6 2.1 Atomic Theory

Learning Objectives

By the end of this section, you will be able to:

  • State the postulates of Dalton’s atomic theory
  • Use postulates of Dalton’s atomic theory to explain the laws of definite and multiple proportions

The core ideas of chemistry are used in many disciplines, ranging from medicine to engineering to forensics to art. Chemical symbols are used to represent atoms and elements. Chemical formulas depict molecules as well as the composition of compounds. Chemical equations provide information about chemical reactions.

The most foundational chemistry idea of all is atomic theory. As we consider this theory in some detail we will identify the composition and mass of atoms, the variability of the composition of isotopes, ion formation, and chemical compounds held together by ionic and covalent bonding. We will also introduce one of the most powerful tools for organizing chemical knowledge: the periodic table.

Dalton’s Atomic Theory

Modern understandings of chemistry can be traced back to John Dalton who, in 1807, first published his hypothesis that the behavior of matter could be explained using an atomic theory. Dalton’s atomic theory states the following:

  1. Matter is composed of exceedingly small particles called atoms. An atom is the smallest unit of an element that can participate in a chemical change.
  2. An element consists of only one type of atom, which has a mass that is characteristic of the element and is the same for all atoms of that element (Figure 1). A macroscopic sample of an element contains an incredibly large number of atoms, all of which have identical chemical properties.
    The left image shows a photograph of a stack of pennies. The right image calls out an area of one of the pennies, which is made up of many sphere-shaped copper atoms. The atoms are densely organized.
    Figure 1. A pre-1982 copper penny (left) contains approximately 3 × 1022 copper atoms (several dozen are represented as brown spheres at the right), each of which has the same chemical properties. (credit: modification of work by “slgckgc”/Flickr)
  3. Atoms of one element differ in properties from atoms of all other elements.
  4. A compound consists of atoms of two or more elements combined in a small, whole-number ratio. In a given compound, the numbers of atoms of each of its elements are always present in the same ratio (Figure 2).
    The left image shows a container with a black, powdery compound. The right image calls out the molecular structure of the powder which contains copper atoms that are clustered together with an equal number of oxygen atoms.
    Figure 2. Copper(II) oxide, a powdery, black compound, results from the combination of two types of atoms—copper (brown spheres) and oxygen (red spheres)—in a 1:1 ratio. (credit: modification of work by “Chemicalinterest”/Wikimedia Commons)
  5. Atoms are neither created nor destroyed during a chemical change, but are instead rearranged to yield substances that are different from those present before the change (Figure 3).
    The left stoppered bottle contains copper and oxygen. There is a callout which shows that copper is made up of many sphere-shaped atoms. The atoms are densely organized. The open space of the bottle contains oxygen gas, which is made up of bonded pairs of oxygen atoms that are evenly spaced. The right stoppered bottle shows the compound copper two oxide, which is a black, powdery substance. A callout from the powder shows a molecule of copper two oxide, which contains copper atoms that are clustered together with an equal number of oxygen atoms.
    Figure 3. When the elements copper (a shiny, red-brown solid, shown here as brown spheres) and oxygen (a clear and colorless gas, shown here as red spheres) react, their atoms rearrange to form a compound containing copper and oxygen (a powdery, black solid). (credit copper: modification of work by http://images-of-elements.com/copper.php)

Dalton’s atomic theory provides a microscopic explanation of the many macroscopic properties of matter. For example, if an element such as copper consists of only one kind of atom, then it cannot be broken down into simpler substances, that is, into substances composed of fewer types of atoms. And if atoms are neither created nor destroyed during a chemical change, then the total mass of matter present when matter changes from one type to another will remain constant (the law of conservation of matter).

Many of Dalton’s hypotheses about the microscopic features of matter are still valid in modern atomic theory. The shortcomings of his model, however are that it does not account for WHY elements combine as they do and does not explain the observed electrical charge of particles. More discoveries have grown out from, and improved upon his model.

Example 1

In the following drawing, the green spheres represent atoms of a certain element. The purple spheres represent atoms of another element. If the spheres touch, they are part of a single unit of a compound. Does the following chemical change represented by these symbols violate any of the ideas of Dalton’s atomic theory? If so, which one?


This equation shows that the starting materials of the reaction are two bonded, green spheres, which are being combined with two smaller, bonded purple spheres. The product of the change is one purple sphere that is bonded to one green sphere.

 

Solution
The starting materials consist of two green spheres and two purple spheres. The products consist of only one green sphere and one purple sphere. This violates Dalton’s postulate that atoms are neither created nor destroyed during a chemical change, but are merely redistributed. (In this case, atoms appear to have been destroyed.)

 

Test Yourself

In the following drawing, the green spheres represent atoms of a certain element. The purple spheres represent atoms of another element. If the spheres touch, they are part of a single unit of a compound. Does the following chemical change represented by these symbols violate any of the ideas of Dalton’s atomic theory? If so, which one?


This equation shows that the starting materials of the reaction are two sets of bonded, green spheres which are each being combined with two smaller, bonded purple spheres. The products of the change are two molecules that each contain one purple sphere bonded between two green spheres.

 

Answer

The starting materials consist of four green spheres and two purple spheres. The products consist of four green spheres and two purple spheres. This does not violate any of Dalton’s postulates: Atoms are neither created nor destroyed, but are redistributed in small, whole-number ratios.

The Laws of Constant Composition and Multiple Proportions

Dalton knew of the experiments of French chemist Joseph Proust, who demonstrated that all samples of a pure compound contain the same elements in the same proportion by mass. This statement is known as the law of constant composition. The suggestion that the numbers of atoms of the elements in a given compound always exist in the same ratio is consistent with these observations. For example, when different samples of isooctane (a component of gasoline and one of the standards used in the octane rating system) are analyzed, they are found to have a carbon-to-hydrogen mass ratio of 5.33:1, as shown in Table 1.

Sample Carbon Hydrogen Mass Ratio
A 14.82 g 2.78 g [latex]\frac{14.82 \text{g carbon}}{2.78 \text{g hydrogen}} = \frac{5.33 \text{g carbon}}{1.00 \text{g hydrogen}}[/latex]
B 22.33 g 4.19 g [latex]\frac{22.33 \text{g carbon}}{4.19 \text{g hydrogen}} = \frac{5.33 \text{g carbon}}{1.00 \text{g hydrogen}}[/latex]
C 19.40 g 3.64 g [latex]\frac{19.40 \text{g carbon}}{3.64 \text{g hydrogen}} = \frac{5.33 \text{g carbon}}{1.00 \text{g hydrogen}}[/latex]
Table 1. Constant Composition of Isooctane

Dalton also used data from Proust, as well as results from his own experiments, to formulate another interesting law. The law of multiple proportions states that when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small, whole numbers. For example, copper and chlorine can form a green, crystalline solid with a mass ratio of 0.558 g chlorine to 1.00 g copper, as well as a brown crystalline solid with a mass ratio of 1.116 g chlorine to 1.00 g copper. These ratios by themselves may not seem particularly interesting or informative; however, if we take a ratio of these ratios, we obtain a useful and possibly surprising result: a small, whole-number ratio.

[latex]\frac{\frac{1.116 \text{g Cl}}{1.00 \text{g Cu}}}{\frac{0.558 \text{g Cl}}{1.00 \text{g Cu}}} = \frac{2}{1}[/latex]

This 2-to-1 ratio means that the brown compound has twice the amount of chlorine per amount of copper as the green compound.  This can be explained by atomic theory if the copper-to-chlorine ratio in the brown compound is 1 copper atom to 2 chlorine atoms, and the ratio in the green compound is 1 copper atom to 1 chlorine atom. The ratio of chlorine atoms (and thus the ratio of their masses) is therefore 2 to 1 (Figure 4).

 

Figure A shows a pile of green powder. A callout shows that the green powder is made up of a lattice of copper atoms interspersed with chlorine atoms. The atoms are color coded brown for copper and green for chlorine. The number of copper atoms is equal to the number of chlorine atoms in the molecule. Figure B shows a pile of brown powder. A callout shows that the brown powder is also made up of copper and chlorine atoms similar to the molecule shown in figure A. However there appears to be two chlorine atoms for every copper atom in this molecule. The copper atoms in figure B bond with both the chlorine atoms and the other copper atoms. The copper atoms in figure A only bond with the chlorine atoms.
Figure 4. Compared to the copper chlorine compounds, where copper is represented by brown spheres and chlorine by green spheres, in (a) where the copper chlorine compound is CuCl and (b) where the copper chlorine compound CuCl2 which has twice as many chlorine atoms per copper atom. (credit a: modification of work by “Benjah-bmm27”/Wikimedia Commons; credit b: modification of work by “Walkerma”/Wikimedia Commons)

Example 2

A sample of compound A (a clear, colorless gas) is analyzed and found to contain 4.27 g carbon and 5.69 g oxygen. A sample of compound B (also a clear, colorless gas) is analyzed and found to contain 5.19 g carbon and 13.84 g oxygen. Are these data an example of the law of definite proportions, the law of multiple proportions, or neither? What do these data tell you about substances A and B?

 

Solution
In compound A, the mass ratio of carbon to oxygen is:

[latex]\frac{1.33 \text{g O}}{1.00 \text{g C}}[/latex]

In compound B, the mass ratio of carbon to oxygen is:

[latex]\frac{2.67 \text{g O}}{1.00 \text{g C}}[/latex]

The ratio of these ratios is:

[latex]\frac{\frac{1.33 \text{g O}}{1.00 \text{g C}}}{\frac{2.67 \text{g O}}{1.00 \text{g C}}} = \frac{1}{2}[/latex]

This supports the law of multiple proportions. This means that A and B are different compounds, with A having one-half as much oxygen per amount of carbon as B.  A possible pair of compounds that would fit this relationship would be A = CO and B = CO2.

 

Test Yourself
A sample of compound X (a clear, colorless, combustible liquid with a noticeable odor) is analyzed and found to contain 14.13 g carbon and 2.96 g hydrogen. A sample of compound Y (a clear, colorless, combustible liquid with a noticeable odor that is slightly different from X’s odor) is analyzed and found to contain 19.91 g carbon and 3.34 g hydrogen. Are these data an example of the law of definite proportions, the law of multiple proportions, or neither? What do these data tell you about substances X and Y?

 

Answers

In compound X, the mass ratio of carbon to hydrogen is [latex]\frac{14.13 \text{g C}}{2.96 \text{g H}}[/latex] . In compound Y, the mass ratio of carbon to oxygen is [latex]\frac{19.91 \text{g C}}{3.34 \text{g H}}[/latex] . The ratio of these ratios is [latex]\frac{\frac{14.13 \text{g C}}{2.96 \text{g H}}}{\frac{19.91 \text{g C}}{3.34 \text{g H}}} = \frac{4.77 \text{g C/g H}}{5.96 \text{g C/g H}} = 0.800 = \frac{4}{5}[/latex] . This small, whole-number ratio supports the law of multiple proportions. This means that X and Y are different compounds.

Key Concepts and Summary

Dalton postulated that each element has a characteristic type of atom that differs in properties from atoms of all other elements, and that atoms of different elements can combine in fixed, small, whole-number ratios to form compounds. Samples of a particular compound all have the same elemental proportions by mass. When two elements form different compounds, a given mass of one element will combine with masses of the other element in a small, whole-number ratio. During any chemical change, atoms are neither created nor destroyed.

Review-Reflect, Extend

Review-Reflect

1. In the following drawing, the green spheres represent atoms of a certain element. The purple spheres represent atoms of another element. If the spheres of different elements touch, they are part of a single unit of a compound. The following chemical change represented by these spheres may violate one of the ideas of Dalton’s atomic theory. Which one?This equation contains the starting materials of a single, green sphere plus two smaller, purple spheres bonded together. When the starting materials are added together the products of the change are one purple sphere bonded with one green sphere plus one purple sphere bonded with one green sphere.

2. Which postulate of Dalton’s theory is consistent with the following observation concerning the weights of reactants and products? When 100 grams of solid calcium carbonate is heated, 44 grams of carbon dioxide and 56 grams of calcium oxide are produced.

Do these data provide example(s) of the law of definite proportions, the law of multiple proportions, neither, or both? What do these data tell you about compounds X, Y, and Z?

Extendthree Duplo building blocks of varying size and color

  1. Building blocks such as Duplo® can be used as a rough model for the formation of compounds from atoms of various elements. Using this idea, come up with a scheme for representing the following compounds, with atoms of various elements keyed to a particular type of block:

H2O, representing water with 2 hydrogen atoms and one oxygen atom

CO2, carbon dioxide, with one carbon atom and two oxygen atoms

C3H8, propane, with 3 carbon atoms and 8 hydrogen atoms

 

Answers to Review-Refresh
1. The starting materials consist of one green sphere and two purple spheres. The products consist of two green spheres and two purple spheres. This violates Dalton’s postulate that that atoms are not created during a chemical change, but are merely redistributed.

2. The law of conservation of matter – the total mass of matter present when matter changes from one type to another remains constant.

3. This statement violates Dalton’s fourth postulate: In a given compound, the numbers of atoms of each type (and thus also the percentage) always have the same ratio.

4.  Samples X and Z provide an example of the law of definite proportions (law of constant composition) for both samples are found to have a carbon-to-hydrogen mass ratio of 12:1.

Samples X and Y provide an example of the law of multiple proportions,

since in sample X, the mass ratio of carbon to hydrogen is:

[latex]\frac{0.0833 \text{g H}}{1.00 \text{g C}}[/latex]

and in sample Y, the mass ratio of carbon to hydrogen is:

[latex]\frac{0.167 \text{g H}}{1.00 \text{g C}}[/latex]

The ratio of these ratios is:

[latex]\frac{\frac{0.0833 \text{g H}}{1.00 \text{g C}}}{\frac{0.167 \text{g H}}{1.00 \text{g C}}} = \frac{1}{2}[/latex]

This supports the law of multiple proportions. This means that sample X and Y are different compounds, with X having one-half as much hydrogen per amount of carbon as Y.

Samples Z and Y also provide an example of the law of multiple proportions,

since in sample Z, the mass ratio of carbon to hydrogen is:

[latex]\frac{0.0833 \text{g H}}{1.00 \text{g C}}[/latex]

and in sample Y, the mass ratio of carbon to hydrogen is:

[latex]\frac{0.167 \text{g H}}{1.00 \text{g C}}[/latex]

The ratio of these ratios is:

[latex]\frac{\frac{0.0833 \text{g H}}{1.00 \text{g C}}}{\frac{0.167 \text{g H}}{1.00 \text{g C}}} = \frac{1}{2}[/latex]

This supports the law of multiple proportions. This means that sample Z and Y are different compounds, with Z having one-half as much hydrogen per amount of carbon as Y.

Note: Samples X and Z may be the same compound or they may be different (isomers – a topic to be discussed later).

Glossary

Dalton’s atomic theory: set of postulates that established the fundamental properties of atoms

law of conservation of matter: the total mass of matter present when matter changes from one type to another remains constant.

law of constant composition: (also, law of definite proportions) all samples of a pure compound contain the same elements in the same proportions by mass

law of definite proportions: (also, law of constant composition) all samples of a pure compound contain the same elements in the same proportions by mass

law of multiple proportions: when two elements react to form more than one compound, a fixed mass of one element will react with masses of the other element in a ratio of small whole numbers

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