Chapter Three

ATOMIC THEORY NOTES

Important Concepts in a Nutshell

First person to theorize that matter was made up of tiny particles was a Greek philosopher (algebra teacher), DEMOCRITUS around 400 B.C.

Later Aristotle theorized that matter was continuous and couldn’t be broken down into smaller parts. (He was Wrong! He set atomic theory back thousands of years)

Neither Democritus nor Aristotle had any experimental evidence to support their theories.

It would be another 2000 years before our modern atomic theory would evolve.

Serious Atomic Research Begins

In the 1790’s, because of the development of accurate measuring instruments, scientists began to collect data that would help develop atomic theory.

In 1806 John Dalton, an English school teacher, developed a theory that is almost completely accepted today.

Dalton’s Atomic Theory All matter is composed of extremely small

particles called atoms Atoms of a given element are identical in size,

mass and other properties. Atoms of different elements are different in size, mass and properties.

Atoms cannot be subdivided, created or destroyed.

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

In chemical reactions, atoms combine, are separated or rearrange to form products.

Other laws developed in the 1790’s include:Other laws developed in the 1790’s include:

LAW OF CONSERVATION OF MASS: Matter can LAW OF CONSERVATION OF MASS: Matter can neither be created nor destroyed. (neither be created nor destroyed. (In other words; the In other words; the mass of the reactants = the mass of the products)mass of the reactants = the mass of the products)

LAW OF DEFINITE PROPORTIONS: Compounds LAW OF DEFINITE PROPORTIONS: Compounds contain the same elements in the same proportions by contain the same elements in the same proportions by mass no matter the size of the sample. mass no matter the size of the sample. EXAMPLEEXAMPLE---- ---- In 18 grams of water, there are 2 g of hydrogen and 16 g In 18 grams of water, there are 2 g of hydrogen and 16 g of oxygen. A 1: 8 ratio. of oxygen. A 1: 8 ratio. In 50 grams of water there are 5.56 g of hydrogen and In 50 grams of water there are 5.56 g of hydrogen and 44.44 g of oxygen which reduces down to a 1:8 ratio. 44.44 g of oxygen which reduces down to a 1:8 ratio. No No matter how large or small the sample of water, the ratio matter how large or small the sample of water, the ratio of the mass of hydrogen to the mass of oxygen is of the mass of hydrogen to the mass of oxygen is ALWAYS 1:8.ALWAYS 1:8.

Other “Laws” continuedOther “Laws” continued

LAW OF MULTIPLE PROPORTIONS: If two or more LAW OF MULTIPLE PROPORTIONS: If two or more compounds are composed of the same two compounds are composed of the same two elements, then the amount of the second element in elements, then the amount of the second element in each separate compound which combines with the each separate compound which combines with the same amount of the first element, form small, whole-same amount of the first element, form small, whole-number ratios. number ratios. EXAMPLEEXAMPLE: In compound : In compound A, 1 g nitrogen combines with 1.14 g of oxygen. In A, 1 g nitrogen combines with 1.14 g of oxygen. In compound B,1 g nitrogen combines with 2.28 g compound B,1 g nitrogen combines with 2.28 g oxygen. oxygen. The ratio of oxygen in compound A to the The ratio of oxygen in compound A to the oxygen in compound B is …….. oxygen in compound B is …….. 1.14 g : 2.28 g which reduces to a 1:2 ratio 1.14 g : 2.28 g which reduces to a 1:2 ratio (a simple whole number ratio)(a simple whole number ratio)

Cathode Ray Tubes Cathode Ray Tubes AKA Crookes TubeAKA Crookes Tube

In the late 1880’s, many experiments were In the late 1880’s, many experiments were performed in which electric current was passed performed in which electric current was passed through various gases at low pressures. They through various gases at low pressures. They were carried out in glass tubes known as cathode-were carried out in glass tubes known as cathode-ray tubes.ray tubes.

Inferences Made From Cathode Inferences Made From Cathode RaysRays Cathode rays were deflected by a Cathode rays were deflected by a

negative magnetic field. negative magnetic field. Conclusion: Conclusion: Cathode rays are negatively chargedCathode rays are negatively charged

Paddle wheel moved when placed in Paddle wheel moved when placed in front of cathode ray. front of cathode ray. Conclusion: Conclusion: Cathode rays are made up of particles Cathode rays are made up of particles with masswith mass

Cathode ray tubes were made with Cathode ray tubes were made with different metals and gases and the different metals and gases and the results were always the same. results were always the same. Conclusion: Electrons are present in all Conclusion: Electrons are present in all mattermatter

Other Inferences MadeOther Inferences Made Since atoms have negative electrons but Since atoms have negative electrons but

are electrically neutral themselves, they are electrically neutral themselves, they must contain some positive charge to must contain some positive charge to balance the negative electronsbalance the negative electrons..

Since electrons have so much less mass Since electrons have so much less mass than atoms, atoms must have some other than atoms, atoms must have some other particles that account for most of their particles that account for most of their mass.mass.

Rutherford’s Gold Foil Rutherford’s Gold Foil ExperimentExperiment

Rutherford’s Conclusions based onRutherford’s Conclusions based on Gold Foil Experiment Gold Foil Experiment

Atoms are made up of mostly empty space

There must be a very small, positive, dense bundle of matter at the center of the atom (nucleus)

The electrons most likely orbit the nucleus like planets orbit the sun

Other Facts about atomic structure

Atoms are neutral because they contain equal numbers of protons and electrons

Mass of a neutron is 1.675 x 10-27 kg Mass of a proton is 1.673 x 10-27 kg Mass of an electron is 9.109 x 10-31 kg Short-range forces called “nuclear forces” hold

the protons and neutrons together The density of a nucleus is extremely high: 2 x

108 metric tons/ mL

Facts about atomic structure cont.d

The symbol “Z” represents atomic number (number of protons) Z=9 means atomic number 9

The number of protons in the nucleus is what gives an atom its identity. For example: an atom with 9 protons is fluorine. The number of neutrons can vary but the number of protons is always 9. If it doesn’t have 9 protons, THEN IT’S NOT FLUORINE!

ISOTOPES are atoms of the same element (same # of protons) with different numbers of neutrons, therefore different masses and thus different mass numbers! Ex. Copper-63 (29 protons and 34 neutrons) Copper-65 (29 protons and 36 neutrons)

Calculating Average Atomic Mass

Copper is a mix of Cu-63 and Cu-65. 69.17% of the atoms are Cu-63 with a mass of 62.929 599 and the other 30.83% are Cu-65 with a mass of 64.927 793.

Let MAMA help you calculate average atomic mass(it’s really MxA + MxA)

“M” stands for mass of isotope and “A” stands for abundance (percentage)

Multiply the mass times the abundance and add to the other mass x abundance. Don’t forget to change the percent to a decimal

Ex. (62.929 599 x .6917 )+ (64.927 793 x .3083 )= Average Atomic Mass

Definition of Mole

A mole is the amount of a substance that contains as many particles as there are atoms in exactly 12 grams of carbon-12. What does that mean?

There are 6.022 136 7 x 1023 atoms in 12g of carbon. That means….if you have that many atoms or molecules, you have a mole!

This number is called “Avogadro’s Number” One mole contains 6.022 136 7 x 1023 atoms or 6.022 136 7 x

1023 molecules or 6.022 136 7 x 1023 ions or 6.022 136 7 x 1023 particles etc.

Not as weird as you think!!! It’s not any different than the idea of a “dozen”. A dozen

is 12 eggs or 12 doughnuts or 12 chicken wings or 12 pencils…….

A gross is 144 eggs or 144 pencils or 144 sodas….. A ream is 500 sheets of paper. So “MOLE” is the easy way to say ………

6.022 136 7 x 1023 atoms or602 213 670 000 000 000 000 000 atoms

MOLAR MASS The mass of one mole of a pure

substance is called the molar mass of that substance. The molar mass of an element is numerically equal to the average atomic mass of the element contained in the periodic table EXCEPT THE UNITS ARE GRAMS NOT AMU’s.

For example: 1 atom of Mg has a mass of 24.3 atomic mass units1 mole (6.02 x 1023 atoms) of Mg has a mass of 24.3 grams 1 atom of Au has a mass of 196.967 atomic mass units1 mole (6.02 x 1023 atoms) of Au has a mass of 196.967 grams

Gram Mole ConversionsConversions from moles to grams and grams to moles are simple if you use dimensional analysis.Example: How many moles are contained in 57 grams of sodium? Your conversion ratio is the molar mass of sodium which is found on the periodic table. 23g Na

1 mole NaBegin by writing 57 g Na then multiply it by the reciprocal of the molar mass so that the grams units cancel. 57 g Na [ 1 mole Na] Since the 23 is in the

[23 g Na ] denominator, you should

divide 57 by 23 to get your ans.