3-2 Discovering Atomic Structure*

The “Holy Grail” Of Chemistry:

To Understand the Structure of the Atom

*Modified from a presentation by Mr. Matt Davis.Sept 2006

OBJECTIVES Define the term “atom.” Describe the history of Atomic Theory. List the postulates of Dalton’s Atomic Theory. Show how atomic structure and

electricity are related. Explain what studies of cathode rays

and radioactivity revealed about atoms. Discuss Rutherford’s alpha-scattering

experiment and how it showed the existence of the nucleus.

Name/describe the three subatomic particles in an atom.

Determine the number of protons, neutrons and electrons in an atom or ion.

Define “isotope” and atomic mass.

Understanding the Structure of the Atom

Michael Faraday suggested… …atomic structure is somehow related to electricity.

Benjamin Franklin proposed… …there are two kinds of charge (positive & negative); …like charges repel, and opposite charges attract.

Cathode Ray Tube (“CRT”) was developed and… …it showed an ‘odd glow’ in electrified gases. (Demonstration of CRT.)

J.J. Thompson proved… …CRT ‘glow’ came from the negative end (cathode). The glow was actually particles. (It moved a pinwheel.) He is credited with discovering the electron. He measured the ratio of an electron’s charge to its mass.

Understanding the Structure of the Atom (cont’d)

Robert Millikan measured… …the charge of a single electron (Oil Drop Experiment)

as 1.60 X 10-19 coulomb. So the calculated mass of electron is 9.11 X 10-28 g.

Henri Becquerel discovered… …radioactivity is emitted from uranium ore.

Marie Curie & Pierre Curie discovered… ..other radioactive elements, radium & polonium.

J. J. Thomson proposed the “Plum Pudding” model of the atom.

(Think of it as a “chocolate chip cookie” model instead.)

Atoms have negative chargesevenly distributed throughoutthe atom’s positive interior.

Understanding the Structure of the Atom (cont’d)

Ernest Rutherford discovered… …alpha & beta radiation. Both are charged. (Alpha particles have 2+

charge; beta particles have 1- charge.) (Gamma radiation was discovered later.)

Rutherford also did a very important experiment: the Gold Foil Experiment.

Rutherford’s Gold Foil Experiment

Try the following websites at home! (I’ll demo them herejust to show how they work.)

http://www.waoen.screaming.net/revision/nuclear/rsanim.htm

http://www.micro.magnet.fsu.edu

(Use the search tool to find the “Rutherford Experiment.”)

Gold Foil Experiment Observations:

99% of alpha particles went straight through the gold foil.

But, ½% deflected, and ½% reflected! Unexpected & remarkable results! Rutherford said, “It was about as credible

as if you had fired a 15-inch [artillery] shell at a piece of paper and it came back and hit you!”

Rutherford’s Conclusions

The ‘Plum Pudding’ model is wrong. An atom basically consists of a lot of

empty space! All of an atom’s positive charge is

concentrated in a very small core at the atom’s center, which Rutherford called the nucleus.

The negatively charged electrons move around the nucleus.

Note: If the entire atom is the size of a football stadium, the nucleus would be about the size of a marble sitting on the 50-yd line.

OBJECTIVES

Define the term “atom.” Describe the history of Atomic Theory. List the postulates of Dalton’s Atomic Theory. Show how atomic structure and electricity are related. Explain what studies of cathode rays and radioactivity

revealed about atoms. Discuss Rutherford’s alpha-scattering experiment and

how it showed the existence of the nucleus. Name/describe the three subatomic

particles in an atom. Determine the number of protons,

neutrons and electrons in an atom or ion.

Define “isotope” and atomic mass.

Understanding the Structure of the Atom (cont’d)

James Chadwick discovered… …another subatomic particle: the neutron. Neutrons have no charge, but their mass is nearly

equal to that of a proton. Neutrons reside in the nucleus, along with protons. Neutrons act as a ‘glue’ that holds the nucleus

together. Recall that “like charges repel,” so atoms

with lots of protons would be very unstable without lots of neutrons.

Strong Nuclear Force – the name for the attraction that holds a nucleus together, thus preventing it from flying apart.

Understanding the Structure of the Atom (cont’d)

Henry Moseley found that… …atoms of each element contain a unique

positive charge in their nucleus. An atom’s identity comes from the number of

protons in its nucleus. The number of protons in an atom is called its

Atomic Number (Z).

3-3 Modern Atomic Theory “Parts of the Atom”

Atoms are composed of protons, neutrons and electrons. (Plus many other smaller particles that do not impact the chemistry.)

The nucleus contains protons (+) and neutrons (neutral).

Electrons (-) move in space around the nucleus (in the “planetary” model).

The number of electrons always equals the number of protons when an atom is neutral.

Atomic Number (Z): the number of protons in an atom.

Mass Number (A): the total of protons and neutrons.

Atomic mass units (amu) are used to express mass.

Models of the Atom (So far!)

Ancient Greek Model: Tiny particles (atomos).

Thomson Model: Ball of positive charge with embedded electrons.

Rutherford Model: An atom’s mass concentrated in a small, positively charged region (nucleus) with electrons around it.

Other Models to follow?

Subatomic Particles

Particle Location Charge (C)

Mass (g)

p Nucleus +1.602 X10^-19

1.673 X10^-24

n Nucleus 0 1.675 X10^-24

e Outside nucleus

-1.602 X10^-19

9.109 X10^-28

Text Fig. 3-19, page 104.

Atom Notation

XA

Z = Atomic Number (Often omitted. Why?)

ZX = Element Symbol

A = Mass Number

Ions

Atoms are electrically neutral. (Why? Demonstration.) If an atom loses or gains electrons it becomes charged,

forming an ion. ION: an atom or group of atoms that has a

positive or negative charge because it lost or gained electrons.

Lithium: Z = 3, A = 7 Losing one electron, Li forms Li1+ and the e1-

Oxygen: Z = 8, A = 16 Gaining two electrons, O plus 2 e1- forms O2-

Ions are written with chemical symbols by placing its charge on the upper right.

Practice this!

Isotopes

Contrary to Dalton’s idea, all atoms of a given element are NOT identical!

They have the same number of protons, but may not have the same number of neutrons.

Isotopes: atoms of the same element that have different masses. (This is caused by the different numbers of neutrons.)

Examples H-1 (hydrogen or protium, H), H-2 (deuterium, D), H-

3 (tritium, T). C-12, C-13, C-14. U-235, U-238.

Let’s practice p, n, e counting. In your notes, set up the grid on the next slide.

p, n, e Counting

Element At # Mass #

p n e

H

He

Li

B

F

Counting the Mass of Atoms

Recall that total mass of an atom is (p + n + e), but electrons are only 1/2000th the mass of a proton, and may generally be neglected.

So an atom’s mass is basically just the mass of its protons & neutrons.

Very tiny masses; not practical to use. Chemists compare the relative masses of atoms

vs. a carbon-12 atom standard, which has 6p, 6n & 6e.

C-12 defined as exactly 12 atomic mass units (amu), so 1 amu = 1/12th the mass of carbon 12.

All atoms are then compared with this. (We will do a lab to see how this works.)

Let’s complete the Table we saw before.

Subatomic Particles

Particle Location Charge (C)

Mass (g) Mass (amu)

p Nucleus +1.602 X10^-19

1.673 X10^-24

1.0073 = 1

n Nucleus 0 1.675 X10^-24

1.0087 = 1

e Outside nucleus

-1.602 X10^-19

9.109 X10^-28

0.0006 = 0

Text Fig. 3-19, page 104.

Counting the Mass of Atoms (cont’d)

We learned: Mass of a single atom depends on the

number of protons and neutron only. (Why?)

Therefore, mass of an element should just be whole numbers, right?

Then how can the mass of chlorine be 35.453 amu?

Explanation: relative abundance of natural isotopes!

Relative Abundance of Isotopes

Chlorine has two isotopes, Cl-35 (35 amu) & Cl-37 (37 amu).

Fractional abundance of Cl-35 is ~75%, and of Cl-37 is ~25%.

Use a weighted average to get the atomic mass of the element. This reflects the mass and relative abundance of isotopes.

Example: Three Cl-35 atoms for every Cl-37 atom. Total proton mass = 17 + 17 + 17 +17 = 68 amu. Total neutron mass = 18 + 18 + 18 +20 = 74 amu. Weighted average mass of Cl = (68 + 74)/4 = 35.5 amu. Actual value = 35.453 amu if more precise data used. We will always round atomic mass data to the nearest 0.1

amu. What are the most common isotopes of He; Ne; Kr?

VOCABULARY (Chapter 3)

Atom Law of Constant

Composition Atomic Theory of

Matter Cathode Ray Cathode Ray Tube Electron Radioactivity Nucleus

Proton Neutron Atomic Mass Unit

(amu) Atomic Number Ion Isotope Mass Number Atomic Mass Strong Nuclear

Force

3-1

3-2

3-3

Did we meet the OBJECTIVES?

Define the term “atom.” Describe the history of Atomic Theory. List the postulates of Dalton’s Atomic Theory. Show how atomic structure and electricity are related. Explain what studies of cathode rays and radioactivity

revealed about atoms. Discuss Rutherford’s alpha-scattering experiment and

how it showed the existence of the nucleus. Name/describe the three subatomic particles in

an atom. Determine the number of protons, neutrons and

electrons in an atom or ion. Define “isotope” and atomic mass.

NOTE: We will skip Section 3-4 (Changes in the Nucleus). You are NOT responsible for it now.