Group 15 elements include nitrogen, phosphorus, arsenic, antimony and bismuth. Nitrogen is the real constituent of the air and records for 78% of it by volume. It is the primary member of this group and happens in a free state as a diatomic gas, N₂.

Phosphorus is a fundamental constituent of animal and plant matter. Phosphate groups are constituents of nucleic acids, that is, DNA and RNA. Around 60% of bones and teeth are made out of phosphates. Phosphoproteins are available in egg yolk, milk, and bone marrow. The rest of the elements of the group, that is, arsenic, antimony, and bismuth, mostly happen as sulfides. For example, Stibnite, Arsenopyrite, and bismuth glance.

 Packing efficiency is defined as the percentage of space occupied by constituent particles packed inside the lattice. It can be calculated with the help of geometry in three structures namely: HCP and CCP structures. The packing efficiency of simple cubic lattice is 52.4%.

Matters exist in solid state because of close packing of their constituent particles. There are two types of close packing found in solids. These are Cubic Close Packed (ccp) and Hexagonal Close Packed (hcp) lattice.

Enthalpy is defined as the sum of internal energy of a system and the product of its pressure and volume. It is denoted by the symbol E. It is a state function. Units used to express are calorie, BTU, or joules. Below we have given the equation.

Enthalpy Equation:

E= U + PV where,

E is the enthalpy

U is the internal energy of a system

P is the pressure

V is the volume

Entropy is defined as the measure of the thermal energy of a system per unit temperature which is not available for doing useful work. It is denoted as S. The SI unit for Entropy is Joules per Kelvin. Entropy change at constant temperature is calculated as given below:

ΔS_system = q_rev / T

ΔS represents the change in entropy,

q_rev represents the reverse of the heat, and

T is the temperature in the Kelvin scale.

Normality in chemistry is one of the expressions used to measure the concentration of a solution. It is abbreviated as ‘N’ and is sometimes referred to as the equivalent concentration of a solution. It is mainly used as a measure of reactive species in a solution and during titration reactions or particularly in situations involving acid-base chemistry.

Example: 

n the following reaction calculate and find the normality when it is 1.0 M H₃PO₄

H₃AsO₄ + 2NaOH → Na₂HAsO₄ + 2H₂O

Solution:

If we look at the given reaction we can identify that only two of the H⁺ ions of H₃AsO₄ react with NaOH to form the product. Therefore,  the two ions are 2 equivalents. In order to find the normality, we will apply the given formula.

N = Molarity (M) × number of equivalents

N = 1.0 × 2 (replacing the values)

Therefore, normality of the solution = 2.0.

Centered Unit Cells

• If the constituent particles of a crystal lattice are present at positions other than corners in addition to those at corners, it is known as centered unit cell.
• Centered unit cells are of three types:
  • Body-Centred Unit Cells:

If the constituent particles of a unit cell are present at its body-centre besides the ones that are at its corners.

• Face-Centred Unit Cells:

If the constituent particles of a unit cell are present at the center of each face, besides the ones that are at its corners.

• End-Centred Unit Cells:

If the constituent particles of a unit cell are present the center of any two opposite faces besides the ones present at its corners.

Types of Unit Cell

Numerous unit cells together make a crystal lattice. Constituent particles like atoms, molecules are also present. Each lattice point is occupied by one such particle.

1. Primitive Cubic Unit Cell
2. Body-centered Cubic Unit Cell
3. Face centered cubic unit cell

The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements. The periodic structure of an ideal crystal is most easily described by a lattice. The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure. The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.

The ‘crystal lattice’ is the pattern formed by the points and used to represent the positions of these repeating structural elements. The periodic structure of an ideal crystal is most easily described by a lattice. The crystal lattice is the array of points at the corners of all the unit cells in the crystal structure. The crystal lattice is the symmetrical three-dimensional structural arrangements of atoms, ions or molecules (constituent particle) inside a crystalline solid as points. It can be defined as the geometrical arrangement of the atoms, ions or molecules of the crystalline solid as points in space.

Constituent particles in ionic solids of the Crystalline Solids are anions (negatively charged) and cations (positively charged). An ion is surrounded by a typical number of opposite charges. For example, in NaCl, the Na+ ion is surrounded by 6 Cl- ions. Ions in these solids are held together by strong electrostatic forces. They have high melting and boiling points and are soluble in polar solvents but not in non-polar solvents.

CLASSIFICATION OF CRYSTALLINE SOLIDS:-

Based upon nature of constituent particles and binding forces present in them:-

1. Ionic Solids:- 

• In these solids constituent particles are positive and negative ions. (cation or anions). They are held together by strong columbic electrostatic forces of attraction examples are NaCl, CaCl2, MgCl2, BaCl2 etc.

Characteristics of ionic solids:-

1. They have high melting and boiling points
2. They are soluble in polar solvents but are insoluble in non polar solvents.
3. Due to strong electrostatic forces of attraction they are closely packed hence hard but they are brittle.

2. Molecular Solids:-

• In these solids the constituent particles are molecules on the nature of molecules they can further subdivided into following three types:-

1. Non polar molecular solids:-

• • The crystalline solids in which constituent particles are atoms of noble gases [helium, neon] or non polar molecules like [H2, Cl2, I2]

Their characters are:-

1. 1. They are soft due to weak intermolecular forces.
   2. They are non conductors of electricity. They have low melting and boiling points.

2. Polar molecular solids:-

• • The crystalline solids in which constituent particles are polar molecules like HCl, SO2 etc. the intermolecular forces of attraction are dipole – dipole forces of attraction.

Thus their characters are:-

1. 1. They are soft; they are non conductors of electricity.
   2. Their melting and boiling points are high then non polar solids. They exists gases or liquid at room temperature.

3. Hydrogen bonded – molecular solids:-

• • In these solids the constituent particles are which contain hydrogen atom linked to high electronegative atoms as N, O, F

Their characters are:-

1. 1. They exists as volatile liquids or gases at room temperature.
   2. They are non conductor of electricity.
   3. Their melting and boiling points are high.

3. Covalent or network solids:-

• In these crystalline solids the constituent particles are non metal atoms linked to adjacent atom by covalent bond throughout the crystal. They forms a network of covalent bonds and exists as giant molecules. Example: Diamond

Their main characteristics are:-

1. As covalent bond is strong and directional in nature, these solid are very hard and brittle.
2. They have extremely high melting points and decompose before melting.
3. They are insult of and do not conduct electricity one exception is graphite which is covalent solid but is soft and also a good conductor of electricity.

4. Metallic solids:-

• In natural the constituent particles are positively charged metal ions – and free electrons.
• They are formed of metal atoms which lose their valance electrons to left behind positively charged ions.
• These metal atoms are surrounded by the sea of electrons each metal atom contributes one or more electrons to this sea of electrons.
• The electrons are simultaneously attracted by the +ve ions and holds these +ve ions ether

Metallic bond:-

• The force that holds the metal ion together in the crystal is called metallic bond.

Properties of metallic solids:-

1. They possess high electrical and thermal conductivity.
2. They possess lusture and colour in some case due to presence of sea of free electrons.
3. They are highly malleable and ductile.
4. They are closely packed. They exhibit high melting points and high densities.

Most of the solids found in daily life and practice are crystalline in nature. For Example metals like silver, copper, and iron, non-metals like iodine, sulphur and iodine and several compounds like NaCl (Sodium Chloride/Common Salt) etc. fall into the category of crystalline solids. We know that attraction nature of constituent particle is mainly responsible for the existence of solids. On the basis of nature of force operating between constituent particles of matter, crystalline solids are classified into four categories, namely:

• Molecular Solids

• Ionic Solids

• Metallic Solids

• Network or Covalent Solids

Amorphous silicon is a photovoltaic material widely used for conversion of sunlight into electricity.
The glass used in making glassware, rubber used in making hot water bags and cotton candy are amorphous solids

Pseudo-solid- are solids which are considered to be solid though they resembles in many respects liquid. They flow very slowly at room temperature and are considered as super cooled liquids. Amorphous solids are considered to be pseudo solid for e.g. Glass.
 

Amorphous Solids

The solids in which the constituent particles of matter are arranged in a random manner are called amorphous solids. It is a non-crystalline solid with no proper arrangement of atoms in the solid lattice. In other words, we can define amorphous solids as materials which don’t have certain organized arrangement of atoms and molecules. Most solids are amorphous in nature and are utilized in many sectors as well. One of the most common examples of amorphous solids is glass, which is used widely in the manufacturing sector.

Crystalline Solids

The solids in which the constituent particles of matter are arranged and organized in a specific manner are called Crystalline Solids. These solids contain crystals in their structure and each crystal has definite geometry. Adding further, as crystalline solids have low potential energy, they are the most stable form of solids. Almost all solids fall in the category of crystalline solids including metallic elements (iron, silver, and copper) and non-metallic elements (Phosphorus, Sulphur, and iodine). Also several compounds like sodium chloride, zinc sulphide and naphthalene build crystalline solids.

a n s w e r
 Classification of Crystalline Solids
Ionic Solids:- ...
Molecular Solids:- ...
Non polar molecular solids:- ...
Polar molecular solids:- ...
Hydrogen bonded – molecular solids:- ...
Covalent or network solids:- ...
Metallic solids:-

Thermal Energy

• Thermal energy refers to the energy of a body that arisesfrom the motion of its atoms or molecules.
• The thermal energy of a substance is directly proportional to the temperature of the substance.
• It is also defined as the measure of average kineticenergy of the constituent particles of a substance and hence responsible for movement of those particles.