Chapter 6:ELEMENTS

  Introduction  

Metals are malleable [can be beaten into sheets], ductile [pulled into wires] and conductors of heat and electricity, sonorous [produce ringing sounds when struck].

Exceptions:

1. lithium, sodium, potassium [alkali metals] are soft and can be cut;
   
2. mercury is liquid at room temperature.
   
3. Iodine is a non metal but lustrous.
   
4. Carbon exits as different allotropes like diamond [hardest substance, sharpening tools, manufacturing tungsten filaments for light bulbs, cutting, grinding], and graphite [conductor of electricity, lubricant in machines where oil can’t be used due to high temperature].

Facts of Metals

Generally, oxides of metals are basic and non metals are acidic.

Sodium is very reactive with oxygen and hence stored in kerosene. Phosphorous reacts with oxygen so kept in water.

Brass [copper+zinc], bronze [copper+tin], solder [lead+tin] welding electrical wires together.

Metals react with acids to produce hydrogen gas. Non metals don’t react.

More reactive metals can replace less reactive metals in reactions.

Out of 98 naturally occurring elements, 20 are non metals. Most are metals. Bromine is non metal, which is liquid at room temperature. Metalloids have property of both metals and non metals. e.g.: boron, silicon, and germanium.

Coke is a pure form of carbon, used in manufacture of steel.

Coal tar is starting material for synthetic fibers, drugs, naphthalene balls are made from coal tar.

Chemical Properties of Match-stick:

Matchstick has antimony trisulphide and potassium chlorate. The rubbing surface has powdered glass and red phosphorous. On striking the surface, red phosphorous get converted to white phosphorous which reacts with potassium chlorate to create enough heat to ignite the antimony trisulphide.

Carbon dioxide as an extinguisher:

CO2 is a good fire extinguisher if petrol or oil is burning. Even electrical fires are extinguished by using CO2 as it cuts off oxygen supply to fire by making a protective blanket as it’s heavier than O2.

Heat Energy:

The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilo-Joules per kg (kJ/kg).

A paper cup filled with water doesn’t burn as heat is transferred to the water and it doesn’t reach its ignition temperature.

Ethylene Glycol [antifreeze] reduces freezing point of water to -17.2C.

Lavoisier’s laws of chemical compositions:

Mass can neither be created nor destroyed in a chemical reaction.

In chemical substances, the elements are always present in definite proportion by mass.

1 atomic mass unit was equal to exactly one twelfth the weight of one atom of carbon-12.

Ions are charged particles, positively charged are cat ions and negatively charged are anions.

The number of particles present in 1 mole of any substance is fixed as 6.022*10^23. 1 mole of a substance is its relative atomic or molecular mass in grams.

e.g.: 1 mole of carbon atoms = 12 g of carbon atoms.

Atoms and Properties:

Sub atomic particles, electrons were discovered by J.J.Thomson by the cathode ray experiment and protons were discovered by Goldstein. Chadwick discovered neutrons.

Atomic number is the number of protons in an atom. Mass number is the sum of protons and neutrons.

Isotopes have same atomic number [due to same number of protons but different number of neutrons] but different mass numbers. e.g.: isotopes of hydrogen are protium, deuterium and tritium. Chemical properties of isotopes are same [as they depend on electronic structure] but their physical properties are different.

Special isotopes: isotope of cobalt is used to treat cancer. Isotope of iodine is used to treat goiter.

Isobars: atoms of different elements with same mass number but different atomic number.

Isotones have same number of neutrons but different number of protons [and electrons].

Becquerel discovered radioactivity. In this an unstable nucleus undergoes decay.

Types of decay are

1. ά – Decay: helium nucleus is emitted. Heavy, slow, least penetrating, easily blocked, positive charged.
2.       B-decay: electrons or positrons are emitted. Less Heavy, fast, more penetrating [100 times more than alpha], tough to block, negatively / positively [positron] charged.
3.      γ-decay: high energy photons are emitted. Lightest, fastest, highly penetrating [1000 times more than alpha], toughest to block, no charge.

Miscellaneous:

Dihydrogen is the most abundant element in the universe [70% of total mass of the universe]. Oxygen is the most abundant element [up to 47%] on earth.

The mixture of CO and H2 is called water gas or synthetic gas. It’s used in the synthesis of hydrocarbons and methanol.

Hydrogenation of vegetable oils using nickel as catalyst gives edible oils.

Aqua regia is mixture of hydrochloric and nitric acid in ratio of 3:1 it can dissolve even gold and platinum.

Anaesthesia is oxygen and nitrous oxide. Carbogen is 95% oxygen and 5% CO2, it stimulates natural breathing.

Zones of candle flame: Outer zone [blue] – hottest, middle zone [yellow] –less hot, inner zone [black] – least hot.

In limestone caves, stalactites are seen hanging from the roofs and stalagmites are seen at the bottom of the cave.

Official YouTube Channel of UPSCFEVER - #1

Quiz

Score more than 80% marks and move ahead else stay back and read again!