Modern Theories of the Atom

Modern Atomic Theory states that matter is composed of small, indivisible particles called atoms. 

Three Laws of Modern Atomic Theory

Concept: The Conservation of Matter

Video Transcript

We already know that the basic functional unit in chemistry is the atom. But we should realize that associated with the atom are theories that scientists from the past developed to better understand how do these atoms interact with one another, as well as their surroundings.
Now, we're going to say according to the Law of Conservation of Mass, in a chemical reaction, any type of chemical reaction, matter can neither be created nor destroyed. The only thing that happens is that matter changes forms.
Here, when I give us an example, we have CH4 gas, which is called methane gas, reacts with oxygen gas to give us CO2 gas plus water. What we mean by this law of conservation of mass, what it means is let's say we started out with 100 grams of these two reactants. On the left side of the arrow, we call these reactants. Let's say we started out with 100 g of it. At the end of our reaction, we formed products. And at the end of the reaction, we should still end up with 100 g.
We also can say that connected to this Law of Conservation of Mass is that we should realize that at the end of our reaction, we should have equal numbers of each individual element. We're going to say on the left side, we have one carbon, we have four hydrogens and we have two, times another two, we have four oxygens.
At the end of our reaction, we should still have the same number of carbons, hydrogens, and oxygens. This also goes in line with the conservation of mass. We have one carbon. We have two times two, we have four hydrogens. Then this two gets distributed, so we have two times one, we have two oxygens here, plus another two oxygens here, so we have four oxygens total.
Whether you're looking at it in terms of grams produced from grams starting with or you're looking at it in terms of, I started out with this number of elements, I should end with the same number of elements—both are staying the same thing.
What you start in a certain quantity, you should end with at the end of the reaction. Matter doesn't get destroyed. If we start out with 100 grams, we shouldn’t end with less than 100 grams. We should only end with 100 grams because we're conserving the mass that we had. 

The Law of Conservation of Mass states that matter can neither be created nor destroyed, all that happens is that it changes forms. 

No matter where you obtain a compound, whether it’s from a lab experiment or from collection out in the field, the ratio of elements in it will remain constant. 

Concept: Law of Multiple Proportions

Video Transcript

The last important atomic theory is we're going to say according to the Law of Multiple Proportions, when two elements, which we're going to say are A and B, form different compounds. Here in this example, our element A will be nitrogen and our element B could be oxygen. And we're going to say the masses of element B that combine with 1 g of A are a ratio of whole numbers. This one is a little bit trickier.
Here we're dealing with two different compounds. We're dealing with NO, which is nitrogen monoxide, and then NO2 which is nitrogen dioxide. We do the mass ratios of both. Nitrogen is 14 g roughly on our periodic table, oxygen is 16. Their mass ratio to each other in NO is 1.143. Now, if we do NO2, since we have two oxygens, it's 16 times 2, which gives us 32. So 32 divided by 14, give us this new number. That's the mass ratio for each of them.
Now, the Law of Multiple Proportions tells me, if I take those mass ratios and I divide them by each other, it should give me back a whole number as an answer. I'm going to take this 2.286 and I divide it by the 1.143 and it spits back to me, 2. So what that's telling me is that we'd have two oxygens, two grams of oxygen for every 1 g of nitrogen. That's really what it's telling me.
The Law of Multiple Proportions for this type of questions, it's going to be a quick type of question that your professor asks you, nothing in too depth where you have to write long paragraphs or anything. As long as you can grasp the basic concepts of each of these, you'll better be able to answer the questions that they're going to ask you.
Then, again, they're not going to be too in depth with those types of questions. So, all that matters is that you understand the most basic parts of these three modern atomic theories. That's the most important thing. 

When element A and element B combine they can form different compounds in different ratios to one another. Dividing these different ratios should generate whole number answers. 

Example: A 15.39 g sample of iodine reacts with 62.92 g of chlorine to form iodine pentachloride, ICl5. If iodine pentachloride is the only product formed calculate its mass. 


Example: Two samples sodium fluoride decompose into their constituent elements. The first sample produces 15.8 kg of sodium and 20.1 kg of fluorine. If the second sample produces 192.0 g of sodium, how many grams of fluorine were also produced? 


Example: Which of the following is an example of the law of multiple proportions?

a) A sample of bromine (Br) is contains equal amounts of its two isotopes.

b) Two different samples of H2O have the same mass ratio.

c) The atomic mass of sodium (Na) is 22.99 amu.

d) Two different compounds composed of sulfur (S) and oxygen (O) have different mass ratios: 2.48 g O: 1 g S and 1.24 g O: to 1 g S.



Modern Theories of the Atom Additional Practice Problems

A 12.39 g sample of phosphorus reacts with 42.54 g of chlorine to form only phosphorus trichloride (PCl3). If it is the only product, what mass of PCl 3 is formed?

A) 140.01 g

B) 30.15 g

C) 91.86 g

D) 79.71 g

E) 54.93 g

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Which is the correct match of person and the law: when two elements form a series of compounds, the masses of the one element that combine with a fixed mass of the other element stand to one another in the ratio of small whole numbers.

A. Democratus, atomic law

B. Lavosier, law of conservation of mass

C. Berthollet, law of definite proportions

D. Dalton, law of multiple proportions

E. Avagadro, law of moles

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The two compounds, Fe3O4 and Fe2O3, illustrate the law of multiple proportions. For the same mass of iron in samples of each compound, what is the ratio of the masses of the oxygen in the compounds?

A. 1 to 1

B. 2 to 3

C. 3 to 4

D. 7 to 8

E. 8 to 9

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When a log burns, the mass of its products equals the mass of the log and oxygen. This is an example of which law?

A. Law of Definite Proportions

B. Law of Conservation of Mass

C. Law of Multiple Proportions

D. Law of Modern Atomic Theory

E. First Law of Thermodynamics 

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Which of the following statements is TRUE?

A. A scientific law is fact.

B. Once a theory is constructed, it is considered fact.

C. A hypothesis is speculation that is difficult to test.

D. An observation explains why nature behaves as it does.

E. A scientific law summarizes a series of related observations. 

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A compound contains only calcium and fluorine. A sample of the compound is determined to contain 2.00 g of calcium and 1.90 g of fluorine. According to the Law of Definite Proportions, how much calcium should another sample of this compound contain if it contains 2.85 g of fluorine?

a. 2.71 g

b. 4.00 g

c. 3.00 g

d. 4.50 g

e. 6.00 g

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Which of the following is NOT a component of Dalton’s Atomic Theory?

A. A chemical reaction rearranges the grouping of atoms.

B. Atoms are comprised of protons, neutrons, and electrons.

C. Atoms of a given element are chemically and physically identical.

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

E. Matter is composed of atoms, which cannot be created or destroyed.

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The statement, “In a chemical reaction, no matter is neither created nor destroyed.” Is called

a. The Law of Conservation of Mass

b. Dalton’s Atomic Theory

c. The Scientific Methods

d. The Law of Multiple Proportions

e. The Law of Definite Proportions

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Which of the following sets illustrates the Law of Multiple Proportions?


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Drinking alcohol is also called ethyl alcohol or ethanol. Most of the ethanol produced in the USA comes from corn, an obviously natural source. It can also be made in the laboratory from natural gas (CH4) as a starting material. If you wanted to use ethanol from one of these sources, and price were not a consideration, decide which one you would choose. Which of the following laws listed below is most useful to you in making your decision?


a. The Law of Conservation of Mass

b. The Law of Definite Proportions

c. Dalton’s Atomic Theory

d. The Law of Multiple Proportions

e. Avogadro’s Hypothesis

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