All type of matter that generally exist around us can be divided into three states namely; solid, liquid and gas. When we change the physical conditions by changing the surrounding pressure and temperature, we find the states change from one form to another. A state's stability depends on two opposing factors, namely:
• Intermolecular Forces
• Thermal Energy
These two factors primarily determine the characteristics of a state and predict in desired conditions what the nature of the state would be. In addition to these two factors, the formation and existence of solids are also responsible. These two forces keep the solid atoms in a fixed structure that we call a lattice closely packed. The thermal energy of solid material particles is low at low temperatures. So, the movement of particles is minimal, the intermolecular force is high and space between different atoms is very less. This gives solid their basic properties.
The forces that act within a solid between constituent particles of matter are called intermolecular forces. The main role of intermolecular forces is to keep the particles of matter together and close to each other; it can be atoms, molecules or ions. The intermolecular force establishes the state of the matter along with the boiling point, melting point, enthalpy and other factors of the elements. They keep together, the atoms or molecules of the matter in a bond.
Due to their movement, thermal energy is defined as the energy, the particles of matter possess. Thermal energy is a kinetic form of energy. It is an object's internal energy responsible for its temperature. Thermal energy transfer occurs through heat transfer. With a rise in the thermal energy, the particles of matter tend to move faster and vice versa. With rising temperature, thermal energy increases and tends to move the particles faster. Thermal energy, unlike intermolecular forces, moves the particles of matter away from each other.
Solid State at Molecular Level
Due to the close packing of constituent particles of matter (atoms, molecules or ions), solid state is generally characterized by rigidity, mechanical strength and incompressibility. Strong forces of nature hold the atoms, molecules or ions together, which adhere to them in the solid lattice to occupy fixed positions (called lattice sites).
In a three - dimensional structure called lattice, solid state at molecular level is basically an arrangement of constituent particles of matter (atoms, molecules or ions). Inside the lattice, the particles are not free to move.
If we change the physical conditions such as pressure and temperature, the particles tend to change their arrangement, so we find the change in solid state. The conclusion drawn is that a solid's general characteristics depend largely on the position and alteration of particles of constituent particles (atoms, molecules or ions).
How formation of a solid takes place?
A solid is formed at a low temperature and depends on two factors: intermolecular forces and thermal energy. The thermal energy is low at low temperatures and the intermolecular forces are high, which aligns the particles of matter to remain stable and only oscillate about their mean positions. The particles of matter are therefore unable to escape the solid and continue to oscillate about their mean position. Therefore, solid formations take place.
General Characteristics of Solid State
Now if you actually observe all the solid objects around you, you will notice dissimilarities between them. They may vary greatly in shape, texture, weight, size, colors, etc. But there are a few physical properties of a solid that holds true to a few rare exceptions for all solids. Let's look at some of these common / general solids characteristics. Following characteristics determine the solid state. These are the following:
• Definite mass, volume, and shape
• Short Intermolecular distance
• Strong Intermolecular Forces
• The molecules of the solid material remain fixed at their respective positions. They do not actually move but only oscillate about their mean positions.
• Solids are incompressible and rigid
• High Density
Definite Mass, Volume, and Shape
Solids have a definite mass, volume, and shape because strong intermolecular forces hold the constituent particles of matter together. The intermolecular force tends to dominate the thermal energy at low temperature and the solids stay in the fixed state. In a solid and liquid, the mass and volume remain the same. But solids also have a fixed shape unlike liquids. A solid material’s tightly packed atoms ensure that a definite shape is maintained.
Example: Ice cube remains in the solid state at low temperature, but we find the ice to either melt or evaporate away when we change the physical conditions.
Short Intermolecular Distance
Due to tight close packing, the intermolecular distance between constituent material particles in a solid is short. Strong intermolecular forces hold the atoms and molecules together. The various types of intermolecular forces that exist between the molecules of solids are:
• London Dispersion Forces: These forces are also known as Dispersion Force, London force, or van derWaals forces. These attractions are caused by temporary dipole instantaneous dipole) created at any moment by uneven distribution of electrons within the particle. In a nearby particle, the temporary dipole in one particle will induce a temporary dipole opposite. The attractions are formed by the temporary partial positive and partial negative charges which develop between particles. In all molecules and atoms, these attractions are present. This is the only force of attraction found in non-polar molecules and noble gases. It is the weakest of the forces of attraction. Attractions are increased as the electron cloud's molar mass or size increases. As the surface area of the particles is more spread / larger, attractions increase.
• Dipole-Dipole forces: These attractions are caused by permanent dipoles occurring in polar molecules. The line-up of molecules will attract the partial positive charges on one particle to the partial negative charges on a nearby particle. These attractions exist between polar molecules which create partial charges attracting the opposites. These are a bit stronger than dispersion forces attractions. This will cause a higher melting point for the same molar mass polar molecule than the nonpolar molecule. Attractions are increased as the difference in electronegativity increases. Attractions are increased as the size of the atoms attracting an opposite charge gets decreased. The more closer it is possible to get the opposite charges, the more will it make the attraction stronger-like a magnet that has stronger attraction when the distance between the objects is less.
• Hydrogen bonding: The hydrogen bonding is a special case of the Dipole-Dipole Forces. Hydrogen bonding takes place when H is attached directly to a small, highly electronegative atom (F, O and N). H will develop a permanent partial positive charge due to the differences in electronegativity and small size atoms, and the F, O, or N atom will develop a permanent partial negative charge. The opposite charges will be strongly attracted by interactions between two separate particles.
• Induced dipole forces: Attractions are caused by the charge or ion that occurs in polar molecules attracted to the opposite permanent dipoles charge. The particles are ordered to surround the charged ions with molecules with a permanent dipole of the opposite charge. In a polar solvent, these attractions are present in dissolved ion solutions.
Strong Intermolecular Forces
The intermolecular forces are quite strong in solids. This is due to the closeness of constituent material particles. The intermolecular force variation shows why solids behave differently from liquids and gases. Due to the strong intermolecular forces between particles, the tight knit packing of molecules. This also means that there are very small / short intermolecular distances between particles. That's why solids keep their shape and do not spread like liquids or gases.
The molecules of the solid material remain fixed at their respective positions but oscillate at their mean position
Since solid formation and existence happens only at low temperatures, the intermolecular forces are high enough to keep the atoms and molecules in a solid state fixed at their positions. The thermal energy is also quite less and can only oscillate at the positions of atoms and molecules.
Solids are incompressible and rigid
In nature, solids are rigid and incompressible due to the strong intermolecular forces that act on particles that keep them fixed on their positions. As there is already no space between molecules, solids cannot be compressed further.
The solid mass is greater than the liquid and gas mass. Also, solids show high density because they have fixed volume. This is because density is a function of mass and volume as depicted by the following equation: Density = Mass/Volume.
According to this equation, density is directly proportional to the mass and inversely proportional to the volume of the substance under consideration. Since, the mass of the solids materials is high (due to their close inter-packed structure) and volume of the solid materials is less due to the presence of lesser amount of intermolecular spaces, the resulting density becomes high. This property of high density of solids is utilized by metals which are employed for a wide number of purposes due to their mechanical strength.