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• CBSE Class 11 Chemistry Notes

# Dalton’s Atomic Theory

In the year 1808, the English scientist and chemist John Dalton proposed Dalton’s atomic hypothesis, a scientific theory on the nature of matter. It asserted that all matter is made up of atoms, which are tiny, indivisible units. According to Dalton’s atomic theory, all substances are made up of atoms, which are indestructible and indivisible building blocks. While the atoms of one element were all the same size and mass, other elements had atoms of different sizes and weights.

### Dalton’s Atomic Theory

For scientific lovers, one of the most significant areas of inquiry has been mattering. Scientists and philosophers have long sought to make things easier to understand. They were curious about the fundamental particles that make up matter, as well as its properties, structure, and other characteristics. As a result, a variety of atomic theories were developed.

Democritus is credited as being the first to postulate that matter is made up of particles. These particles were given the name atomos, which means indivisible in Greek. Democritus’ Atomic Theory was based on this. Scientists had very little information on this idea at the time due to a lack of technical setup.

Scientist John Dalton manifested the works on simplifying matter over two thousand years later. John Dalton proposed the famous Dalton’s Atomic Theory in 1808. In a paper titled “A New Chemical Philosophy,” he published this idea; certainly, the philosophy was novel at the time. Let’s have a look at the theory’s postulates.

Dalton formulated his theory based on two laws: the law of conservation of mass and the law of constant composition.

1. Law of Conservation of Mass: In 1789, Antoine Laurent Lavoisier discovered the law of conservation of mass. The law of conservation of mass says that matter can neither be not created nor destroyed but it can modified from one for to another in a closed system. We use the law of conservation of mass to balance linear equations.
2. Law of Constant Composition: The law of constant composition says that a pure compound will always have the same proportion of the same elements. For example, table salt, which has the molecular formula NaCl contains the same proportions of sodium and chlorine instead of the fact, how much salt we want to make.

### Postulates of Dalton’s Atomic Theory

The postulates of Dalton’s theory may be stated as follows:

1. All matter is composed of atoms, which are indivisible: According to Dalton, the law of conservation of mass and the law of definite proportions can be explained using the idea of atoms. He proposed that all matter is made of tiny indivisible particles called atoms, which he imagined as “solid and movable particles”
2. All atoms of a given element are identical in mass and properties: Dalton proposed that every single atom of a particular element, such as copper, potassium etc, is the same as every other atom of that particular element. e.g. A potassium atom is different from an oxygen atom. Elements may share some similar boiling points, melting points, but no two elements have the same exact same set of properties.
3. Compounds are combinations of two or more different types of atoms: In the third part of Dalton’s atomic theory, he proposed that compounds are combinations of two or more different types of atoms. An example of such a compound is Common Salt. Common Salt is a combination of two different types of elements with varying physical and chemical properties. The first, sodium, is a highly reactive metal. The second, chlorine, is a toxic gas. When they react, the atoms combine in a 1:1 ratio to form white crystals of NaCl.
4. A chemical reaction is a rearrangement of atoms: In the last postulate, Dalton suggested that chemical reactions neither destroy and nor create atoms. They just rearranged the atoms. Using our salt example again, when sodium combines with chlorine to make salt, both the sodium and chlorine atoms still exist. They simply rearrange to form a new compound.

• Dalton’s theory provides a basic idea to differentiate between elements and compounds.
• Dalton’s atomic theory. doesn’t violate the law of multiple proportions, the law of conservation of mass, and the law of constant proportions.

• Dalton states that atoms are indivisible that they can be further divided into electron, proton and neutron.
• Dalton states that atoms of a given element have exactly the same masses. But, it is known that even atoms of the same element can have different masses just like isotopes.
• Dalton states that atoms of different elements can have different masses. But, it is known that even atoms of different element can have same masses just like isobars.

### Sample Questions

Question 1: Whether it is possible that a molecule is made up of a single atom?

An electrically neutral group of two or more atoms held together by chemical bonds is the most basic definition of molecule. In that sense, no, by nature, a molecule can not be formed from a single atom.

Question 2: Is Salt (NaCl) a molecule?

Molecules are subject to molecular bonds. Something like table salt (NaCl) is a compound because it is made of more than one type of element (sodium and chlorine), but it is not a molecule because it is an ionic bond that holds NaCl together. We can say sodium chloride is an ionic compound.

Question 3: What is the difference between atoms and molecules?

A tiny particle of a chemical element is called an atom, which may or may not exist independently. Molecules refer to the group of atoms that the bond binds together, representing the smallest unit in a compound. Two or more identical or distinct atoms are chemically bonded.

Question 4: What is the mass number?

The complete amount is in the protons and neutrons in the nucleus of an atom. For example, nitrogen has seven protons in its nucleus and seven neutrons, supplying it with 14 masses.

Question 5: Who discovered the atomic number?