AAATOMSTOMSTOMS ANDANDAND M M MOLECULES

1 Ancient Indian and Greek philosophers havealways wondered about the unknown andunseen form of matter . The idea of divisibilityof matter was considered long back in India,around 500 BC. An Indian philosopherMaharishi Kanad, postulated that if we go ondividing matter (padarth), we shall get smallerand smaller particles. Ultimately, a stage willcome when we shall come across the smallestparticles beyond which further division willnot be possible. He named these particlesParmanu. Another Indian philosopher,Pakudha Katyayama, elaborated this doctrineand said that these particles normally existin a combined form which gives us variousforms of the same era, ancient Greekphilosophers Democritus and Leucippussuggested that if we go on dividing matter , astage will come when particles obtainedcannot be divided further.

2 Democritus calledthese indivisible particles atoms (meaningindivisible). All this was based onphilosophical considerations and not muchexperimental work to validate these ideascould be done till the eighteenth the end of the eighteenth century,scientists recognised the difference betweenelements and compounds and naturallybecame interested in finding out how and whyelements combine and what happens whenthey L. Lavoisier laid the foundationof chemical sciences by establishing twoimportant laws of chemical of Chemical CombinationThe following two laws of chemicalcombination were established aftermuch experimentations by Lavoisier andJoseph L. OF CONSERVATION OF MASSIs there a change in mass when a chemicalchange (chemical reaction) takes place?

3 Take one of the following sets, X and Yof chemicals XY(i)copper sulphatesodium g(ii)barium chloridesodium g(iii)lead nitratesodium g Prepare separately a 5% solution ofany one pair of substances listedunder X and Y each in 10 mL in water. Take a little amount of solution of Y ina conical flask and some solution ofX in an ignition tube. Hang the ignition tube in the flaskcarefully; see that the solutions do notget mixed. Put a cork on the flask(see Fig. ).Fig. : Ignition tube containing solution of X, dippedin a conical flask containing solution of ANDANDANDANDAND M M M M MOLECULESOLECULESOLECULESOLECULESOLECULE SC hapter2021-22 SCIENCE32 Weigh the flask with its contentscarefully.

4 Now tilt and swirl the flask, so that thesolutions X and Y get mixed. Weigh again. What happens in the reaction flask? Do you think that a chemical reactionhas taken place? Why should we put a cork on the mouthof the flask? Does the mass of the flask and itscontents change?Law of conservation of mass states thatmass can neither be created nor destroyed ina chemical OF CONSTANT PROPORTIONSL avoisier, along with other scientists, notedthat many compounds were composed of twoor more elements and each such compoundhad the same elements in the sameproportions, irrespective of where thecompound came from or who prepared a compound such as water, the ratio ofthe mass of hydrogen to the mass of oxygen isalways 1:8, whatever the source of water.

5 Thus,if 9 g of water is decomposed, 1 g of hydrogenand 8 g of oxygen are always in ammonia, nitrogen and hydrogenare always present in the ratio 14:3 by mass,whatever the method or the source from whichit is led to the law of constant proportionswhich is also known as the law of definiteproportions. This law was stated by Proust as In a chemical substance the elements arealways present in definite proportions bymass .The next problem faced by scientists wasto give appropriate explanations of these chemist John Dalton provided thebasic theory about the nature of picked up the idea of divisibility ofmatter, which was till then just a took the name atoms as given by theGreeks and said that the smallest particles ofmatter are atoms.

6 His theory was based on thelaws of chemical combination. Dalton s atomictheory provided an explanation for the law ofconservation of mass and the law of Dalton was born ina poor weaver s family in1766 in England. Hebegan his career as ateacher at the age oftwelve. Seven years laterhe became a schoolprincipal. In 1793, Daltonleft for Manchester toteach mathematics,physics and chemistry ina college. He spent most of his life thereteaching and researching. In 1808, hepresented his atomic theory which was aturning point in the study of to Dalton s atomic theory, allmatter, whether an element, a compound ora mixture is composed of small particles calledatoms. The postulates of this theory may bestated as follows:(i)All matter is made of very tiny particlescalled atoms, which participate inchemical reactions.

7 (ii)Atoms are indivisible particles, whichcannot be created or destroyed in achemical reaction.(iii)Atoms of a given element are identicalin mass and chemical properties.(iv)Atoms of different elements havedifferent masses and chemicalproperties.(v)Atoms combine in the ratio of smallwhole numbers to form compounds.(vi)The relative number and kinds ofatoms are constant in a will study in the next chapter that allatoms are made up of still smaller particles. a reaction, g of sodiumcarbonate reacted with 6 g ofacetic acid. The products g of carbon dioxide, gwater and g of sodiumacetate. Show that theseQJohn Dalton2021-22 ATOMS AND MOLECULES33We might think that if atoms are soinsignificant in size, why should we care aboutthem?

8 This is because our entire world ismade up of atoms. We may not be able to seethem, but they are there, and constantlyaffecting whatever we do. Through moderntechniques, we can now produce magnifiedimages of surfaces of elements showing are in agreementwith the law of conservation carbonate + acetic acid sodium acetate + carbondioxide + and oxygen combine inthe ratio of 1:8 by mass to formwater. What mass of oxygen gaswould be required to reactcompletely with 3 g of hydrogengas? postulate of Dalton satomic theory is the result of thelaw of conservation of mass? postulate of Dalton satomic theory can explain the lawof definite proportions? is an Atom?Have you ever observed a mason buildingwalls, from these walls a room and then acollection of rooms to form a building?

9 Whatis the building block of the huge building?What about the building block of an ant-hill?It is a small grain of sand. Similarly, thebuilding blocks of all matter are big are atoms?Atoms are very small, they are smaller thananything that we can imagine or comparewith. More than millions of atoms whenstacked would make a layer barely as thickas this sheet of radius is measured in 9 m=1 nm1 m=109 nmRelative SizesRadii (in m)Example10 10 Atom of hydrogen10 9 Molecule of water10 8 Molecule of haemoglobin10 4 Grain of sand10 3 Ant10 1 AppleFig. : An image of the surface of ARE THE MODERN DAYSYMBOLS OF ATOMS OF DIFFERENTELEMENTS?Dalton was the first scientist to use thesymbols for elements in a very specific he used a symbol for an element healso meant a definite quantity of that element,that is, one atom of that element.

10 Berziliussuggested that the symbols of elements bemade from one or two letters of the name ofthe : Symbols for some elements as proposed byDalton2021-22 SCIENCE34In the beginning, the names of elementswere derived from the name of the place wherethey were found for the first time. For example,the name copper was taken from Cyprus. Somenames were taken from specific colours. Forexample, gold was taken from the English wordmeaning yellow. Now-a-days, IUPAC(International Union of Pure and AppliedChemistry) is an international scientificorganisation which approves names ofelements, symbols and units. Many of thesymbols are the first one or two letters of theelement s name in English. The first letter of asymbol is always written as a capital letter(uppercase) and the second letter as a smallletter (lowercase).