Wednesday, October 16, 2013

IX Science Ch-3- Atoms and Molecules E- Notes

Ancient Greek philosophers – Democritus and Leucippus suggested that if we go on dividing matter, a stage will come when particles obtained cannot be divided further. Democritus called these indivisible particles atoms (meaning indivisible).
How and why elements combine and what happens when they combine. Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.
(a) The Law of conservation of mass was stated by Antoine L. Lavoisier as” Mass can neither be created nor destroyed in a chemical reaction”
Verification of “Law of Conservation of mass”
 A solution of sodium chloride and silver nitrate are taken separately in the two limbs  of an 'H' shaped tube. The tube is sealed and weighed precisely. The two reactants   are made to react by inverting the tube. 
The following reaction takes place.
 AgNO3(aq) + NaCl (aq) à AgCl (s) + NaNO3 (aq)
 The whole tube is kept undisturbed for sometime so that the reaction is complete.  When the tube is weighed again it is observed that:   Weight before the reaction = Weight after the reaction
(b) The law of constant proportions which is also known as the law of definite proportions was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.
E.g. In a compound such as water, the ratio of the mass of hydrogen to the mass of oxygen
is always 1:8, whatever the source of water. Thus, if 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.
According to Dalton’s atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound
Atom: It is the smallest particle of an element which may or may not have independent existence. The atoms of certain elements such as hydrogen, oxygen, nitrogen, etc .do not have independent existence whereas atoms of helium, neon, argon, etc. do have independent existence. Thus we can say that all elements are composed of atoms.
IUPAC (International Union of Pure and Applied Chemistry) approves names of elements. Many of the symbols are the first one or two letters of the element’s name in English. The first letter of a symbol is always written as a capital letter (uppercase) and the second letter as a small letter (lowercase)
For example :  (i) hydrogen, H (ii) aluminium, Al and not AL (iii) cobalt, Co and not CO.
Symbols of some elements are formed from the first letter of the name and a letter, appearing later in the name. Examples are: (i) chlorine, Cl, (ii) zinc, Zn etc.
Other symbols have been taken from the names of elements in Latin, German or Greek. For example, the symbol of iron is Fe from its Latin name ferrum, sodium is Na from natrium, potassium is K from kalium. Therefore, each element has a name and a unique chemical symbol.
Molecule: A molecule is the smallest or the simplest structural unit of an element (or) a compound which contains one (or) more atoms. It retains the characteristics of an element. A molecule can exist freely and it is a combined form of bonded units whereas an atom is a singular smallest form of non bonded unit.
Molecules are of two types, namely homo atomic molecules and hetero atomic molecules.
Homo atomic molecules: These are the molecules which are made up of atoms of the same element. For example hydrogen gas consists of two atoms of hydrogen (H2).Similarly oxygen gas consists of two atoms of oxygen (O2).
HETERO ATOMIC MOLECULES : The hetero atomic molecules are made up of atoms of different elements. They are also classified as diatomic, triatomic, or polyatomic molecules depending upon the number of atoms present. H2O, NH3, CH4, etc., are the examples for hetero atomic molecules.
Atomicity:   The number of atoms present in one molecule of an element is called the atomicity of an element. Depending upon the number of atoms in one molecule of an element, molecules are classified into monoatomic, diatomic, triatomic or poly atomic molecules containing one, two, three, or more than three atoms respectively.
Mon atomic molecules: Helium (He) Neon (Ne) Metals
Di atomic molecules:   Hydrogen H2 Chlorine Cl2
Tri atomic molecules: Ozone (O3)
Poly atomic molecules: phosphorous P4 Sulphur S8
Atomicity = Molecular Mass/Atomic mass
MORE TO KNOW  :
Isotopes   These are the atoms of same element with same atomic number (Z) but different mass number (A). Example (17Cl35,17Cl37 )
Isobars   These are the Atoms of the different element with same mass number but different atomic number. Example (18Ar4020Ca 40 )
 Isotones  These are the atoms of different elements with same number of neutrons Example : (6C137N14 )
AVOGADRO’S HYPOTHESIS  : Amedeo Avogadro put forward hypothesis and is based on the relation between number of molecules and volume of gases that is “volume of a gas at a given temperature and pressure is proportional to the number of particles”.
Avogadro’s Law: Equal volumes of all gases under the same conditions of temperature and pressure. contain the equal number of molecules.
TEST YOUR UNDERSTANDING SKILL
 (a) Find the atomicity of chlorine if its atomic mass is 35.5 and its molecular mass is 71 
(b) Find the atomicity of ozone if its atomic mass is 16 and its molecular mass is 48
WHAT IS AN ION? 
An ion is a charged particle and can be negatively or positively charged.
A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. For example, sodium chloride  (NaCl). Its constituent particles are positively charged sodium ions (Na+) and negatively charged chloride ions (Cl–).
Ions may consist of a single charged atom or a group of atoms that have a net charge on them.
A group of atoms carrying a charge is known as a polyatomic ion e.g. Calcium oxide (Ca+2 O-2)
Ionic compound A compound which consist of ions is called ionic compound like NaCl (Na+1  Cl-1   Read more of this post   Read more »
Related post to 9th  Atoms and Molecules

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