Chapter 12Chapter 12

Introduction to AtomsIntroduction to Atoms

Development of the Atomic Theory

Development of the Atomic Theory

Atom - the smallest particle into which an element can be divided and still be the same substance It still retains its characteristic properties

Atoms make up elements Elements combine to form compounds

Because all matter is made up of elements or compounds, atoms are considered the building blocks of matter.

Atom - the smallest particle into which an element can be divided and still be the same substance It still retains its characteristic properties

Atoms make up elements Elements combine to form compounds

Because all matter is made up of elements or compounds, atoms are considered the building blocks of matter.

Democritus Proposes the Atom

Democritus Proposes the Atom

In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further. This was against popular belief held by

Aristotle He called this particle an atomos (Greek

for atom), which means indivisible He believed these particles were

constantly moving and they form different materials by joining together

In 440 B.C. Democritus believed that there was a point where a particle could not be divided any further. This was against popular belief held by

Aristotle He called this particle an atomos (Greek

for atom), which means indivisible He believed these particles were

constantly moving and they form different materials by joining together

Dalton Creates an Atomic Theory Based on

Experiments

Dalton Creates an Atomic Theory Based on

Experiments John Dalton, a British chemist,

wanted to know why elements combine in specific proportions to form compounds i.e. water (H2O) always has two hydrogen

and one oxygen atoms bound togetherHe performed experiments on

compounds and concluded that they combined in specific proportions because of atoms

John Dalton, a British chemist, wanted to know why elements combine in specific proportions to form compounds i.e. water (H2O) always has two hydrogen

and one oxygen atoms bound togetherHe performed experiments on

compounds and concluded that they combined in specific proportions because of atoms

After many experiments and observations Dalton published his atomic theory in 1803.

He stated the following: 1) All substances are made of atoms.

ATOMS Are small particles that cannot be created, divided, or destroyed

2) Atoms of the same element are exactly alike, and atoms of different elements are different

3) Atoms join with other atoms to make new substances

This was the first step in our understanding of the atom and our current atomic theory

After many experiments and observations Dalton published his atomic theory in 1803.

He stated the following: 1) All substances are made of atoms.

ATOMS Are small particles that cannot be created, divided, or destroyed

2) Atoms of the same element are exactly alike, and atoms of different elements are different

3) Atoms join with other atoms to make new substances

This was the first step in our understanding of the atom and our current atomic theory

Thomson Finds Electrons in the Atom

Thomson Finds Electrons in the Atom

In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles. This was contrary to Daltons atomic theory

Thomson experimented with a cathod ray

He noticed that the ray was affected (bent) by a positive charge

He concluded that the ray contained negatively charged particles

In 1897, a British scientist named J. J. Thomson discovered that atoms are made of small particles. This was contrary to Daltons atomic theory

Thomson experimented with a cathod ray

He noticed that the ray was affected (bent) by a positive charge

He concluded that the ray contained negatively charged particles

These negatively charged particles are now called electrons

Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atom

He came up with the plum pudding modelModel - is a representation of an object or

systemThomson’s model illustrated positively

charged material with negatively charged particles located throughout.

These negatively charged particles are now called electrons

Thomson revised Dalton's atomic theory to include the presence of electrons even though he did not know how they were arranged in the atom

He came up with the plum pudding modelModel - is a representation of an object or

systemThomson’s model illustrated positively

charged material with negatively charged particles located throughout.

Rutherford Opens an Atomic “Shooting Gallery”

Rutherford Opens an Atomic “Shooting Gallery”

In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theory

He aimed positively charged particles, larger than protons, at gold foil

Most particles passed straight through But to Rutherford’s amazement;

Some were deflected at various angles And a few were bounced straight back

It became obvious to Rutherford that Thomson’s model was wrong

In 1909, a former student of Thomson’s named Ernest Rutherford tested Thomson’s theory

He aimed positively charged particles, larger than protons, at gold foil

Most particles passed straight through But to Rutherford’s amazement;

Some were deflected at various angles And a few were bounced straight back

It became obvious to Rutherford that Thomson’s model was wrong

Rutherford Presents a New Atomic Model

Rutherford Presents a New Atomic Model

In 1911 Rutherford revised the atomic theory

He concluded that atoms are mostly empty space with lightweight negative electrons moving around

And in the center of the atom is a tiny, extremely dense, positively charged region - Nucleus

He calculated that the diameter of the nucleus was 100,000 times smaller than

the atom itself

In 1911 Rutherford revised the atomic theory

He concluded that atoms are mostly empty space with lightweight negative electrons moving around

And in the center of the atom is a tiny, extremely dense, positively charged region - Nucleus

He calculated that the diameter of the nucleus was 100,000 times smaller than

the atom itself

Bohr States That Electrons Can Jump Between LevelsBohr States That Electrons Can Jump Between Levels

In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite paths These paths are located at levels at certain

distances from the nucleus He also believe that electrons could jump

from one level to the next Bohr’s model was valuable in predicting

some atomic behavior, but was too simple to explain all atomic behavior

In 1913 a Danish scientist who worked with Rutherford, Niels Bohr, suggested that electrons travel around the nucleus in definite paths These paths are located at levels at certain

distances from the nucleus He also believe that electrons could jump

from one level to the next Bohr’s model was valuable in predicting

some atomic behavior, but was too simple to explain all atomic behavior

The Modern Theory: Electron Clouds Surround

the Nucleus

The Modern Theory: Electron Clouds Surround

the Nucleus Many scientists have contributed

to our current understanding of the atom

This led to the following change to the atomic theory; Electrons do not travel in definite

paths around the nucleusThe exact paths cannot be predicted

The regions around the nucleus where electrons are likely to be found are called - Electron Clouds

Many scientists have contributed to our current understanding of the atom

This led to the following change to the atomic theory; Electrons do not travel in definite

paths around the nucleusThe exact paths cannot be predicted

The regions around the nucleus where electrons are likely to be found are called - Electron Clouds

www.csmate.colostate.edu/#A1D07www.csmate.colostate.edu/#A1D07

Chapter 12 Quiz 1Chapter 12 Quiz 1 1) __________is the smallest particle into

which an element can be divided and still be that element

2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing

3) __________ are the negatively charged particles found in all atoms

4) ___________ discovered the nucleus 5) ___________ are regions where electrons

are likely to be found Bonus) __________ are atoms that have the

same number of protons, but different numbers of neutrons

1) __________is the smallest particle into which an element can be divided and still be that element

2) __________ is a unifying explanation for a broad range of hypothesis and observations that have been supported by testing

3) __________ are the negatively charged particles found in all atoms

4) ___________ discovered the nucleus 5) ___________ are regions where electrons

are likely to be found Bonus) __________ are atoms that have the

same number of protons, but different numbers of neutrons

Section 2: The AtomSection 2: The Atom

How Small Is an Atom?How Small Is an Atom? An average sized atom (i.e. aluminum)

has a diameter of 0.00000003 cm That’s three hundred millionths of a

centimeter It would take a stack of 50,000 aluminum

atoms to equal the thickness of a sheet of aluminum foil

There are 2 x 1022 atoms in one penny! 20,000,000,000,000,000,000,000 atoms That’s twenty thousand billion billion atoms That is 4,000,000,000,000 times more atoms

than people on Earth!

An average sized atom (i.e. aluminum) has a diameter of 0.00000003 cm That’s three hundred millionths of a

centimeter It would take a stack of 50,000 aluminum

atoms to equal the thickness of a sheet of aluminum foil

There are 2 x 1022 atoms in one penny! 20,000,000,000,000,000,000,000 atoms That’s twenty thousand billion billion atoms That is 4,000,000,000,000 times more atoms

than people on Earth!

What’s Inside an Atom?What’s Inside an Atom?

As tiny as an atom is, it consists of even smaller particlesProtons, Neutrons, and Electrons

Protons and Neutrons make up the nucleus, which is at the center of the atom

As tiny as an atom is, it consists of even smaller particlesProtons, Neutrons, and Electrons

Protons and Neutrons make up the nucleus, which is at the center of the atom

Protons - the positively charged particles of the nucleusAll protons are identicalEach has an approximate mass of 1.7 x

10-24 grams or 1 amu Neutrons - particles of the nucleus

that have no chargeNeutrons have a slightly greater mass

than protons The nucleus of the atom is small, but

very dense

Protons - the positively charged particles of the nucleusAll protons are identicalEach has an approximate mass of 1.7 x

10-24 grams or 1 amu Neutrons - particles of the nucleus

that have no chargeNeutrons have a slightly greater mass

than protons The nucleus of the atom is small, but

very dense

Outside the NucleusOutside the Nucleus

Electrons - negatively charged particles in atomsElectrons are found moving around

the nucleus within electron cloudsElectrons are very small in mass

compared to protons and neutrons (0.000546 amu)

It takes more than 1,800 electrons to equal the mass of one proton

Electrons - negatively charged particles in atomsElectrons are found moving around

the nucleus within electron cloudsElectrons are very small in mass

compared to protons and neutrons (0.000546 amu)

It takes more than 1,800 electrons to equal the mass of one proton

The charges of protons and electrons are opposite, but equal in strength

Therefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutral i.e. 10 protons - 10 electrons = 0

If the number of protons and electrons differ, the atom becomes a charged particle - Ion

Positively charged atoms are CationsNegatively charged atoms are Anions

The charges of protons and electrons are opposite, but equal in strength

Therefore if there are equal amounts of protons and electrons, the overall charge of the atom is zero - neutral i.e. 10 protons - 10 electrons = 0

If the number of protons and electrons differ, the atom becomes a charged particle - Ion

Positively charged atoms are CationsNegatively charged atoms are Anions

How Do Atoms of Different Elements Differ?

How Do Atoms of Different Elements Differ?

There are 117 different elements, each made of different atoms

The number of Protons determines the elementThis is called the Atomic number

Each element is composed of atoms with the same atomic number

There are 117 different elements, each made of different atoms

The number of Protons determines the elementThis is called the Atomic number

Each element is composed of atoms with the same atomic number

Are All Atoms of an Element the Same?Are All Atoms of an Element the Same?

Isotopes - are atoms that have the same number of protons, but have different number of neutronsAtoms that are isotopes of each other

are the same element because they have the same number of protons

Some isotopes have unstable nuclei and become radioactive

Isotopes - are atoms that have the same number of protons, but have different number of neutronsAtoms that are isotopes of each other

are the same element because they have the same number of protons

Some isotopes have unstable nuclei and become radioactive

How Can You Tell One Isotope from Another?How Can You Tell One Isotope from Another?

You can identify one isotope from another by its mass number Mass number - is the sum of the protons and

neutrons in an atom Electrons are not included in this calculation because

their mass too small to affect it

To identify a specific isotope write the name of the element followed by a hyphen and the mass number i.e. carbon - 12 (C-12) has contains 6 protons

and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons

You can identify one isotope from another by its mass number Mass number - is the sum of the protons and

neutrons in an atom Electrons are not included in this calculation because

their mass too small to affect it

To identify a specific isotope write the name of the element followed by a hyphen and the mass number i.e. carbon - 12 (C-12) has contains 6 protons

and 6 neutron, while carbon - 13 has 6 protons and 7 neutrons

How Do You Calculate the Mass of an Element

How Do You Calculate the Mass of an Element

Most elements found in nature contain a mixture of different isotopes i.e. All copper is composed of copper-63 and

copper-65 atoms Atomic Mass Unit (amu) - is the

weighted average of the masses of of all naturally occurring isotopes

To calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form)

Most elements found in nature contain a mixture of different isotopes i.e. All copper is composed of copper-63 and

copper-65 atoms Atomic Mass Unit (amu) - is the

weighted average of the masses of of all naturally occurring isotopes

To calculate the amu of an element, multiple the mass of each element by its percentage of abundance (in decimal form)

Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper?

(63 x .69) = 43.47 (65 x .31) = 20.17

63.62 amu

* So the amu of copper is 63.62 amu

Copper consists of copper-63 at 69% abundance, and copper-65 at 31% abundance. What is the amu of copper?

(63 x .69) = 43.47 (65 x .31) = 20.17

63.62 amu

* So the amu of copper is 63.62 amu

Now You Try OneNow You Try One

Chlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine

(35 x .76)= 26.6 (37 x .24)= 8.9

35.5 amu

Chlorine consists of chlorine-35 at 76%, and chlorine-37 at 24%. What is the amu of chlorine

(35 x .76)= 26.6 (37 x .24)= 8.9

35.5 amu

What Forces Are at Work in Atoms?

What Forces Are at Work in Atoms?

There are four basic forces at work everywhere, including in atoms.

They include: 1) Gravity - the attraction between two

objects based on their masses Because the masses of the particles in atoms is so

small, the gravity between them is small 2) Electromagnetic Force - particles of the

same charge repel, while opposite charges attract.

The electromagnetic force holds electrons around the nucleus

There are four basic forces at work everywhere, including in atoms.

They include: 1) Gravity - the attraction between two

objects based on their masses Because the masses of the particles in atoms is so

small, the gravity between them is small 2) Electromagnetic Force - particles of the

same charge repel, while opposite charges attract.

The electromagnetic force holds electrons around the nucleus

3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protons

This keeps the nucleus together4) Weak Force - In certain unstable, a

neutron can change into a proton and a electron.

It is an important force in radioactive atoms

3) Strong Force - At the close distances between protons in the nucleus, the strong force is greater than the electromagnetic force between the protons

This keeps the nucleus together4) Weak Force - In certain unstable, a

neutron can change into a proton and a electron.

It is an important force in radioactive atoms