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Dalton's Atomic Theory

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 Sub Topics Leucippus and Democritus argued for a limit to the divisibility of matter, which was counter to the prevailing view of Greek philosophers that matter is endlessly divisible. Democritus used the Greek word atoms which literally means "uncut table" to describe the ultimate particles of matter, particles that could not be divided further. However, it wast until John Dalton (1760-1844) introduced his atomic theory in 1803 that the importance of using atoms to explain properties of matter was recognized. Dalton atomic theory and the development of the periodic table by Mendeleev in 1969 led to rapid growth of chemistry as a science. In particular the influence of the location and number of electrons in atoms on the properties of elements has become one of the essential ideas of chemistry.

What is Dalton's Atomic Theory?

John Dalton (1766-1844) was an English scientist who made his living as a teacher in Manchester. He is best known for his atomic theory.

1. A given compound always contains the same proportions of the elements.
2. Most natural materials are mixtures of pure substances.
3. Pure substances are either elements or combination of elements called compounds.
John Dalton was aware of these observations and in about 1808 he offered an explanation for them that became known as Dalton's atomic theory.

Dalton's Atomic Theory Definition

An atom is the smallest particle of an element which may or may not be capable of independent existence. Dalton's idea of matter is known as John Dalton's atomic theory. Dalton first proposed his atomic theory in 1803.

On the basis of Dalton's atomic theory, "an atom is defined as the basic unit of a substance that can take part in chemical reactions."

The present definition of an atom is as follows. "An atom is a particle indivisible in chemical changes."

Matter is composed of very tiny indivisible particles called atoms. Atoms are incapable of being destroyed or created. According to Dalton's atomic theory the atom is the basic unit from which matter is constructed.

Dalton's Atomic Theory States

Dalton's Atomic theory states that
1. Matter is made of tiny particles called atoms.
2. Atoms are indivisible. During a chemical reaction, atoms are rearranged, but they do not break apart, nor are they created or destroyed.
3. All atoms of a given element are identical in mass and other properties.
4. The atoms of different elements differ in mass and other properties.
5. According to Dalton's Atomic theory atoms of one element can combine with atoms of another element to form "compounds" - new complex particles. In a given compound however the different types of atoms are always present in the same relative numbers.

Dalton's atomic theory states that all matter is made of small particles called atoms. But he wrongly assumed that atoms cannot be divided. In the late nineteenth century, it was proved that atom can be further divided into three subatomic particles called electron, proton and neutron.

Dalton's Atomic Theory Model

Dalton's theory accounts for chemical reactions. As the reaction proceeds the atoms rearranged to form the products. The atoms are neither created nor destroyed, but simple arranged differently.

Dalton's model successfully explained important observations such as the law of constant composition. This law makes sense because if a compound always contains the same relative numbers of atoms, it will always contain the same proportions by mass of the various elements.

Dalton's atomic model picture was not accepted immediately. However, Dalton was convinced he was right and used hie model to predict how a given pair of elements might combine to form more than one compound.

For example, nitrogen and oxygen might form a compound containing one atom of nitrogen and one atom of oxygen, a compound containing two atoms of nitrogen and one atom of oxygen, a compound containing one atom of nitrogen and two atoms of oxygen and so on. When the existence of these substances was verified, it was a triumph for Dalton's model. Because Dalton was able to predict correctly the formation of multiple compounds between two elements, his atomic theory became widely accepted.

Dalton's Model of the Atom

John Dalton revived the concept of atoms and proposed an atomic model based on facts and experimental evidence are shown below.

Dalton's atoms were individual particles the atoms of each element being in mass and size but different in mass and size from other elements and Dalton's atoms combine in specific ratios to form compounds.

Dalton's atomic model is used to explain the behavior of atoms. Dalton's atomic model stands as a landmark in the development of chemistry. The major premises of his model are still valid, but some of his statements must be modified or qualified because later investigations have shown that
1. Atoms are composed of subatomic particles.
2. Not all the atoms of a specific element have the same mass.
3. Atoms under special circumstances can be decomposed.

Postulates of Dalton's Atomic Theory

The Dalton's atomic theory postulates are summarized as follows.
1. The basic unit of an element is an extremely small, indivisible particle called an atom.
2. Atoms are hard spheres with a characteristic size and mass.
3. Each atom of the same element has the same mass.
4. Atoms of one element are different from atoms of another element.
5. Atoms of one element can combine with atoms of a second element, usually in small whole number ratios, to form "compound atoms" or molecules - the characteristic units of a compound.

Dalton's Atomic Theory Experiment

According to Dalton's atomic theory atoms of different elements combine with each other in a simple whole number ratio whereas according to Gay Lussac's law of combining volumes gases combine with each other in a simple whole number ratio by their volumes. Berzelius correlated these two laws and put forward a generalization known as Berzelius hypothesis.

This hypothesis when applied to a number of reactions involving gases, came in direct conflict with Dalton's atomic theory.

For example, it has been experimentally found that one volume of hydrogen combines with one volume of chlorine to form two volumes of hydrochloric acid gas.

Hydrogen + Chlorine $\rightarrow$ Hydrochloric acid gas

Applying Berzelius hypothesis

Let n atoms of each gas be present in unit volume

Hydrogen + Chlorine $\rightarrow$ Hydrochloric acid gas
n atoms + n atoms $\rightarrow$ 2n compound atoms
1 atom + 1 atom $\rightarrow$ 2 compound atoms
$\frac{1}{2}$ atom + $\frac{1}{2}$ atom $\rightarrow$ 1 compound atom

From the above it is obvious that one compound atom of hydrochloric acid contains $\frac{1}{2}$ atom of hydrogen and $\frac{1}{2}$ atom of chlorine.

From this it may be concluded that atoms may undergo division during chemical reactions. But Dalton's atomic theory incorrect according to which atom is the smallest unit of an element that takes part in chemical reactions.

Dalton's Atomic Theory vs Modern Atomic Theory

 S.No Dalton's Atomic Theory Modern Atomic Theory 1 All matter is made up of extremely small particles called atoms. An element is made up of extremely small particles called atoms. 2 Atoms are indivisible and indestructible. Atoms are divisible and are no more indestructible. 3 Atom is the smallest particle of an element. Atoms consist of still smaller particles called protons, neutrons and electrons. 4 All atoms of an element are alike in all respects. All atoms of an element may not be alike they can have different masses. 5 Atoms take part in all chemical reactions the relative numbers and kinds of atoms are constant in a given compound. Atoms take part in all chemical reactions the relative numbers and kinds of atoms are constant in a given compound.

Main Points of Dalton's Atomic Theory

John Dalton statements regarding the structure of atom are grouped together and known as Dalton's atomic theory. The main points of Dalton's atomic theory are listed below.
1. Atom is the smallest particle of matter but cannot be seen.
2. Atom is indestructible, that is it can neither be created nor destroyed.
3. Atoms of the same element are identical in all respects.
4. Atom of different elements differ in all respects.
5. Whole number of atoms of different elements combine together to give compound atoms (molecules).
6. Atom is the smallest unit of matter that takes part in a chemical reaction.

Summary of Dalton's Atomic Theory

• Each element is made up of tiny individual particles called atoms.
• Atoms are indivisible they cannot be created or destroyed.
• All atoms of each element are identical in every respect.
• Atoms of one element are different from atoms of any other element.
• Atoms of one element may combine with atoms of other elements, usually in the ratio of small whole numbers to form chemical compounds.

Problems with Dalton's Atomic Theory

The Dalton's atomic theory was propose by Dalton on the basis of laws of chemical combination. Solved problems based on the concept Dalton's atomic theory are given below.

Solved Examples

Question 1: If a piece of copper weights 0.635g, how many atoms does it contain?
Solution:

Number of moles of Cu in 0.635g
= $\frac{0.635g}{63.5g\ mol^{-1}}$
= 10-2 mol

As 1 mole Cu contains 6.023 $\times$ 1023 atoms of Cu.
So 10-2 mole Cu contains = 6.023 $\times$ 1023 $\times$ 10-2
= 6.023 $\times$ 1021 atoms of Cu.

Question 2: How many atoms of carbon is there in a diamond ring of 0.5 carat? (1 carat = 200 mg)
Solution:

Mass of diamond (C) = 0.5 $\times$ 200 mg
= 100 mg = 100 $\times$ 10-3g = 0.1g

Number of mole of C = $\frac{0.1g}{12g\ mol^{-1}}$

= $\frac{1}{120}$ mole

Number of C atoms = $\frac{1}{120} \times 6.023 \times 10^{23}$

Number of C atoms = 5.02 $\times$ 1021