
of matter. According to the model, all known matter consists
of quarks and leptons that interact by exchanging bosons,
which transmit fundamental forces. The standard model is a
good theory because all of its predictions have been verified
by experimental data. However, the model doesn’t explain
everything, including the force of gravity and why matter has
mass. Scientists continue to search for evidence that will allow
them to explain these aspects of force and matter as well.
A lepton is a weak fundamental particle or fermion. There
are also six types of leptons, including electrons. Leptons
have an electric charge of either -1 or 0. Electrons, for exam
ple, have a charge of -1. Leptons have mass, although the mass
of electrons is extremely small.
There are four known types of bosons, which are forcecarrying particles. Each of these bosons carries a different
fundamental force between interacting particles. In addition,
there is a particle called the "Higgs Boson", which gives
objects the masses they have. Some types of bosons have
mass; others are massless. Bosons have an electric charge of
+1, -1, or 0.
Based on their knowledge of subatomic particles, scientists
have developed a theory called the standard model to
explain all the matter in the universe and how it is held
together. The model includes only the fundamental particles
in the table. No other particles are needed to explain all kinds
3
Section Review:
1. Make a table comparing and contrasting the three types
of fundamental particles. Include an example of each type in your table.
2. In what ways is the standard model incomplete?
4
S ECTION 9.2
Atomic Structure
Atoms are the smallest particles of an element that still have
the element’s properties. Individual atoms are extremely
small. In fact, they are so small that trillions of them would fit
inside the period at the end of this sentence. Although atoms
are very tiny, they consist of even smaller particles. Three
main types of particles that make up all atoms are: protons,
neutrons, and electrons.
Protons, elections, and neutrons are subatomic particles
which can be distinguished by mass, charge, and location in
an atom.
O BJECTIVE :
1. Identify three subatomic particles and
compare their properties.
2. Distinguish the atomic number of an element
from the mass number of an isotope, and use
these numbers to describe the structure of
atoms.
Vocabulary:
atoms
proton
neutron
electron
atomic number
mass number
isotope
Symbol
Relative
Charge
Relative
Mass
Location
Electron
e-
1-
1/1836
Outside the
Nucleus
Proton
p+
1+
Neutron
n
0
1
Nucleus
1
Nucleus
A proton is a positive charged subatomic particle that is
found in the nucleus of an atom. Protons have two up quarks
and one down quark. Each nucleus must contain at least one
particle with a positive charge.
A neutron is a neutral subatomic particle that is found in the
nucleus of an atom. It has a mass almost exactly equal to that
of a proton. Neutrons have one up quark and two down
quarks.
An electron is a negatively subatomic particles that is found
in the space outside the nucleus. It would take 1836 electrons
5
to equal to the mass of a proton. Thus people say it does not
have enough mass to be considered or almost 0. Electrons are
one type of lepton thus have very difference properties than
protons and neutrons.
The mass number of an atom is the sum of the protons and
neutrons in the nucleus on that atom. If you know the atomic
number and the mass number of an atom, you can find the
number of neutrons by subtracting.
The atoms of any given elements always have the same
number of protons. For example, there is one proton in the
nucleus of each and every hydrogen atom. Atoms of different
elements have different number of protons, therefore, the
number of protons determines the element.
Number of Neutrons = Mass number – Atomic number
Atomic number of an element equals the number of
protons in an atom of that element. Atoms of different
elements have different atomic numbers. The number of
protons is the atomic number.
Each positive charge in an atom is balanced by a negative
charge because atoms are neutral. So the atomic number of
an element also equals the number of electrons in an atom.
Electrons have almost no mass. Instead, almost all the mass of
an atom is in its protons and neutrons in the nucleus. The
nucleus is very small, but it is densely packed with matter.
The SI unit for the mass of an atom is the atomic mass unit
(amu). One atomic mass unit equals the mass of a proton,
which is about 1.7 × 10−24 g. Each neutron also has a mass of
1 amu. Therefore, the sum of the protons and neutrons in an
atom is about equal to the atom’s total mass in atomic mass
units.
Every atom of a given element has the same number of
protons and electrons. Some atoms of the same element may
have different numbers of neutrons. For example, some
carbon atoms have seven or eight neutrons instead of the
usual six. Isotopes are atoms of the same element that have
different numbers of neutrons and different mass numbers.
Isotopes of an element have the same atomic number but
different mass numbers because they have different numbers
of neutrons.
Some atoms of hydrogen have no neutrons and a mass
number of one. Some atoms of hydrogen have one neutrons
and a mass number of two. Some atoms of hydrogen have two
neutrons and a mass number of three. When it is important
to distinguish one hydrogen isotope from another, the
isotopes are referenced to as hydrogen-1, hydrogen-2, and
hydrogen-3. The number after the element is the mass
number.
6
Section Review:
1. How are protons, neutrons and electrons distinguished?
2. Which subatomic particles are in the nucleus?
3. Which subatomic particles have charge?
4. Since the up quark has a charge of +2/3 and a down
quark has a charge of -1/3, explain how the proton has a
1+ charge?
5. Since the up quark has a charge of +2/3 and a down
quark has a charge of -1/3, explain how the neutron has a
0 charge?
6. Why do some say that the electron does not have mass?
7. Why do electrons have very different properties than
protons and neutrons?
8. How are atoms of one element different from atoms of
other elements?
9. How do you determine the number of protons?
10.How do you determine the number of electrons?
11.How do you determine the number of neutrons?
12.What is the difference between two isotopes of the same
element?
13.How do you distinguish between different isotopes of the
same element?
7
S ECTION 9.3
Atomic Models
O BJECTIVES :
1. Describe ancient Greek models of matter.
2. Describe Dalton’s atomic theory and his
evidence for the existence of atoms.
3. Explain Thomson’s, Rutherford’s, and Bohr’s
atomic models as well as the electron cloud
model.
4. Distinguish the ground state from excited
states of an atom based on electron
configurations.
Vocabulary:
energy levels
electron cloud
orbital
electron configuration
ground state
excited state
The history of the atom begins around 450 B.C. with a Greek
philosopher named Democritus. Democritus wondered what
would happen if you cut a piece of matter, such as an apple,
into smaller and smaller pieces. He thought that a point
would be reached where matter could not be cut into still
smaller pieces, and believed that all matter consists of
extremely small particles that could not be divided. Thus he
called these "uncuttable" pieces atoms form the Greek word
atomos – which means uncut. This is where the modern term
atom comes from. Additionally, he thought each type of atom
had a specific set of properties. For example, solid atoms
were rough and prickly while liquid atoms were round and
smooth.
Democritus was an important philosopher. However, he was
less influential than the Greek philosopher Aristotle, who
lived about 100 years after Democritus. Aristotle rejected
Democritus’s idea of atoms. In fact, Aristotle thought the idea
of atoms was ridiculous. He did not think there was a limit to
the number of times matter could be divided. He thought that
all substances were built up from only four elements – earth,
air, fire and water. These elements were a combination of
four properties – hot, cold, dry and wet. Unfortunately,
Aristotle’s ideas were accepted for more than 2000 years.
During that time, Democritus’s ideas were more or less
forgotten.
8
all matter is made up of individual particles called atoms,
which cannot be divided. An atom is the smallest unit of an
element. These atoms are represented by tiny, solid sphere
with a different mass.
Dalton proposed the theory with four parts:
1.
All elements are composed of atoms.
2. All atoms of the same element have the same mass, and
atoms of different elements have different masses.
3. Compounds contain atoms of more than one element
• Air was a combination of hot and wet.
• Water was a combination of wet and cold.
• Earth was a combination of cold and dry.
• Fire was a combination of dry and hot.
Until the 1800’s, most people accepted Aristotle’s model, until
Dalton. Dalton, a teacher and amateur scientist, was
interested in the behavior of gases in air. Based on the way
gasses exert pressure, he concluded that gas consist of
individual particles, atoms. He gathered evidence for the
existence of atoms by measuring the masses of elements that
combine when compounds form, and noticed that all no
matter how large or small the sample, the ratio of the masses
of the elements in the compound is always the same. Thus
compounds have a fixed composition. Dalton concluded that
4. In a particular compound, atoms of different elements
always combine in the same way.1. What is the evidence
for atoms?
When some objects are rubbed, they gain the ability to attract
or repel other materials. Based on their behavior, such
materials are said to have either a positive or negative electric
charge. Objects with like charge or the same charge, repel or
push apart. Objects with opposite charge or different charges,
attract or pull together. Some charge can flow from one
location to another. A flow of charges particle is called an
electric current. If the particle has a positive charge, it will be
represented by a +. If the particle has a negative charge, it will
be represented by a -.
J.J. Thomson used a devise below. When it was turned on, a
beam formed following path A. He hypothesized that the
beam was a stream of charge particles.
9
gold foil would be too small to change the path of an alpha
particle.
In one experiment, Thomson tested his hypothesis by placing
charge plates on either side of the beam. The plates caused
the beam to deflect, or bend, from its straight path. He
observed that the beam was repelled by the negatively charged
plate and attracted by the positively charge plate, thus the
beam had negative charge. Thomson’s experiments provided
the first evidence that atoms are made of even smaller
particles.
An atom is neutral, meaning it has neither a negative or
positive charge. In Thomson’s atomic model, the negatively
charges were evenly scattered throughout an atom filled with
a positively charge mass of matter. Think of it like chocolate
chip ice cream, the chocolate chips are the negatively charges
and the vanilla ice cream is the positively charge mass. The
model is called the “plum pudding” model, after a traditional
English dessert.
The screen flashed when an alpha particle struck it. Some of
the location of the flashes on the screen did not support
Rutherford’s hypothesis. Some of the particles behaved as
though they had struck an object and bounced straight back.
The alpha particles whose paths were deflected must have
come close to another charged object. The closer they came to
the other charge object, the greater the deflection was;
however, many alpha particles passed through the gold
without being deflected.
From these results, Rutherford concluded that the positive
charge of an atom is NOT evenly spread throughout the atom,
instead it is in a very small, central area called the nucleus.
The nucleus is a dense, positively charge mass located in the
center of the atom.
Earnest Rutherford discovered that uranium emits fast
moving particles that have a positive charge. He called them
alpha particles. Based on Thomson’s model, Rutherford
hypothesized that the mass and charge at any location in a
10
According the Rutherford’s model, all the of an atom’s
positively charge is in its nucleus.
Niels Bohr agreed with Rutherford’s model of the nucleus
surrounded by a large volume of space. But he added to
Rutherford’s model and focused on the electrons.
Something else was missing, however, as the mass of the
heavier nuclei is greater than could be explained by just
protons alone. The mystery was solved when James
Chadwick discovered the neutron, an almost “twin” of the
proton with roughly the same mass but no charge. The
neutron had to be in the nucleus.
In Bohr’s model, electrons move with constant speed in fixed
orbits around the nucleus. Each electron in an atom has a
specific amount of energy. The possible energies that
electrons in an atom can have are called energy levels. An
electron in an atom can move from one energy level to
another when the atom gains or loses energy.
When atoms absorb energy, three things happen, it increases
its kinetic energy, a phase change, or emits light. The last
option is important in understanding the atoms. The energy
is temporarily absorbed by the atom and then emitted as light.
11
Different energy levels have different maximum number of
electrons.
ENERGY
LEVEL
MAXIMUM
NUMBER OF
ELECTRONS
1
2
2
8
3
18
4
32
Like earlier model’s, Bohr’s model was improved as scientists
made further discoveries. Bohr was incorrect in assuming that
electrons traveled in fixed orbits. Today, scientists know that
electrons move in a less predicted way.
An electron cloud is a visual model of the most likely
locations for electrons in an atom. Scientists use the electron
cloud to describe the possible locations of electron around the
nucleus, and it is a good approximation of how electrons
behave in their orbitals. An orbital is a region of space
around the nucleus where an electron is likely to be found.
An electron configuration is the arrangement of electrons
in the orbitals of an atom. The most stable electron
configuration is the one in which the electrons are in orbitals
with the lower possible energies.
When all the electrons in an atom have the lowest possible
energies, the atom is said to be in its ground state. When an
electron moves to an orbital with a higher energy, it is in an
excited state. An excited state is less stable than the ground
state.
12
Section Review:
Section Review Continued:
1. What did the philosopher Democritus believe about all
matter?
11.How will a positive charge be represented?
2. What did Democritus call the particles?
12.What did Thomson observe about the beam?
3. What were Aristotle’s four elements?
13.In Thomson’s experiment, why was the glowing beam
repelled by a negative charged plate?
4. What were Aristotle’s four properties which the elements
were combinations of?
14.How did Thomson’s experiments change how people
thought about the atom?
5. How combination of properties was water?
15.Describe Thomson’s atomic model.
6. What did Dalton conclude about the evidence he
gathered?
16.What did Rutherford conclude about the structure of an
atom?
7. In the spaces provided, write which part (number) of the
atomic theory supports the given statement.
17.In the Rutherford experiment, why weren’t all the alpha
particles deflected?
_______a)
18.What is Rutherford’s model?
The element helium is composed of atoms.
_______b)
Salt, a compound, contains sodium and
chlorine.
19.What is Bohr’s model of the atom?
20.How does Bohr’s model differ from Rutherford’s model?
_______c)
Hydrogen atoms can combine with oxygen
atoms to form the compound water.
21.What are energy levels?
_______d)
An atom of hydrogen weighs much less
than an atom of silver.
22.How can electrons move from one energy level to
another?
8. How do like electric charged particles behave?
23.How do scientists use the electron cloud model?
9. How do opposite electric charged particles behave?
24.What does it mean to say that an atom is in an excited
state?
10.What is electric current?
13