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PERIODIC TABLE OF ELEMENTS

The Periodic Table of elements is a form of classifying elements systematically by placing elements with similar features in the same group. The elements are arranged in the PT in ascending order of their proton number. number.

Historical Development of the Periodic Table Antoine Lavoisier (1743-1794) (1743He was the first person to attempt to classify elements in the form of a table. He classifed all elements into 4 groups. This classification failed as many elements showing different chemical properties were classified together.

J. W. Dobereiner (1780-1849) (1780He introduced the Triad Law where he classified the elements with similar properties in the same group. Each group comprised only 3 elements, thus the name triad. The relative atomic mass of the second element in each triad was equal to or almost equal to the average relative atomic mass of the other 2 elements. This law failed too because it was limited to some elements only.

John Newlands (1838-1898) (1838He proposed the Octave Law, which classified elements according to increasing relative atomic mass. He grouped the elements under horizontal groups containing 7 elements. The chemical and physical properties of each element were similar to the eight elements. This law could be applied to the first 17 elements only, thus it failed too.

Lothar Meyer(1830-1895) Meyer(1830He successfully showed that the properties of elements varied periodically by plotting the graph of atomic volume against the atomic mass of each element. He found that elements occupying corresponding positions on the graph showed similar chemical properties.

Dmitri Mendeleev (1834-1907) (1834He arranged according to increasing atomic numbers in horizontal groups called periods. According to his Periodic Law, the Law, properties of elements varied periodically. Elements with similar chemical properties were placed in vertical rows called groups.

Mendeleev was more successful than Newlands because: a) he realised several elements were still undiscovered and so left empty spaces for them. b) Elements like iron, cobalt and nickel which had completely different properties from the other elements were placed in a separate group. Mendeleev was able to predict the properties of the undiscovered elements, such as germanium which was only discovered years later. He also predicted Iodine, Nickel, Cobalt and Tellurium.

Henry Moseley(1887-1915) Moseley(1887He discovered the proton number of elements by investigating the frequency of X-rays Xproduced by each element. He proposed that it was the proton number that determined the position of elements in the PT. He arranged the elements according to ascending proton numbers. Moseley also proved that Mendeleev s prediction were correct especially regarding the position of Nickel, Cobalt, Iodine and Tellurium in the table. Mosely s contribution formed the base for the Modern Periodic Table.

MODERN PERIODIC TABLEThis is arranged according to ascending proton numbers of the elements. There are 18 groups called Groups, which are named Group 1,2, 3 ..18. Elements exhibiting similar chemical properties are placed in the same group.. They physical properties change gradually as we go down the group. There are 7 horizontal rows called Periodic, which are named Period 1, 2,3 .7. The elements of each period are placed according to ascending proton number.s

Period 1 has only 2 elements, Periodic 2 and 3 contains 8 elements, Period 4 and 5 have 18 elements and are shown seperately as the Lantanide and Actinide Series. Period 1, 2 and 3 are called short Periods. Period 4 and 5 are called long periods. Some of the Groups have special name: Eg: Group 1-Alkali metals 1The elements in Groups 1, 2 and 13 and the transition groups are metals, while elements in Group 15-18 are non metals.Group 14 has 2 non 15metals namely carbon and silicon.

Electron ArrangementThe number of electrons in a neutral atom is equivalent to its number of proton. The electrons are arranged in shells around the nucleus. The maximum number of electrons which can occupy each shell : 1-2 electrons 2-8 electrons 3- 8 or 18 electrons 4-46 electrons

All elements in a group Have the same valence electrons Have the same chemical properties Exhibit physical properties which change gradually down the group. By knowing the proton number of an element, we can determine the group and Period it is placed in.

An element W has a proton number of 15. What group and Period does it occupy in the PT of elements? Number of protons= number of electrons=15 The electron configuration= 2.8.5 Group???? Period????

Group 18Elements: Helium (He) Neon (Ne) Krypton (Kr) Xenon (Xe) Radon (Rn) They are called noble or inert gases. They are single atoms and so are monoatomic gases.

Physical PropertiesThe atomic size of the elements increase as going down the group.This is because an additional shell is added on each of the element. All noble gases cannot dissolve in water and cannot conduct electricity and heat. The m.p and b.p increase down the group as the atomic size or radius increases. The Van der Waals forces become stronger and more heat energy is needed to overcome these forces. Density of elements increase as going down the group.This is because of the proton number increase when going down the group. This causes an increase in atomic size.

Stability of Noble GasesNoble gases are not chemically reactive because they exist as monoatomic gases which cannot combine ionically or covalently with any other element. This is because their outermost shells are completely filled. He has the duplet (2 electrons) while the others have the octet( 8 electrons) arrangement. As their outermost shells are complete, these atoms cannot accept, donate or share electrons with other elements.Thus, they are chemically not reactive or known as inert.

Uses of Group 18 elementsHeliumHelium- to fill air ships and weather ballons. NeonNeon-to light up billboards on hotels, building and airport runways. ArgonArgon-used in filament bulb to prevent the burning of the tungsten filament. KryptonKrypton- to fill high- speed flash bulb in highphotographs.

Group 1 elementsElements: Lithium (Li) Sodium (Na) Potassium (K) Rubidium (Rb) Caesium (Cs) Fransium (Fr) Are known as alkali metals. metals.

Pyhsical PropertiesAll group 1 elements are solid metal which soft and shiny. They can conduct electricity and heat. They have high m.p and b.p.

Changes in physical property down the groupAtomic radius or atomic size of element increases Melting and boiling point decrease

Brief explanation

Number of atomic shells in increases Atomic size increases causing the metal bond between atoms to become weaker. Increase in atomic mass more than the increase in atomic radius Tendency to donate electrons to form positive ions increases

Density increases

Electropositivity increases

Chemical properties of Group 1 elementsAll atoms of alkali metals have one valence electrons which is one electron at the outermost shell. They donate this 1 electron to attain gas noble stability. A cation (positive ion) +1 is formed. Reactivity of alkali metals increases when going down the group. As atomic radius increases, the distance between valence electrons and nucleus increases. There are weak force attraction between nucleus and valence electrons.It is easier to release the valence electron as the atomic size inreases.

A. Reaction of group 1 with waterAll group 1 elements reacts with water. 2M + 2H O 2MOH + H

Example: 2Li + 2H O

2LiOH + H

Results???? Li-moves slowly on water surface. Burns Liwith red flame. SodiumSodium- moves quickly on water surface. Burns with bright yellow flame. PotassiumPotassium- moves vigorously on water surface.Emits a brilliant purple flame.

B. Reaction Group 1 with Oxygen4M(s) + O Eg: 4Li(s) + O 2M2O(s) 2Li2O(s)

C. Reaction Group 1 with Chlorine2M(s) + Cl2 Eg: 2Na(s) + Cl2 2MCl(s) 2NaCl(s)