The Evolution of Atomic Theory

Chapter 3

Law of conservation of Matter

• When a chemical reaction takes place, matter is neither created nor destroyed

Law of Constant Composition

• Multiple Samples of any pure chemical compound always contain the same percent by mass of each element making up the compound

– Example: if a 50 g sample of pure water is decomposed into it’s component elements, you will obtain 5.6 g of hydrogen and 44.4 g oxygen gas

– The percent by mass of these elements is therefore:

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

particles called atoms.

2 Atoms can neither be created nor destroyed (they persist unchanged for all eternity)

3 Atoms of the same element are identical (in size, mass, and properties), those of different atoms are different.

Dalton’s Atomic Theory

4 A chemical reaction involves either the union or the separation of individual atoms.

Law of Multiple Proportions• A law proposed by Dalton

which states that when elements combine, they do so in the ratio of small whole numbers.  For example carbon and oxygen react to form CO or CO2, but not CO1.8.   

Parts of an Atom

• 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 Experiment

Voltage source

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

- +

Thomson’s Experiment

Voltage source

+

-

By adding an electric field By adding an electric field

Thomson’s Experiment

Voltage source

+

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

What Did Thomson Demonstrate?

• Cathode rays:• Travel in straight lines

• Are negatively charged

• Are deflected by electric and magnetic fields

Thomson’s Model

• Found the electron• Said the atom was

like plum pudding• A bunch of positive

stuff, with the electrons able to be removed

Rutherford’s Experiment

• Ernest Rutherford English physicist. (1910)

• Believed in the plum pudding model of the atom.

• Used uranium to produce alpha particles.

Rutherford’s Experiment

Lead block

Uranium

Gold Foil

Florescent Screen

What he expected

Why ??

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

• The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles

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

+

How He Explained It

Density and the Atom

• Since most of the particles went through, it was mostly empty.

• 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

Modern View

• The atom is mostly empty space

• Two regions

• Nucleus- protons and neutrons

• Electron cloud- region where you might find an electron

Structure of Atom

• There are two regions:– The nucleus: with protons and neutrons

• Almost all the mass

– Electron cloud- Most of the volume of an atom

• The region where the electron can be found

Subatomic Particles

 

Name Symbol Charge

Electron e- -1

Proton p+ +1

Electron n0 0

Size of Atom

• Nucleus tiny compared to atom

• IF the atom was the size of a stadium, the nucleus would be the size of a marble.

• Radius of the nucleus near 10-15m.

• Density near 1014 g/cm

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in their nuclei

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

Isotopes

• Dalton was wrong.

• Atoms of the same element can have different numbers of neutrons

• different mass numbers

• called isotopes

The Periodic Table

The Periodic Table

•Each column is called a group.

•Each row is called a period.

Period

Group

Alkali M

etal

Noble G

as

Halogen

Alkali E

arth Metal

2.4

Non-metals

• Not conductors of heat or electricity

• Brittle cannot be rolled into wires or pounded into sheets.

• Exist in two of the three states of matter at room temperature:• Gases- Oxygen• Solids- Carbon

• No metallic luster and do not reflect light

• You are to know the names (correct spelling) and symbols for these elements.

Metalloids

• Properties of both metals and non-metals

• Silicon and Germanium are semi-conductors.

Alkali Metals

Li

Na

K

Rb

Cs

Fr

IA

Alkali Metals

• Very react-active metals that do not occur freely in nature.

• They are Malleable, ductile and good conductors of heat an electricity

• Softer than most other metal

• Explode if they are exposed to water

• Color flames• Li-red Na- Yellow K- lilac Rb- red Cs-blue

Alkaline Earth Metals

Be

Mg

Ca

Sr

Ba

Ra

Alkaline Earth Metals

• Very reactive but not as reactive as the alkali metals

• All are found in the earth’s crust but not in elemental form. Instead they are widely distributed in rock structures

Halogens

F

Cl

Br

I

At

Halogens

• Halogen means “salt-former” therefore compounds containing halogens are called “salts.”

• All halogens exist as diatomic molecules, X2

• The halogens exist, at room temperature, in all three states of matter

• Solid-Iodine, Astatine• Liquid- Bromine• Gas- Fluorine, Chlorine

• The halogens are too reactive to occur free in nature.

Transition metals

Sc Ti V Cr Mn

Y

La

Ac

Zr

Hf

Nb

Ta

Mo

W

Tc

Re

Fe

Ru

Os

Co Ni Cu Zn

Rh

Ir

Pd Ag Cd

Pt Au Hg

Transition Metals

• Ductile

• Malleable

• Conductors of heat and electricity

• Iron, Cobalt and Nickel are they only elements known to produce a magnetic field

Noble GasesHe

Ne

Ar

Kr

Xe

Rn

Noble Gases

• Very un-reactive

• Very small quantities in the atmosphere

• Monatomic

• Low boiling points

• He, Ne, and Ar form no known compounds

Rare Earth Metals

Rare Earth Metals

Lanthanide SeriesHigh LusterConductors of electricityTarnish rapidly in airMany ignite and burnvigorously at elevatedtemperatures

Actinide SeriesAll isotopes of all actinidesare radioactiveActinides are very denseTypically react with:

air (tarnishing)boiling water or dilute acidmost non-metals in direct combination

Atomic Mass

• How heavy is an atom of oxygen?

• There are different kinds of oxygen atoms.

• More concerned with average atomic mass.

• Based on abundance of each element in nature.

• Don’t use grams because the numbers would be too small

Measuring Atomic Mass

• Unit is the Atomic Mass Unit (amu)

• Each isotope has its own atomic mass we need the average from percent abundance.

Calculating averages

• You have five rocks, four with a mass of 50 g, and one with a mass of 60 g. What is the average mass of the rocks?

• Total mass = 4 x 50 + 1 x 60 = 260 g

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5

Calculating averages

• Average mass = 4 x 50 + 1 x 60 = 260 g 5 5 5

• Average mass = .8 x 50 + .2 x 60

• 80% of the rocks were 50 grams

• 20% of the rocks were 60 grams

• Average = % as decimal x mass + % as decimal x mass + % as decimal x mass +

Atomic Mass

• Calculate the atomic mass of copper if copper has two isotopes. 69.1% has a mass of 62.93 amu and the rest has a mass of 64.93 amu.

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 25 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