can someone say me about piezoelectricty in few words and also about structure of atom

 

 

 

 

 

 

 

 

 

 

 

Pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particles. Most of the matter around us exist as mixtures of different substances and hence not pure. For example, milk is actually a mixture of water, fat, proteins etc.

Mixtures are constituted by more than one kind of pure form of matter, known as a substance. A substance cannot be separated into other kinds of matter by any physical process. Example: sugar is a substance because it contains only one kind of pure matter and its composition is the same throughout.

Depending upon the nature of the components that form a mixture, we can have different types of mixtures.

Mixture which has a uniform composition throughout is called homogeneous mixtures or solutions. A homogeneous mixture can have a variable composition.

Mixtures, which contain physically distinct parts and have non-uniform compositions are called heterogeneous mixtures. Example: Mixtures of sodium chloride and iron filings.

 

          A solution is a homogeneous mixture of two or more substances. Examples: Lemonade, soda water etc. We can also have solid solutions (alloys) and gaseous solutions (air). In a solution there is homogeneity at the particle level. Example: particles of sugar or salt in lemonade are evenly distributed in the solution.

A solution has a solvent and a solute as its components. The component of the solution that dissolves the other component in it is called the solvent. The component of the solution that is dissolved in the solvent is called the solute.

Example: 

In a solution the relative proportion of the solute and solvent can be varied. Depending upon the amount of solute present in a solution, it can be called a dilute, concentrated or a saturated solution.

When no more solute can be dissolved in a solution at a given temperature, it is called a saturated solution. The amount of the solute present in the saturated solution at this temperature is called its solubility. Different substances in a given solvent have different solubilities at the same temperature.  If the amount of solute contained in a solution is less than the saturation level, it is called an unsaturated solution.

The concentration of a solution is the amount of solute present per unit volume or per unit mass of the solution.

 

Mass of solvent (water)      =                              320 g

Mass of solution                   =                                 mass of solute + mass of solvent

                                                    =                                 40 g + 320 g  =   360 g

Mass percentage by solution              =             mass of solute / mass of solution × 100

                                                                        =             40 / 360  × 100   =  11.11 %

Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition. For example, brass is a mixture of approximately 30% zinc and 70% copper.

Solids that are insoluble in a solvent and have particles that are visible to naked eyes form a suspension. A suspension is a heterogeneous mixture where solute remains suspended throughout the medium.

Colloids are heterogeneous mixtures in which the particle size is too small to be seen with the naked eye, but is big enough to scatter light. This scattering of a beam of light is called the Tyndall effect after the name of the scientist who discovered this effect. The particles are called the dispersed phase and the medium in which they are distributed is called the dispersion medium. The particles of a colloid are uniformly spread throughout the solution. Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.

The components of a colloidal solution are the dispersed phase and the dispersion medium. The solute-like component or the dispersed particles in a colloid form the dispersed phase, and the component in which the dispersed phase is suspended is known as the dispersing medium. Colloids are classified according to the state (solid, liquid or gas) of the dispersing medium and the dispersed phase.

 

Different methods of separation are used to get individual components from a mixture. Heterogeneous mixtures can be separated into their respective constituents by simple physical methods like handpicking, sieving, filtration that we use in our day-to-day life. Sometimes special techniques have to be used for the separation of the components of a mixture

 

 

 

 

 

 

Air is a homogeneous mixture and can be separated into its components by fractional distillation.

 

 

 

                                Compress and cooled by increasing pressure and decreasing temperature

 

 

 

                                Allowed to warm up slowly in fractional distillation column          

                                                               

                                                Gases get separated at different heights

 

 

                          The air is compressed by increasing the pressure and is then cooled by decreasing the temperature to get liquid air. This liquid air is allowed to warm-up slowly in a fractional distillation column, where gases get separated at different heights depending upon their boiling points.

 

 

The crystallisation method is used to purify solids. For example, the salt we get from sea water can have many impurities in it. To remove these impurities, the process of crystallisation is used. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as –

Thus, by choosing one of the above methods according to the nature of the components of a mixture, we get a pure substance. With advancements in technology many more methods of separation techniques have been devised.

 

 

The physical properties of matter that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properties. The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance. Although ice, water and water vapour all look different and display different physical properties, they are chemically the same.

We know that oil burns in air whereas water extinguishes fire but both are liquid. It is this chemical property of oil that makes it different from water. Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition. Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a chemical reaction. During burning of a candle, both physical and chemical changes take place.

On the basis of their chemical composition, substances can be classified either as elements or compounds.

Robert Boyle was the first scientist to use the term element in 1661. Lavoisier, a French chemist, defined an element as a basic form of matter that cannot be broken down into simpler substances by chemical reactions.

Elements can be normally divided into metals, non-metals and metalloids. There are more than 100 elements known. Ninety-two elements are a occurring and the rest are manmade. Majority of the elements are solid.

 

Eleven elements are in gaseous state at room temperature. Two elements are liquid at room temperature–mercury and bromine. Elements, gallium and cesium become liquid at a temperature slightly above room temperature (303 K).

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.

Atoms and molecules are the fundamental building blocks of matter. Dalton’s atomic theory explained various laws of chemical combination but Dalton’s idea that the atom is indivisible particle has been disproved by the discovery of various subatomic particles. Atoms are found to be mainly composed of three fundamental particles (electrons, protons and neutrons). A major challenge before the scientists at the end of the 19th century was to reveal the structure of the atom.

The electron was the first fundamental particle which was discovered by J.J Thomson based on the experiments carried out in a discharged tube.

                Sir William Crookes was the first scientist who designed the discharged tube which was called Crooke’s discharged tube or cathode ray tube. It is a long glass tube having two metal plates connected to the opposite charged poles of the battery. The pressure inside the discharge tube can be adjusted by means of an exhaust pump.

This discharge tube was slightly modified by J.J Thomson. When high voltage was applied between cathode and anode with a small hole at the centre of the partially evacuated tube at a pressure of 0.01 mm of Hg, a bright spot of light was formed on a zinc sulphide screen kept at the opposite end of the discharge tube. This was caused by the rays which originated from the cathode called cathode rays. J.J Thomson conducted some experiments with a discharge tube for studying the properties of the cathode rays.

The properties do not depend on the nature of gas taken in discharge tube. Specific charge(e/m value) remains same

Some fine oil droplets were allowed to be sprayed into the chamber by the atomizer. The air in chamber is subjected to ionization by X-rays. The electrons produced by the ionization of air attach themselves to the oil drops. When sufficient amount of electric field is applied which can just balance the gravitational force acting on the oil drop, the oil drop remains suspended in air. From this experiment Millikan observed that the electrons produce 1.59 × 10^—19 coulombs and the charge on each drop was always an integral multiple of that value. On the basis of the observation, he concluded that the 1.59 × 10^—19 coulombs is the smallest possible charge and considered that value as the charge of electron  

The presence of positively charged particles has been predicted by E. Goldstein based on the electrical neutrality of the atom. The discovery of proton by Goldstein was done on the basis of cathode ray experiment conducted by using a perforated cathode.

Just like cathode rays, some rays were found to be emanate from an anode. These are called anode rays or canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle which had a charge, equal in magnitude but opposite in sign to that of the electron. Its mass was approximately 2000 times as that of the electron. It was given the name of proton.

In general, an electron is represented as ‘e^–’ and a proton as ‘p⁺’. The mass of a proton is taken as one unit and its charge as plus one. The mass of an electron is considered to be negligible and its charge is minus one. It seemed highly likely that an atom was composed of protons and electrons, mutually balancing their charges.

For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.

 

 

Thomson proposed the model of an atom is popularly known as apple pic or plum pudding model or watermelon model.

Thomson proposed that:

Although Thomson’s model explained that atoms are electrically neutral, but it failed to explain how the protons are shielded from the electrons without getting neutralized.

 

Rutherford designed an experiment to find the model of the atom called α - particle scattering experiment in 1911.

 

In this experiment, a stream of high energy alpha particles emanating from radium was directed at a thin foil of gold metal. The thin gold metal foil had a circular fluorescent zinc sulphide screen around it. Whenever alpha particles struck the screen, a tiny flash of light was produced at that point.

The gold foil was about 1000 atoms thick approximately thickness of 100 nanometre. Alpha particles are doubly-charged helium ions and highly energetic particles. Since they have a mass of 4 u, the fast-moving α-particles have a considerable amount of energy. It was expected that alpha particles would be deflected by the sub-atomic particles in the gold atoms. But, the particle scattering experiment gave totally unexpected results. The following observations were made:

Rutherford concluded from the a particle scattering experiment that

 

The atom is mostly composed of empty space. Very few alpha particles were deflected by 180⁰. The deflection must be due to enormous repulsive force showing that positive charge of the atom is not spread throughout the atom. The entire positive charge and most mass of the atom has to be densely concentrated in a very small volume known as nucleus that repelled and deflected the positively charged alpha particles.

The diameter of the nucleus is estimated by Rutherford as 10^-15 cm in contrast to that of an atom to be 10^-10 cm. The nucleus is surrounded by electrons that move around the nucleus with very high speed in circular paths called orbits. Electrons and nucleus are held together by the electrostatic forces of attraction

Rutherford’s atomic model is like a small scale solar system with the nucleus playing the role of sun and electrons to planets. When classical mechanics is applied to the solar system it shows that planets describe well defined orbits around the sun.

The similarity between the solar system and the nuclear model suggest that electrons should move around the nucleus in well defined orbits. However when a body is moving in an orbit, it undergoes acceleration. So an electron moving in an orbit undergoes acceleration. According to electromagnetic theory of Maxwell, charged particles when in acceleration emit electromagnetic radiation. Therefore an electron in the orbit emit radiation, the electronic radiation comes from electronic motion. Thus the orbit continue to shrink. Calculations show that it would take an electron only 10^-8 s to spiral into nucleus. But this does not happen. Thus the Rutherford’s model of atom cannot explain the stability of the atom.

          If the electrons were stationary electrostatic attraction between the dense nucleus and electrons would pull the electrons toward the nucleus to form a miniature version of Thomson’s atomic model. This model could not explain how the electrons are distributed around the nucleus and what are the energies of these electrons.          

 

To overcome the above defects of Rutherford’s model, Niels Bohr in 1913 gave a modification based on Quantum theory of radiation. The important postulates are:

`The exchange of energy is possible only when electron jumps from one energy level to another energy level.

  E = E₂ – E₁ = hv = hc/ʎ  joule

Where h is planck’s constant having fixed value

                h = 6.63 × 10^-34 J / second  and ʎ is wavelength.

                When an electron falls from an orbit of high energy level to lower energy level, the difference in energy is radiated in the form of electromagnetic radiation of particular wavelength. Since each atom has its specific energy levels, it can energy levels, it can emit radiations of specific wavelength. Ground state means the lowest energy state.  When the electrons absorb energy and jump to outer orbits, this state is called excited state.

HOW ARE ELECTRONS DISTRIBUTED IN DIFFERENT SHELLS

 

              The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The following rules are followed for writing the number of electrons in different energy levels or shells:

first orbit or K-shell will be = 2 × 1² = 2, second orbit or L-shell will be = 2 × 2² = 8, third orbit or M-shell will be = 2 × 3² = 18, fourth orbit or N-shell will be  2 × 4²= 32, and so on.

 

LIMITATIONS OF BOHR’S THEORY

 

Piezoelectricity is the electric charge that accumulates in certain solid materials in response to applied mechanical 

In practice, the crystal becomes a kind of tiny battery with a positive charge on one face and a negative charge on the opposite face; current flows if we connect the two faces together to make a circuit. Now, by crystals, scientists don't mean intriguing bits of rock we find in gift shops: a crystal is the regular, repeating arrangement of atoms in a solid in a three dimensional manner. The atoms are essentially fixed in place but can vibrate slightly.

In most crystals such as metals the atoms are symmetrical; in piezoelectric crystals, it isn't. Normally, piezoelectric crystals are electrically neutral: the atoms inside them may not be symmetrically arranged, but their electrical charges are perfectly balanced: a positive charge in one place cancels out a negative charge nearby. However, if you squeeze or stretch a piezoelectric crystal, you deform the structure, pushing some of the atoms closer together or further apart, upsetting the balance of positive and negative, and causing net electrical charges to appear. This effect carries through the whole structure so net positive and negative charges appear on opposite, outer faces of the crystal. The reverse-piezoelectric effect occurs in the opposite way.

In what way piezoelectricity is used for?

There are all kinds of situations where we need to convert mechanical energy  into electrical signals or vice-versa. Often we can do that with a piezoelectric transducer. A transducer is simply a device that converts small amounts of energy from one kind into another.

Piezoelectricity is found in many useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation.