Unit #3 Atomic Structure Review from 8 th Grade OBJECTIVES: Identify the subatomic particles ...

Unit #3 Atomic Structure

Review from 8th Grade OBJECTIVES:

Identify the subatomic particles

Explain how the atomic number identifies an element.

Use the atomic number and mass number of an element to find the numbers of protons, electrons, and neutrons

Review key terms

Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

n0

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Counting the Pieces Atomic Number = number of

protons in the nucleus # of protons determines kind of

atom (since all protons are alike!)

the same as the number of electrons in the neutral atom.

Mass Number = the number of protons + neutrons.

These account for most of mass

Counting the Pieces

Protons: equal to atomic number

Neutrons: Mass Number – Atomic Number

Electrons: In a neutral atom equal to atomic number

Symbols

Contain the symbol of the element, the mass number and the atomic number.

Symbols Contain the symbol of the

element, the mass number and the atomic number.

X Massnumber

Atomicnumber

Symbols Find the

number of protons

number of neutrons

number of electrons

Atomic number Mass Number

F19 9

SymbolsSymbols Find the Find the

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Atomic numberAtomic number

–Mass NumberMass Number

Br80 35

SymbolsSymbols if an element has an atomic if an element has an atomic

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

–number of protonsnumber of protons

–number of neutronsnumber of neutrons

–number of electronsnumber of electrons

–Complete symbolComplete symbol

SymbolsSymbols if an element has 91 protons and if an element has 91 protons and

140 neutrons what is the 140 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–number of electronsnumber of electrons

–Complete symbolComplete symbol

SymbolsSymbols if an element has 78 electrons and if an element has 78 electrons and

117 neutrons what is the 117 neutrons what is the

–Atomic numberAtomic number

–Mass numberMass number

–Number of protonsNumber of protons

–Complete symbolComplete symbol

What if Atoms Aren’t Neutral Ions: charged atoms resulting

from the loss or gain of electrons

What if Atoms Aren’t Neutral Anion: negatively charged ion;

result from gaining electrons Take the number of electrons in a

neutral atom and add the absolute value of the charge

81

35 Br1- Identify:

Number of ProtonsNumber of NeutronsNumber of Electrons

What if Atoms Aren’t Neutral Cation: positively charged ion;

result from the loss of electrons Take the number of electrons in a

neutral atom and subtract the value of the charge

27

13 Al 3+ Identify:Number of ProtonsNumber of NeutronsNumber of Electrons

Distinguishing Between Atoms

OBJECTIVES: Explain how isotopes differ, and why the atomic masses of elements are not whole numbers.

Distinguishing Between Atoms

OBJECTIVES: Calculate the average atomic mass of an element from isotope data.

Isotopes

Atoms of the same element can have different numbers of neutrons

Different mass numbers Called isotopes

Naming Isotopes

We can also put the mass number after the name of the element.

carbon- 12 carbon -14 uranium-235

Atomic Mass How heavy is an atom of oxygen?

There are different kinds of oxygen atoms

We are more concerned with average atomic mass

Average atomic mass is based on abundance of each element in nature.

We don’t use grams because the numbers would be too small

Measuring Atomic Mass

Unit is the Atomic Mass Unit (amu)

It is one twelfth the mass of a carbon-12 atom

Each isotope has its own atomic mass, thus we determine the average from percent abundance

Calculating averages Convert the percent to a decimal

(move the decimal 2 places to the left or divide by 100)

Multiply the atomic mass by it’s percent (expressed as a decimal), then add the results.

Calculating Averages

Calculating the Average Atomic Mass is similar to calculating your grade in this class

Percent of GradeStudent's Average

Tests 35% 89Quizzes 25% 95

Labs 20% 35Homework 20% 100

Calculating Averages

If Bromine-79 has an abundance of 50.69% and Bromine-81 has an abundance of 49.31% what is the average atomic mass of Bromine?

Why is it better to use the mass of an individual isotope for the mass number rather than round the number from the periodic table?

Atomic Mass Magnesium has three isotopes.

78.99% magnesium-24 with a mass of 23.9850 amu, 10.00% magnesium-25 with a mass of 24.9858 amu, and the rest magnesium- 26 with a mass of 25.9826 amu. What is the atomic mass of magnesium?

If not told otherwise, the mass of the isotope is the mass number in amu

Atomic Mass

Is not a whole number because it is an average.

are the decimal numbers on the periodic table.

History of the atom Not the history of atom, but the

idea of the atom. Original idea Ancient Greece (400

B.C.) Democritus and Leucippus- Greek

philosophers.

Democritus and Leucippus

Leucippus was the first person to come up with the idea of the atom

Democritus was a student of Leucippus and expanded on his idea

Democritus

Democritus added: Matter is composed of atoms

which move through empty space Atoms are solid, homogeneous,

indestructible, and indivisible Different atoms have different

shapes and sizes The size, shape, and movement

of atoms determine their properties

Greek Aristotle - Famous philosopher All substances are made of 4

elements Fire - Hot Air - light Earth - cool, heavy Water - wet Blend these in different

proportions to get all substances

Aristotle

There was no scientific evidence to back up either Democritus or Aristotle, but people believed Aristotle based on reputation

Aristotle’s theory persisted for 2,000 years

Leading to the modern theory Late 1700’s - John Dalton- England. Teacher- summarized results of his

experiments and those of others. Dalton’s Atomic Theory Combined ideas of elements with

that of atoms. Saw atoms as small solid spheres.

Billiard Ball Model.

Dalton’s Atomic Theory All matter is made of tiny indivisible

particles called atoms. Atoms of the same element are

identical, those of atoms of different elements are different.

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

Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed.

Laws Related to Atomic Theory Law of Conservation of Mass-

Antoine Lavoisier; states that matter is neither created nor destroyed in chemical reactions

Law of Definite Proportions- Joseph Proust; states that all compounds contain the same elements in the same ratio

Law of Multiple Proportions- John Dalton; elements combine in small whole number ratios

Just How Small Is an Atom? Think of cutting a piece of lead into smaller and smaller pieces

How far can it be cut? An atom is the smallest

particle of an element that retains the properties of that element

Atoms-very small

Structure of the Nuclear Atom

OBJECTIVES: Distinguish among protons, electrons,

and neutrons in terms of relative mass and charge.

Describe the structure of an atom, including the location of the protons, electrons, and neutrons with respect to the nucleus.

Parts of Atoms

J. J. Thomson - English physicist. 1897

Made a piece of equipment called a cathode ray tube.

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

Thomson’s Experiment

Voltage source

+-

Vacuum tube

Metal Disks

Thomson’s Experiment

Voltage source

+-

Thomson’s Experiment

Voltage source

+-

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Passing an electric current makes a Passing an electric current makes a beam appear to move from the beam appear to move from the negative to the positive endnegative to the positive end

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s Experiment

By adding an electric field

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric fieldBy adding an electric field

+

-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field he found By adding an electric field he found that the moving pieces were negativethat the moving pieces were negative

+

-

Plum Pudding Model

Proposed by JJ Thomson

Said the atom had a uniform positive charge in which the negatively charged electrons resided

Other particles

Proton - positively charged pieces 1840 times heavier than the electron – by E. Goldstein

Neutron - no charge but the same mass as a proton – by J. Chadwick

Where are the pieces?

Millikan’s Oil Drop

JJ Thomson used the cathode ray to find the mass to charge ratio of an electron.

He did not know the mass or charge, only the ratio

Millikan’s Oil Drop experiment determined the charge of an electron

Millikan’s Experiment

Oil

Atomizer

Oil droplets

Telescope

-

+

Millikan’s Experiment

X-rays

X-rays give some electrons a charge.

Millikan’s Experiment

Some drops would hoverFrom the mass of the drop and the charge on the plates, he calculated the mass of an electron

Rutherford’s experiment Ernest Rutherford -English physicist.

(1910) Believed in the plum pudding model of

the atom Wanted to see how big they are. Used radioactivity. Alpha particles - positively charged

pieces- helium atoms minus electrons Shot them at gold foil which can be

made a few atoms thick.

Rutherford’s experiment

When an alpha particle hits a fluorescent screen, it glows.

Here’s what it looked like

Lead block

Uranium

Gold Foil

Fluorescent Screen

He Expected

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

Because…? …the positive charges were

thought to be spread out evenly. Alone they were not enough to stop the alpha particles.

What he expected

Because

He thought the mass was evenly distributed in the atom

Since he thought the mass was evenly distributed in the atom

What he got

How he explained it

+

Atom is mostly empty. Small dense,

positive piece at center.

Alpha particles are deflected by

it if they get close enough.

+

Density and the Atom Since most of the particles went

through, it was mostly empty space.

Because the pieces turned so much, the positive pieces were heavy.

Small volume, big mass, big density.

This small dense positive area is the nucleus.

Bohr Model

Bohr worked with the concepts of energy, wavelength and frequency

Each color of light is associated with a different energy

Each atom gives off its own unique color so..

Electrons of diiferent atoms have different energies

Bohr Model

Each atom has its own specific electron arrangement

Electrons are in Energy levels in the atom

When an electron goes from an excited state back down to its ground state the atom emits light

The Bohr Ring Atom

n = 3n = 4

n = 2n = 1

The Bohr Model

Doesn’t work. Only works for hydrogen atoms. Electrons don’t move in circles. The quantization of energy is right,

but not because they are circling like planets.

Back to the drawing board

The Quantum Mechanical Model A totally new approach. De Broglie said matter could be like

a wave, like standing waves. The vibrations of a stringed

instrument. More on this in Unit # 4

Models of the Atom to Know Daltons Thomsons Rutherford Bohr Quantum