Atoms and Their Atoms and Their StructureStructure

History of the AtomHistory of the Atom

Original idea (400 B.C.) came from Democritus, a Greek philosopher

Democritus expressed the belief that all matter is composed of very small, indivisible particles, which he named atomos.

Who’s Next?Who’s Next?

John Dalton (1766-1844), an English school teacher and chemist, studied the results of experiments by other scientists.

Dalton’s Atomic TheoryDalton’s Atomic Theory

Dalton proposed his atomic theory of matter in 1803.

Although his theory has been modified slightly to accommodate new discoveries, Dalton’s theory was so insightful that it has remained essentially intact up to the present time.

Dalton’s Atomic TheoryDalton’s Atomic Theory

1.1. All All mattermatter is made of tiny is made of tiny indivisibleindivisible particles called atoms.particles called atoms.

2.2. Atoms of the same element are Atoms of the same element are identical; those of different atoms are identical; those of different atoms are different.different.

Dalton’s Atomic Theory, cont.Dalton’s Atomic Theory, cont.

3.3. Atoms of different elements combine in Atoms of different elements combine in whole number ratios to form whole number ratios to form compoundscompounds

4.4. Chemical reactions involve the Chemical reactions involve the rearrangement of atoms. No new atoms rearrangement of atoms. No new atoms are created nor destroyed.are created nor destroyed.

Parts of the AtomParts of the Atom Because of Dalton’s atomic theory, most

scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts.

In 1897, a British physicist, J.J. Thomson, made discoveries that required Dalton’s 1st postulate to be modified

He discovered that atoms are made of smaller (subatomic) particles.

Parts of the AtomParts of the Atom

Thomson’s experiments used a cathode ray tube.

It is a vacuum tube - all the air has been pumped out.

Thomson’s ExperimentThomson’s Experiment

Voltage source

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Vacuum tube

Metal Disks

Thomson’s ExperimentThomson’s Experiment

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At each end of the tube is a metal piece called an electrode, which is connected through the glass to a metal terminal outside the tube.

Thomson’s ExperimentThomson’s Experiment

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When the electrodes are charged, rays travel in the tube from the negative electrode, which is the cathode, to the positive electrode, the anode.

Thomson’s ExperimentThomson’s Experiment

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Because these rays originate at the cathode, they are called cathode rays.

Thomson’s ExperimentThomson’s Experiment

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Thomson’s ExperimentThomson’s Experiment

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Thomson’s ExperimentThomson’s Experiment

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Thomson’s ExperimentThomson’s Experiment

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Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field,,

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Voltage source

Thomson’s ExperimentThomson’s Experiment

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Voltage source

Thomson’s ExperimentThomson’s Experiment

Thomson found that the rays bent toward a positively charged plate and away from a negatively charged plate.

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Thomson’s ExperimentThomson’s Experiment

He knew that objects with like charges repel each other, and objects with unlike charges attract each other.

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Thomson’s ExperimentThomson’s Experiment

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By adding an electric field he found that By adding an electric field he found that the moving rays were negative.the moving rays were negative.

Voltage source

Thomson’s ExperimentThomson’s Experiment

Thomson concluded that cathode rays are made up of invisible, negatively charged particles.

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Cathode Ray TubeCathode Ray Tube

Thomson’s ModelThomson’s Model

From Thomson’s experiments, scientists had to conclude that although atoms are neutral, some of the subatomic particles have a charge (positive or negative).

Thomson’s ModelThomson’s Model

Matter is not negatively charged, so atoms can’t be negatively charged either.

If atoms contained extremely light, negatively charged particles, then they must also contain positively charged particles — probably with a much greater mass than electrons.

Thomson’s ModelThomson’s Model

J.J. Thomson said J.J. Thomson said the atom was like the atom was like plum pudding, a plum pudding, a popular English popular English dessert.dessert.

Thomspon’s ModelThomspon’s Model

In 1886, scientists discovered that a cathode-ray tube emitted rays not only from the cathode but also from the positively charged anode.

Years later, scientists determined that the rays were composed of positively charged subatomic particles.

IsotopesIsotopes In 1910, J.J. Thomson discovered

evidence that atoms of the same type (elements) can have different masses.

Neon consisted of atoms of two different masses.

IsotopesIsotopes

Atoms of an element that are chemically alike but differ in mass are called isotopes of the element.

Ernest RutherfordErnest Rutherford

In 1909, a team of scientists led by Ernest Rutherford in England carried out the first of several important experiments that revealed an arrangement far different from the plum pudding model of the atom.

Rutherford’s ExperimentRutherford’s Experiment

The experimenters set up a lead-shielded box containing radioactive polonium, which emitted a beam of positively charged subatomic particles through a small hole.

Rutherford’s ExperimentRutherford’s Experiment

The sheet of gold foil was surrounded by a screen coated with zinc sulfide, which glows when struck by the positively charged particles of the beam.

Lead block

Polonium

Gold Foil

Florescent Screen

What Rutherford ExpectedWhat Rutherford Expected

The alpha particles to pass through The alpha particles to pass through without changing direction very much.without changing direction very much.

Because he thought the mass was evenly distributed in the atom.

What Rutherford ObservedWhat Rutherford Observed

How Rutherford Explained ItHow Rutherford Explained It

To explain the results of the experiment, Rutherford’s team proposed a new model of the atom:

Because most of the particles passed through the foil, they concluded that the atom is nearly all empty space.

How Rutherford Explained ItHow Rutherford Explained It

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Because so few particles were deflected, they proposed that the atom has a small, dense, positively charged central core, called a “nucleus.”

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Alpha particles are deflected by it if they Alpha particles are deflected by it if they get close enough to the nucleus.get close enough to the nucleus.

The Nuclear Model of the The Nuclear Model of the AtomAtom

The new model of the atom as pictured by Rutherford’s group in 1911 is shown below.

Explanation of 4 ObservationsExplanation of 4 Observations

Three subatomic particles were proposed to explain the four observations made by Thomson and Rutherford: protons, electrons, and neutrons.

The ElectronThe Electron

The first to be discovered, electrons have a negative charge and almost no mass (compared to protons and neutrons).

Electrons account for the volume of an atom.

The ProtonThe Proton

Protons are positively charged, neutralizing the charge of electrons.

They have mass, and are located in the nucleus.

The NeutronThe Neutron Because of the discovery of isotopes,

scientists predicted that a third particle would be discovered that exists in the nucleus, has a mass equal to that of a proton, but has no charge.

The existence of this neutral particle, called a neutron, was confirmed in the early 1930s.

Subatomic ParticlesSubatomic Particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

e-

p+

n0

-1

+1

0

1/2000

1

1

Symbol ChargeRelative mass

Modern View of the AtomModern View of the Atom

The atom has two The atom has two regions and is 3- regions and is 3- dimensional.dimensional.

The The nucleusnucleus is at is at the center and the center and contains the contains the protons and protons and neutrons.neutrons.

Modern View of the AtomModern View of the Atom

The The electron electron cloudcloud is the is the region where you region where you might find an might find an electron and electron and most of the most of the volume of an volume of an atom.atom.

Atomic NumberAtomic Number

The atomic number of an element is the number of protons in the nucleus of an atom of that element.

The number of protons determines identity of an element, as well as many of its chemical and physical properties.

Atomic NumberAtomic Number

Because neutral atoms have no overall electrical charge, a neutral atom must have as many electrons as there are protons in its nucleus.

Therefore, the atomic number of an element also tells the number of electrons in a neutral atom of that element.

MassesMasses

The mass of a neutron is almost the same as the mass of a proton.

The sum of the protons and neutrons in the nucleus is the mass number of that particular atom.

Isotopes of an element have different mass numbers because they have different numbers of neutrons, but they all have the same atomic number.

MassesMasses Isotopes of an element have different mass

numbers because they have different numbers of neutrons, but they all have the same atomic number.

Isotopes are always identified by their mass number, and can be represented in two ways:

Two equivalent representations of the carbon – 12 isotope:

C-12 12C

SymbolsSymbols

Elements can be represented by using Elements can be represented by using the symbol of the element, the mass the symbol of the element, the mass number and the atomic number.number and the atomic number.

X Massnumber

Atomicnumber

The mass number is the number of The mass number is the number of protons + the number of neutrons.protons + the number of neutrons.

SymbolsSymbols

Mass number is represented by the Mass number is represented by the letter A.letter A.

X Massnumber

Atomicnumber

Atomic number is represented by the Atomic number is represented by the letter Z.letter Z.

A

Z

Symbols ExampleSymbols Example

Determine the following for the Determine the following for the fluorine atom depicted belowfluorine atom depicted below..

F19 9

e) mass numbere) mass number

d)d) atomic numberatomic number

c)c) number of electronsnumber of electrons

b)b) number of neutronsnumber of neutrons

(9)

(10)

(9)

(9)

(19)

a)a) number of protonsnumber of protons

Symbols ProblemSymbols Problem

Determine the following for the Determine the following for the bromine atom depicted belowbromine atom depicted below..

Br8035

e) mass numbere) mass number

d)d) atomic numberatomic number

c)c) number of electronsnumber of electrons

b)b) number of neutronsnumber of neutrons

(35)

(45)

(35)

(35)

(80)

a)a) number of protonsnumber of protons

Symbols ProblemSymbols Problem If a neutral element has an atomic If a neutral element has an atomic

number of 34 and a mass number of number of 34 and a mass number of 78 what is the78 what is the

Se or Se-7878

34

d)d) complete symbol complete symbol (2 ways)(2 ways)

c)c) number of electronsnumber of electrons

b)b) number of neutronsnumber of neutrons

(34)

(44)

(34)

a)a) number of protonsnumber of protons

Symbols ProblemSymbols Problem If a neutral element has 91 protons If a neutral element has 91 protons

and 140 neutrons what is the and 140 neutrons what is the

Pa or Pa-231231

91

d)d) complete symbol complete symbol (2 ways)(2 ways)

c)c) number of electronsnumber of electrons

b)b) mass numbermass number

(91)

(231)

(91)

a)a) atomic numberatomic number

Symbols ProblemSymbols Problem If a neutral element has 78 electrons If a neutral element has 78 electrons

and 117 neutrons what is theand 117 neutrons what is the

Pt or Pt-195195

78

d)d) complete symbol complete symbol (2 ways)(2 ways)

c)c) number of protonsnumber of protons

b)b) mass numbermass number

(78)

(195)

(78)

a)a) atomic numberatomic number

Information in the Periodic TableInformation in the Periodic Table

The average atomic mass is the weighted average mass of all the naturally occurring isotopes of that element.

The unit is the Atomic Mass Unit (amu).

Calculating Atomic MassCalculating Atomic Mass

Calculating Atomic MassCalculating Atomic Mass

Copper exists as a mixture of two isotopes.

The lighter isotope (Cu-63), with 29 protons and 34 neutrons, makes up 69.17% of copper atoms.

The heavier isotope (Cu-65), with 29 protons and 36 neutrons, constitutes the remaining 30.83% of copper atoms.

Calculating Atomic MassCalculating Atomic Mass

To determine the average atomic mass, first calculate the contribution of each isotope to the average atomic mass, being sure to convert each percent to a fractional abundance.

Will the average atomic mass of copper Will the average atomic mass of copper be closer to 63 amu or 65 amu?be closer to 63 amu or 65 amu?

Calculating Atomic MassCalculating Atomic Mass

Mass contribution = mass of isotope x abundance of isotope

For Cu-63:

Mass contribution = 62.930 amu x 0.6917 = 43.529 amu

For Cu-65:

Mass contribution = 64.928 amu x 0.3083 = 20.017 amu

Calculating Atomic MassCalculating Atomic Mass

The average atomic mass of the element is the sum of the mass contributions of each isotope.

Atomic mass Cu = mass contribution Cu-63 + mass contribution Cu-65

Atomic mass Cu = 43.529 + 20.017 = 63.546 amu

IonsIons

While changing the number of neutrons While changing the number of neutrons in an atom changes the in an atom changes the mass mass and and creates an creates an isotopeisotope, changing the , changing the number of electrons changes the number of electrons changes the chargecharge of an atom and creates an of an atom and creates an ion.ion.

IonsIons The charge is the sum of the charges The charge is the sum of the charges

contributed by the protons and contributed by the protons and electrons. Charge = (pelectrons. Charge = (p++) – (e) – (e--))

The charge of an atom is written as a The charge of an atom is written as a superscript on the right side of an superscript on the right side of an element’s symbol:element’s symbol:Ex. Fluorine has 9 protons with 10 Ex. Fluorine has 9 protons with 10

electrons: electrons: FF1-1- or simply or simply FF--

Sodium has 11 protons with 10 Sodium has 11 protons with 10 electrons: electrons: NaNa1+1+or simply or simply NaNa++

IonsIonsWrite the symbol for the following ions:Write the symbol for the following ions:

1.1. oxygen (O) with 10 electronsoxygen (O) with 10 electrons

2.2. beryllium (Be) with 2 electronsberyllium (Be) with 2 electrons

3.3. zirconium (Zr) with 37 electronszirconium (Zr) with 37 electrons

4.4. bromine (Br) with 36 electronsbromine (Br) with 36 electrons

O2-Be2+Zr3+Br-

Summary of Subatomic ParticlesSummary of Subatomic Particlessubatomic particle mass charge location

changing the number creates…

proton, p+ 1 +1 nucleusa different element

neutron, n0 1 0 nucleus an isotope

electron, e- ~0 -1electron

cloudan ion

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