ATOMS AND MOLECULE PART -1
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The document discusses the history of ideas about atoms and molecules from ancient Indian and Greek philosophers to modern atomic theory. Some key points: 1) Ancient philosophers proposed ideas of matter being divisible into increasingly smaller particles, but lacked evidence. 2) In the late 18th century, Lavoisier established laws of conservation of mass and definite proportions through experiments. 3) In the early 19th century, Dalton combined previous ideas with evidence to propose atomic theory - that all matter is made of tiny indivisible particles called atoms that combine in whole number ratios. 4) Atoms are represented by symbols, have specific masses, and combine to form molecules - the smallest units of compounds that exhibit chemical properties
ATOMS AND MOLECULE PART -1
- 2. DIFFERENT IDEAS The idea of divisibility by Indian philosophers Maharishi Kanad – He postulated that if we keep on dividing the matter (called as ‘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. .
- 3. . 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.
- 5. Laws of Chemical Combination Antoine Lavoisier gave this law in the year 1789 based on the data he obtained after carefully studying numerous combustion reactions. This law states that matter can neither be created nor destroyed. In other words, the total mass, that is, the sum of mass of reacting mixture and the products formed remains constant.
- 6. Law of Definite Proportions Joseph Proust, a French chemist stated that the proportion of elements by weight in a given compound will always remain exactly the same. In simple terms we can say that, irrespective of its source, origin or its quantity, the percent composition of elements by weight in a given compound will always remain the same.
- 7. The Atomic Theory 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 same nature. They have the same size, mass, and character. 4. Atoms of different elements exhibit variant nature. They do not have same characteristics. 5. Atoms form compounds by combining in a ratio of whole numbers. 6. A compound contains a constant number and kinds of atoms.
- 8. Atoms Atoms as the building blocks of matter. Just like bricks are the building blocks of a building. Atoms are extremely small. Their size is measured in nanometers where 1nm = 1/109 m.
- 9. symbols that are used to represent different atoms
- 10. writing symbol 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.
- 12. The Atomic Mass 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.
- 13. 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.
- 14. Molecules of Elements The molecules of an element are formed by combinations of similar types of atoms. For example, Helium (He) is made up of only one atom while oxygen is made up of two atoms. Atomicity – The number of atoms in a molecule of an element is called its atomicity. For example, helium is monoatomic and oxygen is diatomic. Monoatomic – When an element comprises of a single atom. Example – all metals Diatomic – When an element comprises of two atoms. Example – all gases Triatomic – When an element comprises of three atoms Tetra-atomic – When an element comprises of four atoms Poly-atomic – When an element comprises of more than two atoms
- 16. Molecules of Compounds Molecules of compounds constitute atoms of different elements that combine together in a fixed proportion. For example, water comprises of two atoms of hydrogen and one atom of oxygen.
- 17. Ions Compounds contain metals as well as non-metals. These elements include charged species which are known as ions. Thus, ion is a particle that has a positive or negative charge. Anion – negatively charged ion Cation – positively charged ion There can be a single charged atom in an ion or there may be a group of charged atoms in an ion that have a net charge on the compound. When a group of atoms carries a charge in a compound it is called as a polyatomic ion.
- 18. Chemical Formula We use a chemical formula to represent the composition of a compound in the form of symbols. To write a chemical formula you must know two things: 1. Symbols of elements 2. Valency Valency – It is also known as the combining capacity of an element. In other words, valency explains how atoms of one element will mix with atoms of another element. For example, the hydrogen ion is represented as H+ which means that its valency is 1. Similarly, the oxygen ion is represented as O2- which means that its valency is 2.
- 20. Rules of writing a Chemical Formula Valencies of on the ions must balance. In a case where both metal and non-metal substances are present in a compound, the name of the metal is always written first in the chemical formula. For example, Sodium Chloride is written as NaCl In case of polyatomic ions, the ion is written in brackets before writing the number of ions associated to it. In case of a single ion, there is no need to mention the ion in brackets.
- 21. How to write a Formula of a Compound Write the symbols of the corresponding elements of the compound as explained above. Write the valencies of the elements of the compound. Crossover the valencies of the elements.
- 22. Writing of chemical formula