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Lavoisier is most noted for his discovery of the role oxygen plays in combustion.
He recognized and named oxygen (1778) and hydrogen (1783), and opposed the phlogiston theory.
Lavoisier helped construct the metric system, wrote the first extensive list of elements, and helped to reform chemical nomenclature.

Dmitri Ivanovich Mendeleev was a Russian chemist and inventor.
He is best remembered for formulating the Periodic Law and creating a farsighted version of the periodic table of elements.
Mendeleev classified elements in the increasing order of Atomic Mass.
Atomic number of Mendelevium- 101.
Modern Periodic Law- Henry Moseley.

Valency is the combining capacity of an element. For metals, it is equal to the number of electrons in the outermost shell of their atoms, and for nonmetals it is eight minus the number of electrons in the outermost shell.

The carbon atom’s four valence electrons can be shared by other atoms that have electrons to share, thus forming covalent (shared-electron) bonds.

They can even be shared by other carbon atoms, which in turn can share electrons with other carbon atoms and so on, forming long strings of carbon atoms, bonded to each other like links in a chain.

Electronegativity is a measure of the ability of an atom to attract the electrons.
Fluorine is the most electronegative element.
Fluorine, being assigned an electronegativity value of 3.98.
Pauling scale is the relative scale with values for electronegativity of elements ranging from 0 to 4.

Electropositivity is the measure of the ability of elements (mainly metals) to donate electrons to form positive ions. The elements that can easily accept electrons to form negative ions are called electronegative elements,

The alkali metals are the most electropositive. . Francium, ( unstable ) is theoretically the most electropositive element.

Electropositivity increases down groups and decreases along periods (from left to right) on the periodic table

Cesium is the most electropositive of the stable elements.

Metals are electropositive in nature as they easily lose the electron to form a cation.

The element of group 1 have the highest metallic character and thus are the most electropositive elements.

The alkali metals are the most electropositive.

Cesium is the most electropositive of the stable elements.

Francium, while unstable, is theoretically the most electropositive element.

Near point = 25 cm while far point = infinity

A person with hypermetropia/hyperopia or long sight can see clearly objects far away from them, but not close to them.

This is caused by the shape of the eye – the eyeball is slightly too short.

It is corrected by spectacles or contact lenses with lenses which are ‘plus’ or convex in shape .

Myopia or short sightness can be corrected by using concave lens .

Bifocal lens are required to correct the presbyopia.

Upper point of bifocal lens consists of concave lens used for distant vision while lower point consists of convex lens facilitate near vision.

Ciliary muscles modify the curvature of eye lens.

When eye muscles are relaxed, eye lens becomes thinner thereby are relaxed, eye lens becomes thinner thereby increase in the focal length of eye lens.

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