NCERT Summary: Gist of Chemistry - 2 Notes | Study Science & Technology for UPSC CSE - UPSC

• Atoms are made up of three smaller particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus of the atom. Protons have a single positive charge. This is called the Atomic Number of an atom. The Atomic Number tells us the number of electrons that the atom contains. It is these electrons that determine the chemical properties of the atom and the way it combines with other atoms to form specific compounds. Electrons have a single negative charge. Normally, atoms are electrically neutral so that the number of electrons is equal to the number of protons.
• Electrons orbit around the nucleus. Electrons cannot orbit the nucleus of an atom in any orbit. The electrons are restricted to specific paths called orbitals or shells. Each shell can only hold a certain number of electrons. When a shell is full, no more electrons can go into that shell. The key to the properties of atoms is the electrons in the outer shell. A complete outer shell of electrons is a very stable condition for an atom.
• Valency: Hydrogen is the simplest element. It has one electron. Its outer shell only holds two electrons. Valency can be simply defined as the number of Hydrogen atoms that an element can combine with. The atoms with full electron shells (Helium, Neon, Argon) are chemically inert forming few compounds. The atoms don’t even interact with each other very much. These elements are gases with very low boiling points. The atoms with a single outer electron or a single missing electron are all highly reactive. Sodium is more reactive than Magnesium. Chlorine is more reactive than Oxygen. Generally speaking, the closer an atom is to having a full electron shell, the more reactive it is. Atoms with one outer electron are more reactive than those with two outer electrons, etc. Atoms that are one electron short of a  full shell are more reactive than those that are two short.
• Chemical bonds are what hold atoms together to form the more complicated aggregates that we know as molecules and extended solids. The forces that hold bonded atoms together are basically just the same kinds of electrostatic attractions that bind the electrons of an atom to its positively charged nucleus. chemical bonding occurs when one or more electrons are simultaneously attracted to two nuclei.
• Mainly 3 Types of bonds can be present in Chemical Compounds.
  1. Electrovalent or Ionic Bond: It is formed by Transferring of Electrons between 2 Atoms. These types of bonds are mainly formed between Metals and Non - Metals. These compounds exist in solid form. These compounds have high boiling Point, Melting Point and thermal stability.
  2. Covalent Bond: It is formed by equal sharing of Electrons between 2 Atoms. This type of bond is mainly formed between non - metals. These compounds may be solid, liquid or gas. These compounds have low boiling Point, Melting Point and thermal stability in comparison to Ionic Bond.

3. Co - Ordinate or Dative Bond: It is formed by unequal sharing of Electrons between 2 Atoms. This bond is also called as Semi - Polar bond since; it involves Electro valency and Covalency both. These compounds may be solid, liquid or gas. These compounds are insoluble in H₂O. These compounds do not conduct Electricity. These compounds have high B.P. than Covalent Compounds but less than Electrovalent Compounds.

• Atoms and Molecules, Elements and Compounds: There are about a hundred different types of atoms in the Universe. Substances made up of a single type of atom are called Elements. Some elements are made up of single atoms: Carbon( C), Helium(He), Sodium(Na), Iron(Fe) etc. He, Fe, and Na are the Chemical Symbols of the elements.
• Some elements are made up of groups of atoms: Oxygen(O₂), Ozone(O₃), Chlorine(Cl₃) etc. These groups of atoms are called molecules.
• Molecules can also be made up of combinations of different types of atoms. These substances are called compounds: Common Salt(NaCl), Methane(CH₄), Ammonia(NH₃) etc. O₂, CH₄, NH₃ are the Chemical Formulas of Oxygen, Methane and Ammonia respectively. CH₄ means that a single molecule of methane contains one atom of Carbon and four atoms of Hydrogen. This chemical formula could have been written but the C₁H₄ is never written. Similarly, a molecule of Ammonia (NH₃) contains one atom of Nitrogen and three atoms of Hydrogen.
• A change in which one or more new substances are formed is called a chemical change. A chemical change is also called a chemical reaction. The change may conveniently be represented by a chemical equation.
• Chemical reactions occur when different atoms and molecules combine together and spit apart. For example, if Carbon (C) is burnt in Oxygen (O₂) to form Carbon Dioxide, a Chemical Reaction occurs. This reaction can be written: C + O₂—> CO₂. This is called a Chemical Equation. The substances on the left hand side of the equation are called the Reactants. The substances on the right hand side are called the Products.
• There is one very important rule with chemical equations: The number of individual atoms on each side of the equation must be the same. On the left had side, there is an atom of Carbon and a molecule of Oxygen (containing two atoms). On the right hand side there is a molecule of carbon dioxide (containing one atom of carbon and two atoms of Oxygen). The number of atoms on the left hand side is equal to the number of atoms on the right hand side. All that has changed is the arrangement of the atoms. In a chemical reaction atoms are re-arranged; no atoms are destroyed or created.
• Hydrogen gas is mixed with Oxygen gas. If the mixture is sparked, it explodes to form water. This chemical reaction can be expressed as: H₂ + O₂ → H₂O. On the left had side, there is a molecule of Hydrogen (containing two atoms) and a molecule of Oxygen (also containing two atoms). On the right hand side there is a molecule of water (containing two atoms of Hydrogen and one atom of Oxygen). The left hand side has one extra atom of Oxygen. This is not allowed by the Law of Conservation of Matter. Both sides must contain the same number o  atoms. To make the equation conform, we must balance the equation. It is not possible to change the chemical formulas of the reactants or products. Water will always be H₂O. Balancing the equation is achieved by changing the number of molecules involved. The balanced form of the above equation is: 2H₂ + O₂ → 2H₂O. Now, on the left had side, there are two molecules of Hydrogen (each containing two atoms making four atoms) and a molecule of Oxygen (containing two atoms). On the right hand side there are two molecule of water (each containing two atoms of Hydrogen and one atom of Oxygen making a total of four atoms of Hydrogen and two of Oxygen). The equation is now balanced. In summary, when Hydrogen reacts with Oxygen, two molecules of Hydrogen react with one molecule of Oxygen to give two molecules of water.
• The reaction goes in both directions. While the Nitrogen and Hydrogen are combining to form Ammonia, Ammonia splits to form Hydrogen and Nitrogen. A mixture of all three substances results. This type of reaction is called an Equilibrium and is represented by arrows going in both directions. N₂ + 3H₂ → 2NH₃.
• It is possible to push the reaction in one direction by adding a Catalyst. A catalyst is a substance that helps a reaction without being used up. If Ammonia is removed from the equilibrium mixture, the reaction will move to produce more Ammonia so that equilibrium is attained.
• The total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction.
• During a chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Actually, chemical reactions involve the breaking and making of bonds between atoms to produce new substances.
• In a combination reaction two or more substances combine to form a new single substance.
• Decomposition reactions are opposite to combination reactions. In a decomposition reaction, a single substance decomposes to give two or more substances.
• Reactions in which heat is given out along with the products are called exothermic reactions.
• Reactions in which energy is absorbed are known as endothermic reactions.
• When an element displaces another element from its compound, a displacement reaction occurs.
• Two different atoms or groups of atoms (ions) are exchanged in double displacement reactions.
• Precipitation reactions produce insoluble salts.
• Reactions also involve the gain or loss of oxygen or hydrogen by substances. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. The substance that brings about oxidation and is itself reduced is termed as oxidizing agent and the substance that brings about reduction and is itself oxidized is referred to as reducing agent. There are a number of oxidation reduction reactions that are of industrial use. The production of metals from their ores invariably involves these two processes.

A. Matter and Its Nature

• Anything that possesses mass, occupies space, offers resistance and can be perceived through one or more of our sense is called matter.
• Matter is made up of particles. Particles of matter have space between them and are continuously moving and attract each other.
• Matter can exist in three states-
  I. Solid
  II. Liquid
  III. Gas
• Solid has a definite shape, distinct boundaries and fixed volumes, Solids have a tendency to maintain their shape when subjected to outside force. Solids may break under force but it is difficult to change their shape, so they are rigid.
• Liquids have no fixed shape but have a fixed volume. They take up the shape of the container in which they are kept. Liquids flow and change shape, so they are not rigid but can be called fluid.
• A gas has no definite volume or shape. gases are highly compressible as compared to solids and liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home for cooking or the oxygen supplied to hospitals in cylinders is compressed gas. Compressed natural gas (CNG) is used as fuel these days in vehicles.
• The forces of attraction between the particles(inter-molecular force) are maximum in solids, intermediate in liquids and minimum in gases. The spaces in between the constituent particles and kinetic energy of the particles are minimum in the case of solids, intermediate in liquids and maximum in gases.
• The arrangement of particles is most ordered in the case of solids, in the case of liquids layers of particles can slip and slide over each other while for gases, there is no order, particles just move about randomly.
• In spite of above differences all kinds of matter have a common property, the property of having a mass.
• The states of matter are interconvertible. The state of matter can be changed by changing temperature or pressure.
• On increasing the temperature of solids, the kinetic energy of the particles increases. Due to the increase in kinetic energy, the particles start vibrating with greater speed. The energy supplied by heat overcomes the forces of attraction between the particles. The particles leave their fixed positions and start moving more freely. A stage is reached when the solid melts and is converted to a liquid. The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.
• The process of melting, that is, change of solid state into liquid state is also known as fusion.
• During the melting, the temperature of the system does not change after the melting point is reached, till all the ice melts. This happens even though we continue to heat the beaker, that is, we continue to supply heat. This heat gets used up in changing the state by overcoming the forces of attraction between the particles. As this heat energy is absorbed by ice without showing any rise in temperature, it is considered that it gets hidden into the contents of the beaker and is known as the latent heat.
• The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.
• Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas at atmospheric pressure at its boiling point.
• Sublimation is the change of gaseous state directly to solid state without going through liquid state, and vice versa.
• Evaporation is a surface phenomenon. Particles from the surface gain enough energy to overcome the forces of attraction present in the liquid and change into the vapour state. The rate of evaporation depends upon the surface area exposed to the atmosphere, the temperature, the humidity and the wind speed. Evaporation causes cooling.
• During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool.
• Let us take some ice-cold water in a tumbler. Soon we will see water droplets on the outer surface of the tumbler. The water vapour present in air, on coming in contact with the cold glass of water, loses energy and gets converted to liquid state, which we see as water droplets.
• Properties such as shape, size, colour and state of a substance are called its physical properties. A change , which does not involve any alteration in composition of the substance is called a physical change. A physical change is generally reversible. In such a change no new substance is formed.
• Some substances can be obtained in pure state from their solutions by crystallisation.
• A change that alters the composition of a substance or substances taking part in the change is termed a chemical change. A chemical change is also called a chemical reaction. All new substances are formed as a result of chemical changes.
• Burning of coal, wood or leaves is a chemical change. Explosion of a firework is a chemical change. If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting. The process of rusting can be represented by the following equation: Iron (Fe) + Oxygen (O₂, from the air)water (H₂O) ’! rust (iron oxide- Fe₂O₃) For rusting, the presence of both oxygen and water (or water vapour) is essential. It is a chemical change.
• Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. Another way is to deposit a layer of a metal like chromium or zinc on iron. This process of depositing a layer of zinc on iron is called galvanisation.
• Stainless steel is made by mixing iron with carbon and metals like chromium, nickel and manganese. It does not rust.
• Changes attended with absorption of heat are called endothermic changes, while those which occur with evolution of heat are called exothermic changes. The reactions in which heat is absorbed are known as endothermic reactions, while chemical reactions which evolve heat are called exothermic. The compounds formed from their elements with absorption of heat are called endothermic compounds, whilst those formed from their elements with evolution of heat are called exothermic compounds.