Atoms and Molecules CBSE Class 9 PDF

Atoms and Molecules CBSE Class 9 Science Revision Notes Chapter 3 Class 9 revision notes for Science Chapter 3 Atoms and Molecules are created by askIITians experts for all students. These notes are a consolidated summary of the chapter including all the important points, diagrams and tables. You c...

Atoms and Molecules CBSE Class 9 Science Revision Notes Chapter 3 Class 9 revision notes for Science Chapter 3 Atoms and Molecules are created by askIITians experts for all students. These notes are a consolidated summary of the chapter including all the important points, diagrams and tables. You can use these notes to understand all the topics of the chapter, do a quick revision before the exam or memorise the main points of the chapter. These online revision notes are available for free for all students. Refer to them whenever you want and study them at your own pace. The main topics included in the revision notes for CBSE Class 9 Chapter 3 Atoms and Molecules are the laws of chemical combinations, laws of constant proportions, the structure of atoms, symbols of different elements, atomic mass, introduction to molecules, ions, chemical formulae of different compounds, formulae of simple compounds, molecular mass, formula unit mass, and mole concept. At askIITians, you can also find NCERT Solutions for Class 9 Science Chapter 3 Atoms and Molecules along with Class 6, 7, 8, 9, 10, 11, 12 notes for Science and Maths. Online Revision Notes on Atoms and Molecules The idea of divisibility by Indian philosophers Maharishi Kanad – He postulated that if we keep on dividing the matter (called ‘padarth’) we will get smaller and smaller particles. And soon we will achieve the smallest of particles (called as ‘parmanu’) which may not divide further. Pakudha Katyayama – He postulated that there are various forms of matter because the particles of matter exist together in combinations. The idea of divisibility by Greek philosophers Democritus and Leucippus – They suggested that when we keep on dividing the matter there comes a time when no more division of particles can take place. Such particles are called atoms which means being invisible. But all these ideas were not backed up by many experimental pieces of evidence until Antoine L. Lavoisier provided two laws of chemical combination. Laws of Chemical Combination 1. Law of conservation of mass – mass can neither be created nor destroyed in a chemical reaction 2. Law of constant proportion/Law of definite proportion – the elements are always present in definite proportions by mass in a chemical substance For example, Hydrogen and oxygen are present in water in a ratio of 1:8. So if we decompose 9g of water we will obtain 1g of hydrogen and 8g of oxygen. The Atomic Theory John Dalton proposed an atomic theory that acted as an explanation of the above two laws. As per the theory, all matter whether it is an element, a compound or a mixture consists of tiny invisible particles called ‘atoms’. The postulates of the atomic theory by John Dalton 1. The matter is made up of tiny particles called atoms that cannot be divided. 2. Atoms are never formed or destroyed during a chemical reaction. 3. Atoms of an element exhibit the same nature. They have the same size, mass, and character. 4. Atoms of different elements exhibit variant nature. They do not have the same characteristics. 5. Atoms form compounds by combining them in a ratio of whole numbers. 6. A compound contains a constant number and kinds of atoms Atoms We can call atoms as the building blocks of matter. Just like bricks are the building blocks of a building. What is the size of an atom? Atoms are extremely small. Their size is measured in nanometers, where 1nm = 1/109 m. Atomic radius is measured in nanometers 1/109 = 1nm 1m = 109 nm Relative Sizes Radii (in m) Example 10-10 Atom of hydrogen 10-9 Molecule of water 10-8 Molecule of haemoglobin 10-4 Grain of Sand 10-2 Ant 10-1 Watermelon Symbols for Atoms Here are some examples of the symbols that are used to represent different atoms The symbols for representing an atom are generated from the first two letters of the element’s name. The first letter is always in uppercase (capital letter) while the second letter is written in lowercase. Here are some examples – The Atomic Mass Dalton’s Atomic Theory suggested that each element has a distinguishing atomic mass. With this theory, the law of constant proportions could be explained easily. But it is indeed difficult to evaluate the mass of an atom since the size of an atom is relatively small. Therefore scientists started evaluating the mass of an atom by comparing it with the mass of a standard atom. Earlier 1/16 of the mass of an oxygen atom was used as a standard for calculating the mass of other elements. Now, carbon - 12 is considered a standard atom for calculating the mass. Its atomic mass is 12u (12 atomic mass units). Thus we can say that one atomic mass unit is the mass of 1/12 the mass of a carbon-12 atom. Here is a list of atomic masses of a few elements. Element Atomic Mass Hydrogen 1 µ  Carbon 12 µ Nitrogen 14 µ Oxygen 16 µ Sodium 23 µ Magnesium24 µ Sulphur 32 µ Chlorine 35.5 µ Calcium 40 µ Can atoms exist independently? Atoms cannot survive independently. So, atoms join together and form molecules or ions. Molecule A molecule is a collection of various atoms that combine chemically with each other. These atoms are bound together by certain forces of attraction. Atoms of the same elements or different elements can bind together to form molecules. Therefore, a molecule is the smallest particle of a substance that can exist independently and shows all the properties of that substance.