CHSS CHEM 123 - Atoms and molecules

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IG Avogadra s coaataat or Avogadro s muober The numbs of particles present in o e mole i e 6 023 x l0 particles is called Avogadro s number or Avogadm s constant I mole electrons 6 023 x IN elecooas I mole protons 6 023 x I protons I mole onx 6 023 x IN ione I mole molecules 6 023 x I molecules 17 Mole 1 mole 6 023 x 10 particles 22 4L of a gzs at N T P I mole atoms gram atomic mass I mole molecules gram molecular mass N T P stands For normal temperature 0 Cj and nonnal pressure t I aomspher oi 76 mm of Number of moles Mass of substance in grams grams molecular mass Number of moles Volume of gas in litre at N T P 22 4 Number of moles num6er of panicles 6 023 x 10 i Caiion It is positively charged ion and is formed by ihe loss of electron from an atom e g H Na Ca Al NH etc ii Anion It is negatively charged ion and is formed by the gain of electrons by an atom e g Cl O C F CO PO eic 9 Valency The combining power or capacity of an element is known as its valency 10 Fomiula of simple and molecular campoiuids Binary compounds are those compounds which are made up of two different elements e g NaC1 KBr CaO eic Following rules are io be followed for uniting the formula i The valencies or charges on ihe ions niusi be balanced ii For a compound made up of a meial and a non nieial the symbol of metal is written firsi iii In compounds formed with polyatomic ions the ion is enclosed in a bracket before writing the number to indicate the ratio 11 Atomic mass of an element The atomic mass of an element is the numerical number which indicates how many times an atom of an element is heavier than 1 12 of mass of an aioni 12 of carbon 1 2 For example the atomic mass of magnesium Mg is 24 which indicates that one atom of magnesium is 24 times heavier ihaii 1712 of a carbon 1 2 aioni 12 Caram atomic mass The atomic mass of an element expressed in grants is known as grant atomic mass Grain atomic mass is also known as gram atomic weight 13 Molecular mass The number of iinies a molecule of a compound is heavier ihaii the 1 1 2 of the mass of C 1 2 aiom is known as its molecular mass The molecular mass is equal to ihe sum of ihe atomic masses of all aionis present in one molecule of the substance For example H2 contains two atoms of hydrogen so molecular mass of H is 2 2 x I 2 14 Formula unit mass It is equal io the sum o atomic masses of all ihe aionis in a forniul unit In case of ionic compounds like NaCl forniul mass is SS 5 23 35 5 SS 5 15 Molecular Mass and Mole Concept Mole the unii of a substance It is a collection of fi 023 x 10 particles The mass of I mole particles is equal io its mass in grams Thus I mole fl 023 x 10 1 mole atoms 6 023 x articles Science Class 9 Notes Atoms and Molecules 1 Law of conservation of mass This law was siated by Lavoisier in 1744 It siates ihat In all physical and chemical changes the iotal mass of reacianis is equal io toial mass of products 2 Law of constant proportions or constant com position This lnw w zs first stnied by Proust in 1797 According io the law a chemical compound is alw nys found io he ninde up of the swine elements combined together iii ihe same proportions by w eight e g the ratio of hydrogen and oxygen in pure waier is alw ays 1 S by weight This lnw is also called law of definite proportions 3 Dalton s Atomic theary Postulates of Dalion Atomic Theory Mniter i s innde up of extremely small indix isible particles called atoms Atoms of the same substance are identical in all respects i e ihey possess same size shape mass chemical properties eic Atom s of different substances are different in all respects i e they posses s different size shnpe mass eic Atom is the smallest particle ihni takes pari in a chemical reaction Atoms of different elements May combine with each other iii a fixed simple whole number ratio to form compound atoms Atoms can neither be created nor destroyed i e atom s are indestructible 4 Atorii It is the smallest particle of an element which can take part iii a chemical change It may or mm nor be cnpnble of independent existence 3 Symbol The abbreviation used for lengthy names of elements are kenned as their symbols The symbol of an element i s ihe firsi letter or the first and another leiter of English mine or Latin name of the element While w riting a symbol the firsi letier is alw nys capital and the second i s always small ft Molecule It is the smallest particle of ae element or compound ihat is capable of inde pendent existence and shows all ihe properties of that substance The molecules of an element is made up of only one and same lype of moms while the molecule of n compound is innde up of dissimilar moms 7 Atoriiicity The number of atoms present in a molecule of an element or a compound is known as iis atomicily e g the atomiciiy of oxygen is 2 while nioniicity ozone is 3 8 Ion It is cii electrically charged morn or group of mom It is formed by the loss or gain of electrons by an atom lone are of two types


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CHSS CHEM 123 - Atoms and molecules

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