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Measurement of Matter

Dalton's Atomic Theory and Laws of Chemical Combination-Group A

Dividing Matter

Matter cannot be divided infinite number of times.

 

For example, if we keep chopping a log of wood into smaller and smaller pieces, then we will reach a point when the wood will not be divisible any further. Minute particles of wood will remain and these will not be visible to the naked eye. This is true for all forms of matter.The same was believed by the early Indian and Greek philosophers. In India, around 500 BC, an Indian philosopher named Maharishi Kanad called matter as padarth and these smallest particles (atoms) as ‘parmanu’. The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term. However, for these ancient thinkers, the idea of the minute indivisible particle was a purely philosophical consideration. 

 

 

By the end of the eighteenth century, scientists had begun to distinguish between elements and compounds. Two French chemists named Antoine Lavoisier and Joseph Proust observed that elements combine in definite proportions to form compounds. On the basis of this observation, each of them proposed an important law of chemical combination. The laws proposed by them helped Dalton formulate his atomic theory.

Dalton’s Atomic Theory 

In the early nineteenth century, an English chemist named John Dalton proposed a theory about atoms. Known as ‘Dalton’s atomic theory’, it proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’. He published his atomic theory in 1808 in his book A New System of Chemical Philosophy. The postulates of Dalton’s atomic theory are as follows:

All matter is made up of very tiny particles. These particles are called atoms. An atom cannot be divided further, i.e., atoms are indivisible. Atoms can be neither created nor destroyed in a chemical reaction. All atoms of an compounds . In a given compound, the relative numbers and types of atoms are constant.

Know Your Scientist

John Dalton (1766−1844) was born into the poor family of a weaver in Eaglesfield, England. He was colour-blind from childhood. He became a teacher when he was barely twelve years old. By the time he was nineteen, he had become the principal of a school. In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics at a college. Elected a member of the Manchester Literary and Philosophy Society in 1794, he became its president in 1817 and remained in that position until his death. During his early career, he identified the hereditary nature of red−green colour blindness. In 1803, he postulated the law of partial pressures (known as Dalton’s law of partial pressures). He was the first scientist to explain the behaviour of atoms in terms of relative atomic weight. He also proposed symbolic notations for various elements.

Dividing Matter

Matter cannot be divided infinite number of times.

 

For example, if we keep chopping a log of wood into smaller and smaller pieces, then we will reach a point when the wood will not be divisible any further. Minute particles of wood will remain and these will not be visible to the naked eye. This is true for all forms of matter.The same was believed by the early Indian and Greek philosophers. In India, around 500 BC, an Indian philosopher named Maharishi Kanad called matter as padarth and these smallest particles (atoms) as ‘parmanu’. The word ‘atom’ is derived from the Greek word ‘atomos’ which means ‘indivisible’. It was the Greek philosopher Democritus who coined the term. However, for these ancient thinkers, the idea of the minute indivisible particle was a purely philosophical consideration. 

 

 

By the end of the eighteenth century, scientists had begun to distinguish between elements and compounds. Two French chemists named Antoine Lavoisier and Joseph Proust observed that elements combine in definite proportions to form compounds. On the basis of this observation, each of them proposed an important law of chemical combination. The laws proposed by them helped Dalton formulate his atomic theory.

Dalton’s Atomic Theory 

In the early nineteenth century, an English chemist named John Dalton proposed a theory about atoms. Known as ‘Dalton’s atomic theory’, it proved to be one of the most important theories of science. The various laws of chemical combination also supported Dalton’s theory. Dalton asserted that ‘atoms are the smallest particles of matter, which cannot be divided further’. He published his atomic theory in 1808 in his book A New System of Chemical Philosophy. The postulates of Dalton’s atomic theory are as follows:

All matter is made up of very tiny particles. These particles are called atoms. An atom cannot be divided further, i.e., atoms are indivisible. Atoms can be neither created nor destroyed in a chemical reaction. All atoms of an compounds . In a given compound, the relative numbers and types of atoms are constant.

Know Your Scientist

John Dalton (1766−1844) was born into the poor family of a weaver in Eaglesfield, England. He was colour-blind from childhood. He became a teacher when he was barely twelve years old. By the time he was nineteen, he had become the principal of a school. In 1793, Dalton left for Manchester to teach physics, chemistry and mathematics at a college. Elected a member of the Manchester Literary and Philosophy Society in 1794, he became its president in 1817 and remained in that position until his death. During his early career, he identified the hereditary nature of red−green colour blindness. In 1803, he postulated the law of partial pressures (known as Dalton’s law of partial pressures). He was the first scientist to explain the behaviour of atoms in terms of relative atomic weight. He also proposed symbolic notations for various elements.

Atoms: An Overview 

When we talk about atoms, two questions usually strike our mind... 

Let us go through this lesson to find the answers to these questions. We will also learn how to represent different atoms in symbolic forms. So, in short, we are going to study:

Size of an atom Representation of atoms Atomic mass

Size of an Atom  The size of an isolated atom cannot be measured; however, we can estimate its size by assuming that its radius is half the distance between adjacent atoms in a solid. Atoms are very small in size. They are so small that it is not possible to see them even under a powerful optical microscope. The size of an atom is indicated by its radius, called the nanometre .

   

 

                                          

Surfaces of Silicon Atoms

Atoms cannot be seen with the naked eye, but the use of modern techniques has enabled us to see the surfaces of atoms. The magnified image of the surfaces of silicon atoms is shown in the following figure.

 

Size of an Atom 

Hydrogen atom is the smallest of all atoms. The given figure shows the atomic radii of some elements.

Classical Representation of Atoms A large number of elements are known to us today. It would be cumbersome to refer to them by their names all the time in our studies. For the sake of convenience, we need symbols that represent these elements. Toward the end of the nineteenth century, scientists felt this need to assign standard characteristic symbols to the elements.  John Dalton was the first scientist to use symbols to represent different elements. Dalton’s proposed symbols for some elements are shown in given figure.

Each symbol proposed by Dalton represents an atom of the respective element. For example, if someone wanted to represent two hydrogen atoms, then he would have to draw the symbol of hydrogen atom twice as shown.

Modern Representation of Atoms Many of the symbols proposed by Dalton were difficult to draw and remember. Therefore, an alternative method of representing elements was required. Another scientist named Jӧns Jacob B…

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