Atoms And Molecules Class 9 Notes

Introduction

Philosophers did much research on the atoms and molecules. Atoms are nothing but, it is smaller particles that we can see in their research work. It plays a major role in the formation of the compound. Indian philosopher Pakudha Katyayan stated that the combination of atoms can exist normally. And others have also done so, like Democritus and Leucippus, Greek philosophers combined and did their work, and stated that at one stage of time, particle division will not occur, and it is impossible to progress. Democritus only defined that the individual particles are atoms, which means indivisible. Through all their research today, we all learn what an atom is and how it forms, and also its molecular mass. Many scientists worked on this in the 18th century.

Chemical Combinations

In the 18th century, many experiments were done on chemical combinations. By all these chemical combinations, the scientists become more interested in the work. The people carefully learn how to easily balance chemical reactions. And it contains major progress and many changes occur, like changes in masses and also in reactions. French chemist Antoine Lavoisier discovered many chemical reactions. He once heated the mercury, and he sealed the mercury in the air. In that the air is contained when the chemical combinations happen. After a few days, he observed that it contained the mercury oxide produced due to the chemical reactions. Gas lately remained due to the presence of reduced mass.

Experiments

Lavoisier is doing the chemical reactions, he very carefully doing the experiment and he also weighed the mercury oxide. Then sometimes he heated strongly, the mercury oxide turned red in colour. He finally concluded that in all products, chemical reactions reactants react equally for all the total masses of components, it does not change other way, and it is equal to all the reactants.

Others like Claude Berthollet and Joseph Proust started their work on this, like the ratio of two masses for the combination of the compound. In the year 1808, they stated that the fundamental law is constant proportions. Many scientists thought wisely about the atoms, and also with their scientific thoughts, they learn many modern sciences due to their experimental findings. The scientists worked hard to find the real truth, and also, by doing their work, they discovered many theories and laws for the development of science.

Dalton’s  theory

John Dalton is one of the great English scientists in history. He made many experiments on the existence of atoms in his classical times. AlsoHe contributed to the evidence of atoms and he worked hard for the existence of the atoms in a proper way. He stated a new system of philosophy in the atomic theory of matter in the year 1803, like;

• The matter consists of indivisible atoms in his work
• He noticed that all the atoms in his experimental chemical element are very similar. When compared to others in mass, in different properties.
• Different atoms contain different chemical elements and also have different masses.
• While doing the chemical reactions, the atoms are in an indestructible state and easily regain their identity.
• A combination of atoms makes to helps the formation of compounds. And its elements in their particular ratio.

Dalton clearly stated the law of conservation of mass in his fourth postulate. He also defines that every atom contains a different mass, and different reactions occur for the formation of the compound. The compound is formed due to the mixture of one atom and other atomsand also more atoms unitedly form one compound.

Atom

• An atom is nothing but it is a small particle that cannot be indivisible into smaller particles, according to many Indian and Greek philosophers.
• But after much research, today we know that the articles are not indivisible in nature on the basis of the philosophy
• The atoms are not in same size, shape, and structure, and also it contains different masses.
• Atoms are very small, so we can compare these atoms to anything that we can easily understand about the structure of the atom.
• Many people think that atoms are like in cylindrical, circular, spherical, or rectangular in shape, but based on the many opinions, they think that the atoms are spherical in size.
• The size of an atom is so small that if we could see the atom with our eyes, it would be about a nanometer.

Atomic Mass

The atomic mass concept was given by Dalton. He stated that the atoms’ masses are the same atoms but different in different atomic masses. In those days, he made many proportions and also made each element with water, in various ways. He did his relative atomic masses by taking on the hydrogen scale, but after some days it was replaced by the atomic mass scale.  After it was also replaced by the instrument called the mass spectrometer, it was invented in the 20th century. The mass spectrometer is used to determine atomic masses with clear knowledge. The atomic masses nowadays it is expressed by the letter ‘u’. The atomic mass of an element is called its atomic weight. Dalton also said that atomic masses are equal in all elements.

Molecule

• The definition of a molecule estimates that the combination of two or more atoms containing different elements is known as a molecule.
• Many scientists have defined molecules in their own ways, but Dalton stated that a molecule is the combination of atoms.
• The difference between an atom and a molecule is atom is small but will not exist freely. But if we look at the molecule, it is also small. Therefore, it can easily exist freely in its own conditions.
• The molecule helps us to know about all the chemical particles in the chemical composition. For example, Oxygen has two atoms., Phosphorus has four atoms.
• If the molecule contains four atoms, it’s called a ‘tetratomic molecule’; when the molecule contains more than four atoms, it’s called a ‘Polyatomic molecule’.

Molecular mass

The molecule contains the formula that represents the molecular formula of a compound. The molecular formula is used to determine the molecular mass of the substance. The sum of all atomic masses in the molecule is also known as the molecular mass. If we mix the two atoms, we will get one or more new substances of molecules. Like if we mix the elements, Hydrogen and oxygen, we will get the Hydrogen oxide, named as chemical equation of water. We can also see this concept of  ‘mole’. The count of atoms and molecules can be defined as a mole.  The symbol of the mole can be defined as the  ‘mol’.

Chemical Formula of Compounds

If we consider the compounds, every compound has a chemical formula. Based on the law of atomic theory, the chemical formula is arranged according to the combination of atoms. Na for sodium, H for hydrogen, P for phosphorus, etc.. The chemical formula is the short form of the compounds containing the element name and the number of elements. It can also be called the chemical composition. If we see the atoms and molecular elements contain their numbers and symbols, as well as the subscript at the bottom.

Conclusion

Based on all this information, we can easily learn about the Atoms and Molecules, molecular and atomic masses, and also the structure of the atoms. Therefore, it contains the mole concept, and also we can easily learn about the chemical composition and its formula, and by learning all these, we can easily learn about the valency of the molecules, ionic compound formulas.