Chapter 4

The structure of the atom

AL-COS Objectives 1, 2,3,4,7, 10, 15, 20, 21, 22, 27and 28

You’ll learn to…

• Identify the experiments that led to the development of the nuclear model of atomic structure

• Describe the structure of the atom and the subatomic particles that comprise it

and…

You’ll also learn…

• Explain the relationship between nuclear stability and radioactivity

• Write equations representing nuclear decay

• Discuss the atomic models of Democritus, Thomson, Rutherford, and Bohr

Vocabulary

*atom*atomic mass*atomic mass unit (amu)*atomic number*electron*isotope*mass number*neutron*nucleus*proton

Early Theories of Matter

Democritus ( 460-370 BC)

A man ahead of his time…

•First to believe matter was not infintely divisible

•Believed matter to be made of small particles he called “atomos”

•Believed these particles could not be created or destroyed

Democritus’ Ideas• Matter is composed of empty space through which atoms move • Atoms are solid, homogeneous,

indestructible and indivisible

• Different kinds of atoms have different

sizes and shapes (give matter its

properties) • Changes in matter are due to changes

in grouping of atoms

Aristotle

• One of most influential Greek philosophers

Aristotle

•Believed matter was formed of air, water, earth and fire

•Did not believe in the “nothingness” of space

•All of his ideas were NOT based on empirical (experimental) science – just ideas

Aristotle

Given credit for setting the field of science back nearly 2000 years!

John Dalton(1766-1844)

•Revived the ideas of Democritus

•First atomic model

•A teacher in England

Dalton’s atomic theory

•Matter is made of small particles called atoms

•All atoms of an element are identical **** (isotopes?)

•Atoms of different elements are different

As well as…

• Atoms cannot be created or destroyed or divided into smaller particles ****(nuclear fission?)

• Different atoms combine in simple whole number ratios to form compounds

• In chemical reactions, atoms are separated, combined or rearranged.

The atom

• Smallest particle of an element that retains the properties of that element

Relative size of an atom

How many atoms?

•World population = 6 000 000 000

•Number of atoms in a penny= 29 000 000 000 000 000 000

000

Subatomic particles and the nuclear atom

• The electron- Sir William Crookes noticed the flash of light in a tube

Tube filled with a gas,+ and a – end,

The cathode ray

Scientists were convinced this cathode ray was really a stream of charged particles -> electrons!

Further cathode tube experiments were conducted by J.J. Thomson (1856-1940)

•Was trying to determine the charge/mass ratio of these electrons

•Found the mass of this particles was less than hydrogen’s

J.J. Thomson

• Identified the first subatomic particle – the electron

• Atoms were divisible into smaller particles

J.J. Thomson’s atomic model •A spherical atom composed of evenly distributed positive charges within which the negatively charged electrons were also found

Robert Millikan (1868-1953)

• American physicist

• Determined the charge of an electron

• Calculated the mass of a single electron (e-)

Sir Ernest Rutherford

•Originally from New Zealand •Won a scholarship competition to study at Cambridge Univ. in England •Won the Nobel prize in 1908 for discovering radioactivity

Radioactivity • 3 types

–Alpha particles•Actually the nucleus of a helium atom

–Beta particle •A high speed electron

–Gamma particle •A photon of electromagnetic radiation

Radiation can be shielded by..

Designed an experiment to study the atom

•Experiment was actually conducted by his two graduate students, Geiger (later developed the Geiger counter) and Marsden

•Rutherford was their supervising professor

The gold foil experiment..

Used alpha particles for “bullets”

• Alpha particles = Helium nucleus

2 protons + 2 neutrons

What happened in the experiment

Observed that..

•Most of the alpha particles went straight through

•Some were deflected away from the foil

•Some hit something solid and “bounced back”

What was learned?

• Most of an atom is empty space

• The nucleus has a positive charge (He nucleus was also positive, which caused the deflection)

• The nucleus, when hit straight on, is dense and solid – not a plum pudding!

Other subatomic particles

•Proton –Discovered by Goldstein using a cathode ray tube (1920’s)

•Neutron –Discovered by James Chadwick in 1932

Atomic number

• Henry Moseley discovered that atoms of different elements each have a unique positive charge in their nucleus

Henry Moseley (1887-1915)

•Discovered atomic numbers •Died at a young age in WWI

The atomic number

12

C6

Symbol for element

Atomic number = number of protons in the nucleus

Mass number= Protons + neutrons

Practice writing atomic notation

23 Na

11

What does this tell us?

Write atomic notation

• For an atom of Fe

• For an atom of U

And ……

56 238Fe U

26 92

Isotopes

•Same atomic number but a different atomic mass

•The number of protons remains the same, but the number of neutrons vary.

Isotopes of hydrogen

1 2 3 H H H 1 1 1 Protium deuterium tritium

Calculating Atomic Mass(Turn to pg. 102)

• 1. mass x %abundance (for each isotope)• 2. Add them together to get the

atomic mass• 3. find that atomic mass on the

periodic table and that is the element

VIOLA!

Example Problem: pg. 103

• 1.) 6X =(6.015)(0.075)= 0.451 amu

7X =(7.016)(0.925)= 6.490 amu

• 2.) 0.451 + 6.490 = 6.941 amu

• 3.) The element with an atomic mass of 6.941 amu is Lithium, Li.

• COOL!!!!!!!!!!!!

Look on pg. 104

•Let’s do # 15 and #16 together.

•Now, you do #17 on your own.

Pg. 104, #15

(10.013)(0.198) = 1.982 amu (11.009)(0.802) = 8.829 amu

10.811 amu

Pg. 104, #16

Helium-4 is more abundant in nature because the atomic mass of naturally occurring helium is closer to the mass of helium-4 than to the mass of helium –3.

Pg. 104, #17

• (23.985)(0.7899) = 18.946 amu

• (24.986)(0.1000) = 2.499 amu

• (25.982)(0.1101) = 2.861 amu

24.306 amu

The element with that atomic mass is magnesium, Mg.

Nuclear reactions

•The nucleus of the atoms actually change

•Nuclear stability is based on neutron/proton ratio

•Experience nuclear decay to become more stable

Alpha decay

Nuclear equation becomes

238 234 4 U Th + He92 90 2

Beta decay

Nuclear equation

14 0 15 N e + O 7 -1 8

Another example of Beta decay

Gamma radiation γ

•Electromagentic radiation •Has no charge •Often found along with Beta

or alpha radiation

Gamma radiation

Fission of atoms

The end – for now!