Magnetic properties of solids

Every substance has some magnetic properties associated with it. The origin of these properties lies in the electrons. Each electron in an atom behaves like a tiny magnet.
 Its magnetic moment originates from two types of motions
1. its orbital  motion around the nucleus
2. its spin around its own axis .
 Electron being a charged particle and undergoing these motions can be considered as a small loop of current which possesses a magnetic moment.  Thus, each electron has a permanent spin and an orbital magnetic moment associated with it.  On the basis of their magnetic properties, substances can be classified into five categories:
 (i) paramagnetic
(ii) diamagnetic
(iii) ferromagnetic
(iv) antiferromagnetic
 (v) ferrimagnetic.

 (i) Paramagnetism:
1) Paramagnetic substances are weakly attracted by a magnetic field.
2) They are magnetised in a magnetic field in the same direction.
3) They lose their magnetism in the absence of magnetic field.
4) Paramagnetism is due to presence of one or more unpaired electrons which are attracted by the magnetic field.
 O2, Cu2+, Fe3+, Cr3+  are some examples of such substances.

(ii) Diamagnetism:
1) Diamagnetic substances are weakly repelled by a magnetic field. H2O, NaCl and C6H6 are some examples of such substances.
2) They are weakly magnetised in a magnetic field in opposite direction.
3) Diamagnetism is shown by those substances in which all the electrons are paired and there are no unpaired electrons.
4) Pairing of electrons cancels their magnetic moments and they lose their magnetic character.

(iii) Ferromagnetism:
1) A few substances like iron, cobalt, nickel, gadolinium and CrO2 are attracted very strongly by a magnetic field. Such substances are called ferromagnetic substances.
2) Besides strong attractions, these substances can be permanently magnetised.
3) In solid state, the metal ions of ferromagnetic substances are grouped together into small regions called domains. Thus, each domain acts as a tiny magnet.
4) In an unmagnetised piece of a ferromagnetic substance the domains are randomly oriented and their magnetic moments get cancelled.
5) When the substance is placed in a magnetic field all the domains get oriented in the direction of the magnetic field. and a strong magnetic effect is produced.
6) This ordering of domains persist even when the magnetic field is removed and the ferromagnetic substance becomes a permanent magnet.

(iv) Antiferromagnetism:
1) Anti-ferromagnetism have domain structure similar to ferromagnetic substance, but their domains are oppositely oriented and cancel out each other’s magnetic moment
e.g. MnO

(v) Ferrimagnetism:
1) Ferrimagnetism is observed when the magnetic moments of the domains in the substance are aligned in parallel and anti-parallel directions in unequal numbers .
2) They are weakly attracted by magnetic field as compared to ferromagnetic substances.
Fe3O4 (magnetite) and ferrites like MgFe2O4 and ZnFe2O4 are examples of such substances.
3) These substances also lose ferrimagnetism on heating and become paramagnetic.

Exceptional behaviour of graphite

Graphite’s exceptional properties: –

1. Graphite is soft while covalent solid  are hard.
2. It is a conductor of electricity while covalent solids are non conductors. 

Reason for its exceptional behaviour : –
Its exceptional properties are due to its typical structure.

Structure of graphite 

 Carbon atoms are arranged in different layers and each atom is covalently bonded to three of its neighbouring atoms in the same layer. The fourth valence electron of each atom is present between different layers and is free to move about. These free electrons make graphite a good conductor of electricity. Different layers can slide one over the other. This makes graphite a soft solid and a good solid lubricant

Applications of n-type and p-type semiconductors

1) Various combinations of  n-type and  p-type semiconductors are used for making electronic components.   Diode  is a combination of  n-type and  p-type semiconductors and is used as a rectifier.
2) Transistors are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semiconductor.   npn  and  pnp  type of transistors are used to detect or amplify radio or audio signals.
3)  The solar cell is an efficient photo-diode used for conversion of light energy into electrical energy.
4) Germanium and silicon are group 14 elements and therefore, have a characteristic valence of four and form four bonds as in diamond.
5) A large variety of solid state materials have been prepared by combination of groups 13 and 15 or 12 and 16 to simulate average valence of four as in Ge or Si.  Typical compounds of groups 13 – 15 are InSb, AlP and GaAs.
6) Gallium arsenide (GaAs) semiconductors have very fast response and have revolutionised the design of semiconductor devices.
7) ZnS, CdS, CdSe and HgTe are examples of groups 12 – 16 compounds. In these compounds, the bonds are not perfectly covalent and the ionic character depends on the electronegativities of the two elements. It is interesting to learn that transition metal oxides show marked differences in electrical properties.
8)  TiO, CrO2  and ReO3  behave like metals.  Rhenium oxide, ReO3  is like metallic copper in its conductivity and appearance.

Classification of crystalline solid

Crystalline solids are the solids in which constituent particles are arranged in a regular manner.
Classification of crystalline solid

(A) Molecular solids :
Molecules solids are solids in which the constituent particles are molecular solids.
These are further sub divided into the following categories:
(i) Non polar Molecular Solids:
1. They constituent particles are either atoms,
For example, argon and helium or the molecules formed by non polar
covalent bonds for example H2, Cl2 and I2.
2. In these solids, the atoms or molecules are held by weak dispersion forces or London forces.
3. These solids are soft
4. They are non-conductors of electricity.
5. They have low melting points .
6. They are usually liquid or gaseous state at room temperature and pressure.
(ii) Polar Molecular Solids:
1. The molecules of substances like HCl, SO2,
etc. are formed by polar covalent bonds and these solids are known as Polar Molecular Solids.
2. The molecules in such solids are held together by relatively stronger dipole-dipole
interactions.
3. These solids are soft and non-conductors of electricity.
4. Their melting points are higher than those of non polar molecular
Solids.
5. Most of these are gases or liquids under room
temperature and pressure.
Solid SO2 and solid NH3 are some
examples of such solids.
(iii) Hydrogen Bonded Molecular Solids:
1. The molecules of such solids
contain polar covalent bonds between H and F, O or N atoms.
2. They are non-conductors of electricity.
3. Generally they are volatile liquids or soft solids under room temperature and pressure.

(B) Ionic solid
1. Ions are the constituent particles of ionic solids.
2. Such solids are formed by the three dimensional arrangements of cations and anions bound by strong coulombic (electrostatic) forces.
3. These solids are hard and brittle in nature. They have high melting and boiling points.
4. Since the ions are not free to move about, they are electrical insulators in the
solid state. However, in the molten state or when dissolved in water,
the ions become free to move about and they conduct electricity.

(C) Metallic solids
1. Metals are constituent particles 8n metallic solids.
2. They are orderly collection of positive ions surrounded by and held together by a sea of free electrons. These electrons are mobile and are evenly spread out throughout the crystal. Each metal atom contributes one or more electrons towards this sea of mobile electrons.
3. These free and mobile electrons are responsible for high electrical and thermal conductivity of metals. When an electric field is applied, these electrons flow through the network of positive ions. Similarly, when heat is supplied to one portion of a metal, the thermal energy is uniformly spread throughout by free electrons.
4. Metals are characterised by  their lustre and colour in certain cases. This is also due to the presence of free electrons in them.
5. Metals are highly malleable and ductile.

(D) Covalent or network solids
1. A wide variety of crystalline solids of non-metals result from the formation of covalent bonds between adjacent atoms throughout the crystal such solids are known as covalent solids. They are also called giant molecules.
2. Covalent bonds are strong and directional in nature, therefore atoms are held very strongly at their positions.
3. These solids are very hard and brittle.
4. They have extremely high melting points and may even decompose before melting.
5. They are insulators and do not conduct electricity.
e.g. Diamond, graphite etc.

Alkali metals

Alkali means water soluble base. These elements are collectively known as alkali metals because their oxides and hydroxides form strong alkalies like NaOH, KOH, etc. Alkali metal belongs to group-I elements of periodic table. They have one electron in their valence shell. They do not occur in the native or free state as they are very reactive.

Elements of Alkali metals 

1. General Properties of Alkali metals
https://goyalsacademy.blogspot.in/2017/06/chemical-properties-of-alkali-metals.html?m=1

2. Chemical Properties of Alkali  Metals
http://goyalsacademy.blogspot.com/2017/06/properties-of-allali-metals.html

Chemical Properties of Alkali Metals

(i) Action of air
On exposure to moist air, their surface get tarnished due to the formation of their oxides, hydroxides and carbonates.

Hence they are kept under inert liquid like kerosene oil but lithium is kept wrapped in paraffin wax because it floats on the surface of kerosene oil due to its low density.

(ii) Action of oxygen
All the alkali metals when heated with oxygen form different types of oxides. e.g., lithium forms lithium oxide (Li2O), sodium forms sodium peroxide (Na2O2), while K, Rb and Cs form superoxides MO2 (where, M = K, Rb or Cs)
The stability of peroxides and superoxides increases as the size of alkali metal increases.

(iii) Action of water
Alkali metal react with water to form hydroxide and release hydrogen :-
2M + 2H2O → 2MOH + H2 (where, M = Li, Na, K, Rb, and Cs)
The reactivity order with water is
Li < Na < K < Rb < Cs
This is due to increase in electropositive character in the same order.

(iv)  Reaction with halogens
Alkali metals combine readily with halogens to form ionic halides M+ X- (with the exception of some lithium halides).
2M + X2 → 2M+ X–
(where, M = Li, Na, K etc., and X = F, Cl, Br)
The reactivity of alkali metals towards a particular halogen increase in the order
Li < Na < K < Rb < Cs

(vi) Solubility in liquid ammonia
All alkali metals dissolve in liquid ammonia giving deep blue solution due to formation of ammoniated metal cations and ammoniated electrons in the solution.

The blue colour is due to the excitation of ammoniated electron to higher energy levels and the absorption of photons occurs in the red region of the spectrum.

(vii) Nature of carbonates and bicarbonates
Li2CO3 is unstable toward heat.

The thermal stability of carbonates increases on moving down the group 

All the bicarbonates (except LiHCO3 which exists in solution) exist as solids and on heating form carbonates.

(viii) Nature of nitrates
LiNO3 on heating decomposes to give NO2 and O2, while the nitrates of the other alkali metals decompose on heating and give nitrites and O2.

(ix) Nature of sulphates
Li2SO4 is insoluble in water whereas the other sulphates, i:e., Na2SO4, K2SO4 are insoluble in water.

Physical properties of Allali metals

Alkali Metals or Group-I elements have one electron in their valence shell. They do not occur in the native or free state as they are very reactive. These elements are collectively known as alkali metals because their oxides and hydroxides form strong alkalies like NaOH, KOH, etc.

General Characteristics of Alkali Metals
(i) Electronic configuration  ns1

(ii) Atomic radii
The alkali metals have the biggest atomic radii in their respective periods.
Atomic radii increases as we go down the group due to the addition of a new shell in each subsequent step.

(iii) Ionic radii
Ionic radii of the alkali metals are much smaller than their corresponding metals due to lesser number of shells and increased nuclear charge.
The ionic radii of all these alkali metal ions go on increasing on moving down the group.

(iv) Density
These are light metals with low densities. Lithium is the lightest known metal. On moving down the group, ‘density increases from Li to Cs.
This is because, down the group, both the atomic size and atomic mass increases but the effect of increase in atomic mass is more as compared to increase in atomic size.

(v) Melting and boiling points
The melting and boiling points of alkali metals are quite low and decrease down the group due to weakening of metallic bond.

(vi) Softness
These are soft. malleable and ductile solids which can be cut with knife.

(vii) Atomic volume
Atomic volume of alkali metals is the highest in each period and goes on increasing down the group from top to bottom [Li to Cs].

(viii) Ionisation enthalpy
The ionization energy is the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation .
The ionisation enthalpy of alkali metals is the lowest amongst the elements because they readily loose a electron to gain the stable noble gas electronic gas configuration.

(ix) Electropositive character
The elements that can easily lose electrons to form positive ions are called electropositive elements and this character is known as electropositive character.
Due to low ionisation enthalpies alkali metals are strongly electropositive or metallic in nature. The electropositive nature increases from Li to Cs due to decrease in ionization enthalpy.

(x) Oxidation state
oxidation number is a number assigned to an element in chemical combination which represents the number of electrons lost or gained, by an atom of that element in the compound.
The alkali metal atoms show only +1 oxidation state, because their unipositive ions attain the stable noble gas configuration.

(xi) Hydration of ions
A hydration reaction is a chemical reaction in which a substance combines with water.
The degree of hydration depends upon the size of the cation. Smaller the size of a cation greater is its hydration enthalpy
Relative degree of hydration,
Li+ > Na+ > K+ > Rb+ > Cs+

(xii) Flame colouration
Alkali metals and their salts impart characteristic colours to the flame because the outer electrons get excited to higher energy levels, When the electron return to the original state it releases visible light of characteristic wavelength which provides a colour to the flame.

(xiii) Photoelectric effect
The photoelectric effect is the emission of electrons or other free carriers when light is thrown on a material
Due to very low ionisation enthalpy, alkali metals specially ‘Cs’ exhibit photoelectric effect i.e., eject electrons when exposed to light. so it is used in photoelectric cells.

(xiv) Electrical conductivity
Due to the presence of loosely held valence electrons which are free to move throughout the metal structure the alkali metals are good conductors of heat and electricity. Electrical conductivity increases from top to bottom in the order
Li+ < Na+ < K+ < Rb+ < Cs+

(xv) Reducing character
A reducing agent is an element  or compound that loses or donates an electron to another chemical species in a redox chemical reaction. This character is known as reducing character.
All the alkali metals are good reducing agents due to their low ionisation energies. Their reducing character follows the order
Na < K < Rb < Cs < Li

Laws Of Chemical Combination

1. The Law of Conservation of Mass:
It states
that total mass of the reactants is always equal
to that of the products during any physical or
chemical change.

Or

Matter can neither be created nor destroyed
by any known physical or chemical change.
This law is also known as “The Law of indestructibility of Matter”.

2. The Law of Definite Proportions Or Law
of constant composition:
It states that a
pure chemical compound is always made up
of the same elements combined together in the
same fixed ratio by mass.

3. The Law of Multiple Proportions:
It states
that when two elements combine to form two
or more compounds, the weights of one of the
elements which combine with the fixed weight
of the other element bear a simple whole

number ratio to one another.

4. Law of Reciprocal Proportions:
It states
that when two elements combine separately
with the fixed weight of a third element, the
ratio of their weights is either the same or a
simple whole number multiple of the ratio in
which they also combine with each other.

5.Gay Lussac’s Law:

It states that whenever gases react
with each other they do so in a simple ratio by
volumes to each other as well as to products
formed in the gaseous state all volumes being

measured under similar conditions of

temperature and pressure.

Classification of matter

Matter:
Anything that has mass and occupies space is known as matter. 

Classification of matter :
I. Physical Classification: Depending upon the physical state, matter could be classified into
(a) solid state
(b) liquid state
(c) gaseous state

For properties of solid, liquid and gas see:-

https://goyalsacademy.blogspot.in/2017/06/properties-of-solid-liquid-and-gas.html?m=1

II. Chemical Classification: Based upon its composition, matter could be classified into following three types :
(1) elements
(2) compounds
(3) mixtures.
1. Element. It is a pure substance which can neither be built up from nor decomposed into two or more still simpler substances by any known physical or chemical methods. e.g.
carbon, iron, hydrogen, sulphur etc.
Elements may further be sub-divided into
(a) Metals e.g. Na, Mg, Al, Fe etc.
(b) Metalloids e.g. As, Sb etc.
(c) Non-metals e.g. O, N, S, P, Cl etc.
(d) Noble gases e.g. He, Ne, Ar, Kr etc.
2. Compound: It is a substance which can be obtained by the combination of atoms of two or more, same or different elements combined together in a definite ratio by weight.
e.g., Cl2, SO2, NaCl etc.
3. Mixture: A combination of two or more elements or compounds in any proportion so that they may not lose their identity is known as a mixture.
Mixtures are further of two types :
(a) Homogeneous mixtures: In this case the composition of the mixture is the same throughout the mixture and the constituents are indistinguishable.
e.g. air, gasoline, alloys etc.
(b) Heterogeneous mixtures: In such mixtures the composition is not the same throughout the mixture and the components could be easily separated.
e.g. a mixture of sand and
common salt, iron filings and sulphur.

Faraday’s Law of electrolysis

Michael Faraday was the first scientist who described the quantitative aspects of electrolysis. Now Faraday’s laws also flow from what has been discussed earlier. After his extensive investigations on electrolysis of solutions and melts of electrolytes, Faraday published his results during 1833 34 in the form of the following well known Faraday’s two laws of electrolysis:

(i) First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte.

There were no constant current sources available during Faraday’s times. The general practice was to put a coulometer (a standard electrolytic cell) for determining the quantity of electricity passed from the amount of metal (generally silver or copper) deposited or consumed. However,
coulometers are now obsolete and we now have constant current (I) sources available and the quantity of electricity Q, passed is given by
Q = It
Q is in coloumbs when I is in ampere and t is in second.
The amount of electricity (or charge) required for oxidation or reduction depends on the stoichiometry of the electrode reaction. For example, in the reaction:
Ag +(aq) + e– → Ag(s)
One mole of the electron is required for the reduction of one mole of silver ions.
We know that charge on one electron is equal to 1.602× 10–19C.
Therefore, the charge on one mole of electrons is equal to:
NA × 1.602× 10–19 C = Avogadro’s No. ×1 1.6021 10–19
C = 96487 C mol–1
This quantity of electricity is called Faraday and is represented by the symbol F.
For approximate calculations we use 1F ≃ 96500 C mol–1 .
For the electrode reactions:
Mg2+(l) + 2e–    →     Mg(s)
Al3+(l) + 3e–   →     Al(s)
It is obvious that one mole of Mg2+ and Al3+ require 2 mol of electrons (2F) and 3 mol of electrons (3F) respectively. The charge passed through the electrolytic cell during electrolysis is equal to the product
of current in amperes and time in seconds.

(ii) Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic Mass of Metal ÷ Number of electrons required to reduce the cation).