Atoms and Molecules Important Notes

Law of conservation of Mass
It was given by lavoisiier in 1774, according to the law of conservation of mass:Matter is neither created nor destroyed
It means that in a chemical reaction, the total mass of products is equal to the total mass of reactants, there is no change of mass during chemical reaction

It has been found that if 100 grams of calcium carbonate are decomposed completely then 56 grams of calcium oxide and 44 grams of carbon dioxide are formed

CaCo3------------> Cao +    CO2
(100 g)            (56 g)   (44g)




Here calcium carbonate is reactant and calcium oxide and carbon dioxide are products

Law of Constant proportions
The law of constant proportions was given by Proust in 1779.According to the law.A chemical compound always consists of the same elements combined together in same proportion by mass.For example, water is a compound which always consists of the same two elements hydrogen and oxygen combined together in the same constant proportion of 1:8 by mass.








Dalton's Atomic theory
The various postulates of dalton's atomic theory of matter are as follows:
1.All the matter is made up of very small particles called atoms
2.Atoms can not be divided
3.Atoms can neither be created nor destroyed
4.Atoms of a given element are identical in every respect, having the same mass, size and chemical properties
5.Atoms of different elements differ in mass, size and chemical properties.
6.The number of kind of atoms in a given compoud is fixed
7.During chemical combination, atoms of differnt elements combine in small whole numbers to form compounds
8.Atosm of the same elements can combine more than one ratio to form more than one compounds

Drawbacks of dalton's atomic theory
1.Atoms can be further divided into sub particles known as electron,proton, neutron
2.Atoms of same elements can have different mass
3.Atoms of different elements can have same mass

Atoms
An atoms is the smallest particle of an element that can take part in a chemical reaction.Atoms of most elements are very reactive and do not exist in free state.There are as many kind of atoms as there are elements
Atomic radius is measured in nanometres.Hydrogen atom is the smallest atom of all

Modern Symbols of Elements
It was J.J Berzelius of sweden who proposed that the first letter or the first and another letter of the english name or latin name of an element be used as its symbol.
It should be noted that in a two letter symbol the first letter is the capital letter but the second letter is the small letter.
 
Most of the confusion, arises in the symbols derived from the latin names of the elements

 

Atomic Mass

Actual mass of atoms of the elements are very very small. for e.g, one atom of hydrogen has mas of 0.00000000000000000000001673 gram. it is not convenient to use such small and complicated figure.Therefore relative mass is used to get a simple figure

Relative Atomic Mass

Carbon-12 atom has been assigned an atomic mass of exactly 12 atomic mass units.This means carbon-12 atom has been assigned an atomic mass of exactly 12 u.Therefore atomic mass unit is equal to 1/12 the mass of a carbon-12 atom.That is:

Atomic mass unit=1/12 the mass of a carbon-12 atom

One atomic mass unit is defined as exactly one twelth the mass of an atom of carbon-12

Molecule:

A molecule is an electrically neutral group of two or more atoms chemically bonded together.A molecule can exist in free state

Molecules can be formed by the combination of the same element or of different elements

Molecules of Elements

The molecule of an element contains two or similar atoms chemically combined together.Oxygen gas consists of O2 molecules and not of single atoms H.

Atomicity:

The number of atoms present in one molecule of an element is called its atomicity.The atomicity of some of the common elements is given below

Monoatomic

The atomicity of noble gases (helium, neon, argon and krypton) is 1

All metals are also considered to be monoatomic.

Also carbon element has atomicity of 1

Diatomic:

Hydrogen, oxygen, chlorine, bromine and iodine all have two atoms each in their molecule.

Triatomic:

Ozone (O3) has 3 atoms in its molecule, so the atomicity of ozone is 3.

Tetraatomic:

Phosphorus(P4) has 4 atoms in its molecule, so the atomicity of phosphorus is 4.

Polyatomic:

Solid sulphur (S8) has 8 atoms in its molecule, so the atomicity of sulphur is S8

Molecules of Compounds

The molecule of a compound contains two different types of atoms chemically combined together.A molecule of water is made up of two different types of atoms :Hydrogen and oxygen

Ions:

An ion is positively or negatively charged atom.They are formed when atoms lose or gain electrons in order to attain noble gas configuration. If atoms loses electrons it gets positive charge and when it gains electrons it gets negative charge.

Cations:

A cation is formed by loss of one or more electrons.Since they lose electrons so they acuire positive charge.All the metals are cations.Only hydrogen and ammonium are the cations formed by  non-metals.

Metals are also refer to as electropositive since they lose electrons.

For e.g Potassium atom has 19 protons and 19 electrons so its overall charge is neutral, but when it loses one electrons it is left with 18 electrons and protons remain same so it attains overall charge of +1

 

Anion:

An Anion is formed by gaining two or more electrons.They are negatively charged.Anions are mostly non metals.

Therefore non metals are refered to as electronegative as they are formed by gaining of two or more electrons

Polyatomic Ions:

Group of charged particles are refered to as polyatomic ions. e.g sulphate, phosphate, carbonate

HEre is a simple trick to remember polyatomic Ions

As most of the polyatomic ions are formed with oxygen

just remember this Mnemonic device

Nick the camel ate a clam for supper in Phoenix.

Nick represents Nitrate: it has one vowel (i) and three consonants(3). this means it will have charge of 1 and three oxygen atoms (NO₃) and valency 1

Camel represents Carbonate: it has two vowels (a,e) and three consonants (c,m,l).Its charge or valecy will be 2 and it will have three oxygen atoms  (CO₃)

Clam represents Chlorate:IT has one vowel (a) and has three consonants (c,l,m).It charge or valecy will be 1 and it will have three oxygen atoms (ClO₃)

Supper represents Sulphate: It has two vowels (u,e) and four consonants (s,p,p,r), its charge or valency will be 2 and it will have three oxygen atoms (So₄)

Phoenix represets Phosphate: It has three vowels (o,e,i) and has four consonants (p,h,n,x), its charge will be 3 and and it will h ave four oxygen atoms (PO₄)

for Nitrite sulphite and phosphite one oxygen atom will reduce but the charge will remain same

Valency:The number of electrons which an atom can lose , gain or share to form a bond.It is the combining capacity of an atom of the element.

Common Elements and their Valencies

Molecular Mass

The molecularmass 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).

Formula Mass Unit

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

Mole Concept:

A mole is nothing but a constant number just like a dozen or decade or a score

1 mole=6.022 X 10²³

The mole (mol) is the amount of a substance that contains as many elementary entities as

there are atoms in exactly 12.00 grams of 12C

 The Avogadro constant is named after the early nineteenth century Italian scientist
Amedeo Avogadro

For a Sodium atom, its atomic mass=23 u

So its gram atomic mass=23 g

It means 1 mole of sodium atom weighs 23 g and in other words in 23 g of sodium there are 6.022 X 10²³  atoms

And for an Oxygen molecule

Its molecular mass is 2X16u=32u

Which means 1 mole of oxygen weighs 32 g

And 32 g of oxygen contains 6.022 X 10²³ molecules

And for a compound like water

Its molecular mass will be 2X1u+16u=18u

Which means 1 mole of water weighs 18 g

And 18 g of water contains 6.022 X 10²³ molecules

Converting Mass into Mole

 Converting Mass to particles