ATOMIC STRUCTURE AND CHEMIICAL BONDING [chapter 2 ]

Chapter 2

ATOMIC STRUCTURE AND CHEMIICAL BONDING

ATOMS =

An atoms is the smallest particle which of element which can take part in a chemical reaction

Atoms cosists of three fundamental parts

Proton

Electron

Neutron

Elestron = it a fundamental subatomic particle that carries negative electric charge .

Proton= that carries a positive electrical charge

Neutron = the neutron is a subatomic particle with no net electric charge found in  nucleus .its  antiparticle is called antineutron

Fundamental concepts about atoms

1 number of electron and proton are equal in an atom

2 the charge of proton and electron are equal in magnitude ;yet in opposite side electron and proton are electrically attracted to each other

3 Almost all the mass of an atoms is in its nucleus ; while most of the volume

4 the particle with in an atoms are bound together by powerful forces . in general  electrons are easier to add or remove to an atoms than a proton or a neutron .

5 proton are +charged while electrons are –charged particle .neutrons are neutral particles.

Dalton’s atomic  theory

Matter is made up of small particle ; which cannot be subdivided .

John Dalton carried out a number of experiment with different elements that one another .

postulates of Dalton Atomic theory

1 matter is made up of extremely small invisible particle called atoms

2atoms  of different elements have different weight and passess different properties .

3 atmos can neither be created nor be destroyed

4  atmos of same elements are similar in all respects and have definite weight

Rutherford”s scattering experiment

Alpha particle are positively charged particle with two units of positive charge and mass nearly four times that of a proton

A movable screen coated with zinc sulphide was placed at the back of gold sheet to detect whether @particle under go any deviation in their path on passing though the sheet or not

1 mass of alpha particle pass straight through the gold foil

2 some of these were deflected by small angles <90

3 every few alpha particle suffered major deflections by more than 90 and even reflection back in the same direction

THE SCATTERING OF @PARTICLE BY ATOMS OF GOLD

From these observation

1 most of the atomic space is hollow

2 the central portion of atoms carries positive charge which repels the positively charged @particle

3 the central portion of atoms is solid and carries nearly the whole of the mass of the atoms

RUTHERFORD’s  Nuclear of    model of atoms

On the basis of his observations in gold foil experiment , Rutherford suggested a nuclear model model of atoms called Rutherford  model of atom .  According to this model following picture of structure of atoms comes out

1 Each atoms consist of a small but massive positively charged portion at its centre is called the nucleus .

2 the nucleus of each atoms is surrounded by extremely small negative charged particle called electrons .

3 the number of electrons around the nucleus is equal to the net positive charged

  Bohr’s Model of atoms

New theory of atomic structure known as bohr’s  theory .

According to this theory energy cannot be lost or gained gradually but discontinuously in the form of small packets or bundles called quanta .

The  main postulates of bohr theory as follow

1 an atoms consist of a small heavy positively charged nucleus in the centre and the electrons revolve around iy in certain permitted circular paths  called  orbits.

2 As long as an electrons remain in particular orbit , it does not losse or gain energy even through it continues to resolve around the nucleus .

3 only those orbits are permissible in which the angular momentum of the electron is a whole number multiple of the quantity .

Atomic number

The atomic number of an elements may be defined as the number of protons present in the nucleus .

Mass number

The sum of an elements present in the nucleus of an atoms is called mass number it is denoted by A.

Isotopes

Atoms of an elements which possess the same atomic number but different mass number are called  isotopes.

Isobars

Atoms of different elements which possess the same mass number but different atomic number are called isobars .

Isotones 

The atoms of different elements which contain the same number of neutrons are called isotones .

Orbit

It is defined as which is a well defined circular path traced by the electron .

Orbital

It is the region in space where the probability of fnding the electron is maximum .

Difference between orbit and orbital

Orbit                                     

1 It is well defined circular path traced by the electron .

2 it has two dimensional representations.

3 it is always circular

4 the path of the electron can be exactly followed

5 in it the number of electrons is 2n square

 Orbital

1 it is the regaion in shape where the probability of finding the electron is maximum .

2 it has three dimansional representations .

3 it can have different shape its shape depend on its energy

4 the path of the electron on exact . it is probable

5 the number of electron in it is at the most 2

Shape of orbitals

S shape orbitals

The spatial distribution of  an  electron  is spherical symmetrical .

P orbital

In case of p electron the probability of finding it is more some directions from the nucleus than in others. keeping in view the probability distribution curve of the p electron the p orbital may be thought as made up of two lobes.

Quantum number

Sate of four  numbers  which give information about an electrons

    1 princile quantum number

2  Azimuthal quantum number

3 magnetic quantum number

4 spin quantum number

Principle quantum number

This number largely determines the energy of the electrons in hydrogen atom

 the energy of electron is inversely proportional to n square as stated under  bohr S theory

Azimuthal or secondary quantum number

 the number determine the shape of electron S orbit and to some extent the energy of the atoms due to the angular momentum of the electron .

spin quantum number

this quantum number doe not result from the solution of schrodinger  wave equation

this is due to the spinning of the election about its own axis .

clockwise and anticlockwise spin of electron about their own axis producing opposite  magnetic field

magnetic  quantum number

this quantum number describes the behavior of electron in magnetic field and designed as m.

representation of electronic configrution

the distribution of electron I different shells subshells and orbitals of an atoms is called its electric configuration.

Aufbau Principle

Accourding to this principle ,the electrons tend to occupy orbitals of minimum energy in the ground state of an atoms.

Octet rule

The principle of acquiring 8 electrons in valence shell is called octet rule .

Limitations of octet rule ………………

1 The formation of electron deficient compound suh bf three and ALCL three where central atom have six electrons instead of eight electron , cannot be explained .

 2 the formation of compounds such as pcl five anf sF six where central  atoms have 10 or 12 electron instead of 8 also cannot be explained .

3 formation of compound of noble gas like xef4 cannot be expected though such compounds have been actuall prepared .

Electrovalent or bond

The bond formation by complete transfer of electrons from one atom to other atom to complete thir outer most shell is called ionic bond .

Gernal characteristics of ionic compounds

Physical state

These compounds generally exist in the solid state because their oppositely charged ions attract one another steongly and form a regular crystal structure . the crystal are hard and brittle .

High melting and boiling point

These compounds have melting and boiling point , because the ions are tightly hed together by strong electrostatic  forces of attraction and lot of  energy is required to break crystal lattice .

Solubility

these compounds are polar and hance soluble in polar solvent  like water and other which have high dielectric constant . it is because the polar solvent ineract with the ions of crystal and further high dielectric constant of the solbent cut off the forces of attraction between these ions . further more  the solvent mayreact with ions to liberate energy called hydration energy which is sufficient to overcomes the attraction force between the ions

electric conductivity

these compound conduct electricity in melting state or solution . because in molten state or in solution they form ions having free movements . these ions are attracted towards electrodes and thus acts as carrier of electric current .

covalent bond {lewis Langmuir concept }

the bond formed by mutual sharing of electrons between the combining atoms of the same or different to complete their octetos called a covalent bond

covalency 

the number of electron contributed by each atom during the formation of convalent bond is know covalency.

General characteristics  of covalent compounds

 physical  state

unlike  ionic compounds the convalent  compounds exist in all the three state ;like solid ,liquid and gaseous .

crystal  structure

the  crystal structure of covalent compounds differs from that of ionic compounds . they usually consist of molecules rather than covalent comounds  .

melting  and boiling point

covalent  compound have low melting   and boiling points because the molecules in covalent compounds are held together lass rigidly than n case of ionic compounds .

comparison between the properties of electrovalent and covalent compounds

 electrovalent or ionic compounds

1 they are formed complete transference of electrons from oone atoms to another .

2 these compounds are generally solids .

3 they are made up of ions with powerful electrostatic attractive forces.

4 they have generally high melting and boiling point

Covalent compound

1 they are formed by sharing of electrons between the two atoms

2 these compound may be solid , liquid and or gas

3 they are made up of molecules held together by weak forces of attraction

4 They are generally low melting and boiling point

 Hund S rule of maximum multiplicity

Paring of electrons does not take place in orbits of same energy  until allthe orbits of that kind contain at least one electron each this is called hund S rule .

Pauli s exclusion principle

No two electron in an atom can have all the four quantum number .

N,l,m,and s .

Sigma and pi bond

Sigma bond

When a bond is formed between two atoms bt the overlap of their atomic orbitals along the internuclues axis ,[end to end and hand to hand overlap ] the bond formed is called sigma bond .

S S  overlapping

S p overlapping

P p overlapping

Pi bond

Pi bond is formed by lateral overlapping of p orbitals.

 comparison between sigma and pi bond

sigma bond

1 this bond is formed by overlapping of orbitas along their internuclear axis

2 this is formed by overlapping between ss ,sp, pp orbitals .

3 overlapping is quite large and hence sigma bond is a strong bond

4 free rotation about a sigma bond is possible .

5 sigma bond consist of only one electron cloud symmetrical about the internuclear axis .

Pi bond ….

1 Overlapping is to a small extent hence p bond is a weak bond

 2 electron cloud of p bond is unsymmetrical

3  pi bond consist of two electron cloud , one above the plane of atomic    nucleus  and the other below it .

4 this is formed by sideway overlapping of orbitals

5 this is formed by overlapping of pp  orbitals  only . 

Three isotope of hydrogen

1 protium

2  deuterium

3 trititum