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Contents : Laws of Chemical Combination About Atoms About Molecules About Ions Writing Chemical Formulae Molecular Mass & Mole Concept Multiple Choice Questions Some Interesting Pictures
Laws of Chemical Composition The following two laws of chemical combination were established after much experiments by Lavoisier and Joseph L Proust :(i) Law of conservation of mass : - Law of constant of mass states that mass can neither be created nor destroyed in a chemical reaction.
(ii) Law of Constant Proportions - Lavoisier, along with other scientists, noted that many compounds were composed of two or more elements and each such compound had the same elements in the same proportions, irrespective of where the compound came from or who prepared it.
In a compound such as water, the ratioof the mass of hydrogen to the mass ofoxygen is always 1:8, whatever the sourceof water. Similarly in ammonia, nitrogenand hydrogen are always present in theratio 14:3 by mass, whatever the methodor the source from which it is obtained. This led to the law of constantproportions which is also known as theLaw of Definite Proportions.
About AtomsAccording to Dalton’s atomic theory, allmatter whether an element, a compoundor a mixture is composed of small particlescalled Atoms.
The postulates of this theory may be started 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.
The building blocks of all matter are atoms. Atoms are very small. They are smaller than anything we can imagine or compare with.
► Atomic radius is measured in nanometers.
Relative Sizes :
Modern Day Symbols ofAtoms of Different Elements
Symbols for Some Elements Aluminum : Al
Argon : Ar
Barium : Ba
Boron : B
Bromine : Br
Calcium : Ca
Carbon : C
Chlorine : Cl
Cobalt : Co
• Copper : Cu
► Fluorine :F
► Gold : Au
Hydrogen : H
Iodine : I
Iron : Fe
Lead : Pb
Magnesium : Mg
• Neon : Ne
Nitrogen : N
Oxygen : O
Potassium : K
Silicon : Si
Silver : Ag
Sodium : Na
• Sulphur : S
► Uranium :U
Zinc : Zn
Phosphorus : P
Platinum : Pt
Mercury : Hg
• Nickel : Ni
Tin : Sn
Bismuth : Bi
Antimony : Sb
Arsenic : As
Manganese : Ma
Tungsten : W
Titanium : Ti
• Cerium : Ce
Strontium : Sr
Beryllium : Be
In 1961 a university accepted atomic mass unit, carbon-12 isotope was chosen as the standard reference for measuring atomic masses.
One atomic mass unit is a mass unit equal to exactly one twelfth the mass of one atom of carbon-12.
Atomic Masses of Few Elements Element Atomic Mass (u)(i) Hydrogen 1(ii) Carbon 12(iii) Nitrogen 14(iv) Oxygen 16(v) Sodium 23(vi) Magnesium 24
Element Atomic Mass (u)(vii) Sulphur 32(viii) Chlorine 35.5(ix) Calcium 40
Atoms of most elements are not able to exist independently.
Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.
About Molecules A molecule is a general group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces.
A molecule can be defined as the smallestparticle of an element or a compound & iscapable of an independent existence andshows all the properties of that substance.
Atoms of the same element or of differentelements can join together to formmolecules.
Molecules of Elements• The molecules of an element are constituted by the same type of atoms.
► Moleculesof an element are constituted by the same type of atoms.
► Moleculesof many elements, such as argon(Ar), helium(He),…etc. are made up of only one atom of that element.► But, this is not the case with most of the non-metals.
For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic module O2. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone.
The number of atoms constituting a molecule is known as its Atomicity.
Atomicity of Some Elements Type of Element : Non – Metal Name Atomicity(i) Argon - Monoatomic(ii) Helium - Monoatomic
Type of Element : Non – Metal Name Atomicity(i) Oxygen Diatomic(ii) Hydrogen Diatomic(iii) Nitrogen Diatomic(iv) Chlorine Diatomic
Type of Element : Non – Metal Name Atomicity(i) Phosphorus Tetra - atomic
Type of Element : Non – Metal Name Atomicity(i) Sulphur Poly-atomic
Atoms of different elements join together in definite proportions to form molecules of compounds.
Molecules of Some Compounds Compound : Water Combining Elements : Hydrogen, Oxygen Ratio by Mass : 1:8
Compound : Ammonia Combining Elements : Nitrogen, Hydrogen Ratio by Mass : 14:8
Compound : Carbon Dioxide Combining Elements : Carbon, Oxygen Ratio by mass : 3:8
About Ions Compounds composed of metals and non metals contain charged species. The charged species are known as Ions.
An ion is a charged particle and can benegatively or positively charged.
A negatively charged ion is called an‘Anion’ and the positively charged ion iscalled a ‘Cation’.
Some Ionic Compounds• Ionic Compound : Calcium Oxide Constituting Elements : Calcium & Oxygen Ratio by Mass : 5:2
► IonicCompound : Magnesium Sulphide Constituting Elements : Magnesium & Sulphur Ratio by Mass : 3:4
► IonicSolution : Sodium Chloride Constituting Elements : Sodium & Chlorine Ratio by Mass : 23:35.5
Molecular Mass & Mole Concept The molecular mass of a substance is the sum of the atomic masses of all the atoms in a molecule of the substance.
It is therefore the relative mass of a molecule expressed in atomic mass units (u).
The formula unit mass of a substance is a sum of the atomic masses of all atoms in a formula unit of a compound.
Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions.
EXAMPLE : Reaction of hydrogen and oxygen to form water: 2H2 + O2 2H2O The above reaction indicates that :(i) Two molecules of hydrogen combine with one molecule of oxygen to form two molecules of water. Or,(ii) 4 u of hydrogen molecules combine with 32 u of oxygen molecules to form 36 u of water molecules.
We can infer from the above equation that the quantity of a substance can be characterised by its mass or the number of molecules. But, a chemical reaction equation indicates directly the number of atoms or molecules taking part in the reaction.
Therefore, it is more convenient to refer to the quantity of a substance in terms of the number of its molecules or atoms, rather than their masses.
So, a new unit “Mole” was introduced.
Multiple Choice Questions Q 1 : A molecule does not signify(a) Atomic mass unit(b) 22.4 litres of a gas at STP(c) Gram molecular mass(d) None of the above
Answer :Option : (a) Atomic mass unit
Q 2 : Which of the following does not change when a chemical reaction takes place ?(a) Volume(b) Mass(c) Physical properties(d) Chemical properties
Answer :Option : (b) Mass
Q 3 : Molecules of the following element are made up of only one atom of that element:(a) Iron(b) Sodium(c) Helium(d) Chlorine
Answer :Option : (c) Helium
Q 4 : Law of definite proportion was given by(a) John Dalton(b) Lavoisier(c) Joseph Proust(d) Ritcher
Q 6 : Which of the following supports Daltons atomic theory of matter?(a) Formation of isotopes(b) Formation of isobars(c) Law of multiple proportion(d) Graphite and diamond though made from carbon, show different properties.