Chemistry Chapter – 4 : Periodic Classification of Element
12 August, 2024
Introduction: Periodic Classification of Element
Important data on Modern Periodic Table-
Isotopes are placed at one place in the same group.
There is no element between hydrogen and helium as atomic masses always comes in whole numbers.
Atomic number is represented by Z, and it is equal to the number of protons in the nucleus of the atom.
It also consists of 18 vertical columns known as Groups and 7 horizontal rows known as Periods.
Elements having same number of valence electrons are placed in the same group.
As we go down in a group, number of shell increases.
Elements having same number of occupied shells are placed in same period.
Each period has a new electronic shell getting filled.
Number of elements placed in a particular period depends on the point how electrons are filled into various shell.
To find out the number of electrons in a shell, 2n2 formula can be used, where is n is shell number.
K Shell n = 1 or 2n2 = 2(1)2 = 2
L shell n = 2 or 2n2 = 2(2)2 = 8
Valence electron also determines the number of bonds which is formed by an element.
Valency and Valence Electrons – On moving left to right in a period, valency increases and then it decreases. But it remains same down in a group.
As we move from left to right in a period, valence electron increases and remain same as we go down the group.
Atomic Size – It decreases left to right in a period as the nuclear charge increases due to large positive charges on the nucleus.
Atomic size increases down in a group due to decrease in nuclear charges and addition of new shell.
Metallic Character – Ability of atom to lose the electron is known as MetallicCharacter. Metallic character decreases from left to right in a period.
This is due to increase in nuclear charge. But non-metallic character increases left to right in a period. And metallic character increases down the group as the size increases it can easily lose electron.
Groups in Modern Periodic Table-
The modern periodic table contains 18 vertical columns known as groups.
Group 1 elements are known as alkali metals. Group 2 elements are known as alkaline earth metals. Group 15 elements are known as pnicogens. Group 16 elements are known as chalcogens. Group 17 elements are known as halogens. Group 18 elements are known as noble gases.
Alkali Metals-
The elements in the first group, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) are called alkali metals. They were given the name because they all react with water to form alkalis.
The alkali metals are all shiny, soft, highly reactive solids at standard temperature and pressure and readily lose their outermost electron to form cations with charge +1. Number of valence electrons = 1
Alkali Earth Metals-
The elements in the second group, beryllium(Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra) are called alkaline earth metals.
They were given the name because their oxides are alkaline in nature.
They are all shiny, silvery-white, somewhat reactive hard solids at standard temperature and pressure. They lose two electrons from their outermost shell to form cations with charge +2.
Number of valence electrons = 2
Halogens-
The elements in the seventeenth group (F, Cl, Br, I and As) are called halogens and exist as diatomic molecules. The symbol ‘X’ is often used generically to refer to any halogen.
They were given the name halogen, from the Greek words, Hal (“salt”) and gen (“to produce”), because they all produce a wide range of salts on reacting with metals.
The halogens exist at room temperature in all three states of matter: Solid – Iodine, Astatine. Liquid – Bromine. Gas – Fluorine, Chlorine.
Number of valence electrons = 7
Noble Gases-
The elements in the eighteenth group, helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn) are called noble gases.
They are all odourless, colourless and monatomic gases with very low chemical reactivity.
Since their valence shell is considered to be “full”, they have little tendency to participate in chemical reactions.
When discovered and identified, scientists thought they are exceedingly rare, as well as chemically inert, and therefore these gases were also given the names ‘rare’ or ‘inert’ gases.
Number of valence electrons = 8
Classification of Modern Periodic Table-
Metals-
they are electropositive as they form bonds by losing electrons. In general cases, oxides of metals are basic in nature.
Non-metals-
Nonmetals are electronegative as they form bonds by gaining electrons. In general cases, oxides of non-metals are acidic in nature.
Metalloids-
The elements which show the properties of both metals and nonmetals are called metalloids or semimetals. For example – Boron, silicon, germanium, arsenic, antimony, tellurium and polonium.
Write the answer of the following questions.
Newlands relation is called (a) Musical Law (b) Law of Octaves (c) Periodic Law (d) Atomic Mass Law
Upto which element, the Law of Octaves was found applicable? (a) Oxygen (b) Calcium (c) Cobalt (d) Potassium
In Mendeleev’s Periodic Table, gaps were left for the elements to be discovered later. Which of the following elements found a place in the Periodic Table later? (a) Chlorine (b) Silicon (c) Oxygen (d) Germanium
At the time of Mendeleev, the number of elements known was (a) 63 (b) 65 (c) 62 (d) 64
The properties of eka-aluminium predicted by Mendeleev are the same as the properties of later discovered element: (a) Scandium (b) Germanium (c) Gallium (d) Aluminium
An atom of an element has the electronic configuration 2,8,2. To which group does it belong? (a) 4th group (b) 6th group (c) 3rd group (d) 2nd group
The arrangement of elements in the Modem Periodic Table is based on their (a) increasing atomic mass in the period (b) increasing atomic number in the horizontal rows (c) increasing atomic number in the vertical columns (d) increasing atomic mass in the group
Where would you locate the element with electronic configuration 2, 8 in the Modern Periodic Table? (a) Group 8 (b) Group 2 (c) Group 18 (d) Group 10
Element ‘X’ forms a chloride with the formula XCl2, which is a solid with high melting point. X would most likely be in the same group of the periodic table as: (a) Si (b) Mg (c) Al (d) Na
Which of these belong to the same period? Element Atomic number A 2 B 10 C 5
(a) A, B (b) B, C (c) C, A (d) A, B and C
Carbon belongs to the second period and Group 14. Silicon belongs to the third period and Group 14. If atomic number of carbon is 6, the atomic number of silicon is (a) 7 (b) 14 (c) 24 (d) 16
Pick out the chemically most reactive elements from the given triads. Li, Na, K F, Cl, Br (a) Li and F (b) Li and Br (c) K and F (d) K and Br
What is the atomic number of element of period 3 and group 17 of the Periodic Table? (a) 10 (b) 4 (c) 17 (d) 21
Which one of the following statements is not correct about the trends in the properties of the elements of a period on going from left to right? (a) The oxides become more acidic (b) The elements become less metallic (c) There is an increase in the number of valence electrons (d) The atoms lose their electrons more easily
The elements A, B and C belong to groups 1, 14 and 17 respectively of the Periodic Table. Which two elements will form ionic compounds? (a) A and B (b) A and C (c) B and C (d) None
An element X from group 2 of the Periodic Table reacts with Y from group 17 to form a compound. Give the formula of the compound. (a) XY2 (b) XY (c) X2Y (d) (XY)2
A metal ‘M’ is in the first group of the Periodic Table. What will be the formula of its oxide? (a) MO (b) M2O (C) M2O3 (d) MO2
Name the neutral atom in the Periodic Table which has the same number of electrons as K+ and Cl-. (a) Helium (b) Argon (c) Neon (d) Krypton
An element X combines with oxygen to form an oxide XO. This oxide is electrically con¬ducting. Write the formula of the compound formed when X reacts with chlorine. (a) XCl3 (b) XCl (c) XCl2 (d) XCl5
An element X has mass number 40 and contains 21 neutrons in its atom. To which group of the Periodic Table does it belong? (a) Group 1 (b) Group 4 (c) Group 2 (d) Group 3
Consider the following elements 20Ca, 8Or 18Ar, 16S, 4Be, 2He Which of the above elements would you expect to be in group 16 of the Periodic Table? (a) 20Ca and 16S (b) 20Ca and 8O (c) 18Ar and 16S (d) 8O and 16S
An element ‘A’ belongs to the third period and group 16 of the Periodic Table. Find out the valency of A. (a) Valency = 6 (b) Valency = 2 (c) Valency = 1 (d) Valency = 3
Which one of the following statements is not correct about the trends in the properties of the elements of a group on going down in a group? (a) The chemical reactivity of metals increases. (b) The metallic character of elements increases. (c) The size of the atom increases. (d) The valence electrons increase.
Which of the following set of elements is written in order of their increasing metallic character? (a) Na Li K (b) C Q N (c) Mg Al Si (d) Be Mg Ca
The atom of an element has electronic con-figuration 2, 8, 7. To which of the following elements would it be chemically similar? (a) N(7) (b) P(15) (c) Na(11) (d) F (9)