The three elements A, B and C with similar properties have atomic masses X, Y and Z respectively. The mass of Y is approximately equal to the average mass of X and Z. What is such an arrangement of elements called as? Give one example of such a set of elements.
The arrangement is called a Döbereiner's triad.
Example: Lithium, Sodium and Potassium.
Elements have been arranged in the following sequence on the basis of their increasing atomic masses: F, Na, Mg, Al, Si, P, S, Cl, Ar, K.
(a) Pick two sets of elements which have similar properties.
(b) The given sequence represents which law of classification of elements?
(a) (i) F and Cl; (ii) Na and K.
(b) Newland's law of octaves.
Can the following groups of elements be classified as Döbereiner's triad?
(a) Na, Si, Cl
(b) Be, Mg, Ca
Atomic masses: Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40. Explain by giving reason.
(a) No. These elements do not have similar properties so they cannot be classified as a Döbereiner triad, even though Si's atomic mass lies between Na and Cl.
(b) Yes. Be, Mg and Ca have similar properties and the atomic mass of Mg is approximately the average of Be and Ca, so they form a Döbereiner triad.
In Mendeleev's Periodic Table the elements were arranged in the increasing order of their atomic masses. However, cobalt with atomic mass of 58.93 amu was placed before nickel having an atomic mass of 58.71 amu. Give reason for the same.
Elements were arranged by chemical properties as well as atomic masses. Cobalt was placed before nickel because its chemical properties (and its position in the periodic pattern of properties) fitted that place better despite the small difference in atomic mass.
"Hydrogen occupies a unique position in Modern Periodic Table". Justify the statement.
Hydrogen resembles alkali metals (Group 1) in having one electron and can form H+ but it also resembles halogens in forming H− and covalent compounds. Thus it shows properties of both groups and occupies a unique position.
Write the formulae of chlorides of Eka-silicon and Eka-aluminium, the elements predicted by Mendeleev.
Eka-silicon: GeCl4.
Eka-aluminium: GaCl3.
Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies.
Element A: Group 13 (valency 3).
Element B: Group 14 (valency 4).
Element C: Group 2 (valency 2).
If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride?
Formula: XCl4.
Nature of bonding: Covalent bonding.
Compare the radii of two species X and Y. Give reasons for your answer.
(a) X has 12 protons and 12 electrons
(b) Y has 12 protons and 10 electrons
Y is a cation of X (Y has lost two electrons) and therefore has a smaller radius than X because reduced electron–electron repulsion and greater effective nuclear attraction shrink the radius of Y compared to X.
Arrange the following elements in increasing order of their atomic radii.
(a) Li, Be, F, N
(b) Cl, At, Br, I
(a) F < N < Be < Li.
(b) Cl < Br < I < At.
Identify and name the metals out of the following elements whose electronic configurations are given below.
(a) 2, 8, 2
(b) 2, 8, 1
(c) 2, 8, 7
(d) 2, 1
Metals: (a) 2,8,2 → Magnesium (Mg).
(b) 2,8,1 → Sodium (Na).
(d) 2,1 → Lithium (Li).
Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed?
Product formula: KCl.
Electronic dot structure: K : → :Cl (transfer of one electron from K to Cl forming K⁺ and Cl⁻).
Nature of bond: Ionic bond.
Arrange the following elements in the increasing order of their metallic character: Mg, Ca, K, Ge, Ga.
Increasing metallic character: Ge < Ga < Mg < Ca < K.
Identify the elements with the following property and arrange them in increasing order of their reactivity:
(a) An element which is a soft and reactive metal
(b) The metal which is an important constituent of limestone
(c) The metal which exists in liquid state at room temperature
(a) Soft and reactive metal: Sodium (Na) or Potassium (K).
(b) Important constituent of limestone: Calcium (Ca).
(c) Metal liquid at room temperature: Mercury (Hg).
Increasing reactivity order: Hg < Ca < Na < K.
Properties of the elements are given below. Where would you locate the following elements in the periodic table?
(a) A soft metal stored under kerosene
(b) An element with variable (more than one) valency stored under water
(c) An element which is tetravalent and forms the basis of organic chemistry
(d) An element which is an inert gas with atomic number 2
(e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of "anodising"
(a) Sodium (Na) — Group 1, Period 3 (or Potassium (K) — Group 1, Period 4).
(b) Phosphorus (P) — Group 15, Period 3 (variable valency).
(c) Carbon (C) — Group 14, Period 2 (tetravalent, basis of organic chemistry).
(d) Helium (He) — Group 18, Period 1 (inert gas with atomic number 2).
(e) Aluminium (Al) — Group 13, Period 3 (forms thin oxide layer used in anodising).