Chapter 4Chapter 4

Atomic StructureAtomic Structure

Anything in black letters = write it in your notes (‘knowts’)

4.1 – Defining the Atom4.1 – Defining the Atom

Atom - smallest particle of an element that still has the properties of the element

comes from the Greek word atomos which means uncuttable or indivisible

Democritus (460 B.C. – 370 B.C.) one of the first to propose the idea of the atom; based on pure speculation

John Dalton (~1800) proposed 1st atomic theory

Daltons Atomic Theory (~1800) p. 103)1. All elements are composed of tiny indivisible particles called atoms.

Atoms of element A

Atoms of element B

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

Mixture of atoms of elements A and B

3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

Compound made by atoms of elements A

and B

4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

Caveats of Dalton’s Atomic Theory

1. All elements are composed of tiny indivisible particles called atoms.

2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element.

3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.

4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in different combinations. Atoms of one element are never changed into atoms of another element as a result of a chemical reaction.

Atoms are not indivisible – they are made of subatomic particles

Every atom has at least one isotope; one atom’s isotope is NOT identical to another isotope of the same atom.

This is known as the Law of Definite Proportions – very important.

Atoms of one element can change into an atom of another element as a result of a nuclear reaction.

4.2 – Structure of the Nuclear Atom4.2 – Structure of the Nuclear Atom

Subatomic Particles

Electron – discovered in 1897 by J.J. Thomson while experimenting with cathode ray tubes

Thomson performed experiments that involved passing electric current through gases at low pressure.

The result was a glowing beam, or cathode ray, that traveled from the cathode to the anode.

Thomson found that a cathode ray is deflected by electrically charged metal plates.

Thompson knew that opposite charges attract and like charges repel, so he hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed; now called electrons.

To test his hypothesis, Thompson set up an experiment to measure the ratio of an electron’s charge to its mass.

Also, the charge-to-mass ratio of electrons did not depend on the kind of gas in the cathode-ray tube or the type of metal used for the electrodes.

A cathode ray can also be deflected by a magnet.

Proton – observed in cathode ray tubes also in 1886 by Eugen Goldstein.

Neutron – confirmed in 1932 by James Chadwick.

Properties of Subatomic Particles

Particle SymbolRelative charge

Relative mass (mass of proton = 1)

Actual mass (g)

Electron e– 1– 1/1840 9.11 10–28

Proton p+ 1+ 1 1.67 10–24

Neutron n0 0 1 1.67 10–24

The Atomic Nucleus

How are atoms structured?

DemocritusDalton

Thomson’s Plum Pudding Model

In 1911, Ernest Rutherford and others performed the Gold Foil Experiment to test the plum pudding model

Ernest Rutherford

The Gold Foil Experiment

The Gold Foil Experiment

The results…

It was expected that alpha particles would pass through the plum pudding model of the gold atom undisturbed.

It was observed that a small portion of the alpha particles were deflected, indicating a small, concentrated positive charge (the nucleus!)

Expected

Actual

It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center, carrying a charge.

—Ernest Rutherford

Nucleus – tiny positively charge core of an atom

Rutherford’s Nuclear Model of the Atom

• If an atom were the size of a football stadium, the nucleus would be about the size of a marble

Is this the current model of the atom?

NO…

ASSIGN:

Read 4.2

Lesson Check 4.2; #9-15 (page 109)

4.3 – Distinguishing Among Atoms4.3 – Distinguishing Among Atoms

Atomic Number (Z) -

The number of protons in an atom; identifies the element.

In a neutrally charged atom, the number of protons (p+) equals the number of electrons (e-)

Mass Number (A) -

The number of protons (p+) and neutrons (n0) in an atom.

The mass number is NOT the atomic mass.

Element Atomic Number (Z)

Protons (p+)

Electrons (e-)

Neutrons (n0)

H

O

Ca

1 1 1

8 8 8

20 20 20

???

???

???

The number of n0 depends on the mass number of the isotope

Isotopes -

Atoms of an element that have a different number of neutrons.

Element Atomic Number (Z)

Protons (p+)

Electrons (e-)

Neutrons (n0)

H

O

Ca

1 1 1

8 8 8

20 20 20

Zoom for detail

Chemical Symbols for Isotopes

A is the superscript

Ne2010 Ne21

10 Ne2210

Z is the subscript

Determining the Composition of an Atom

How many protons, electrons, and neutrons are in each atom?

a. Be b. Ne c. Na94

2010

2311

Ne2010 Ne21

10 Ne2210

Naturally Occurring Isotopes of Neon

Percent Abundance in Nature

90.48% 0.27% 9.25%

The masses of atoms are rarely expressed in grams.

The C-12 isotope has been given a mass of exactly 12 atomic mass units (amu)

The masses of all other elements are based on the mass of the C-12 isotope.

Why is the mass of a carbon atom 12.011 amu?

Atomic Mass –

126C 13

6C 146C

12.000 amu

98.93 %

13.003 amu

1.07 %

14.003 amu

0.0000000001 %

Weighted average of all the naturally occurring isotopes of the element.

011.12)0107.0 x 003.12( )9893.0 x 12(

Atomic Mass of Carbon = 12.011 amu

12.000 amu

98.93 %

13.003 amu

1.07 %

14.003 amu

0.0000000001 %

Atomic Mass of Carbon = 12.011 amu

No atom of carbon actually weighs 12.011 amu. But a typical carbon atom averages 12.011 amu.

Atomic masses are weighted averages.

There are 2 stable isotopes of silver

Silver-107; 106.905097 amu; 51.84%

Silver-109; 108.904752 amu; 48.16%

Calculate the atomic weight of silver.

Atomic Weight of Silver = 107.868 amu

Despite differences in the number of neutrons, isotopes of an element are chemically similar.

Neutrons do not determine chemical reactivity; the electrons do.

ASSIGN:

Lesson Check 4.3 (#26-34); p. 119

ASSIGN:

#53-59, 61, 64-67