Periodic Classification Of Elements

Non- metallic character increases from left to right in a period because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the tendency of gaining electrons also increases. So non- metallic character increases from left to right in a period.

Note-

(i) Effective nuclear charge is defined as the charge per unit volume.

(ii) From left to right in a period, the effective nuclear charge increases because charge increases from left to right due to increment of proton. But the size remains same because number of shells remains same from left to right in a period.

(iii) From top to bottom in a group, the effective nuclear charge decreases because charge increases down a group due to increment of proton and the size of elements also increases due to increment in shells.

Electropositive character of elements decreases in a period because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the tendency of losing electrons decreases. Hence the electropositive character of elements decreases in a period.

Note- Non-metals always gain electrons and metals lose their electrons.

Metallic character increases down a group because the size of elements increases down a group due to increment in the shells. So the effective nuclear charge decreases down a group. Due to this effective nuclear charge, the electronegativity decreases and the attraction of nucleus on valence shell electrons also decreases. So the tendency of losing electrons increases. Hence metallic character increases down a group.

Elements are placed in groups have same number of valence electrons.

Ex- Elements Na and K of first group have 1 valence electron.

Elements are placed in period have same number of shells.

Ex- Elements C, N and O of second period have K and L shells.

Na⁺ is smaller than Na because Na⁺ has less number of electrons and its last electrons are in L shell as compared to Na which has last electron in M shell but both have same number of protons. So the attraction of nucleus on valence electrons become strong in Na⁺ than Na. Hence the size of Na⁺ become smaller than Na.

Cl^- has bigger size as compared to Cl because Cl^- has more number of electrons than Cl and nucleus can hold less electrons tightly than more electrons. So the attraction on Cl^- become less. Hence, the Size of Cl^- become bigger than Cl.

Since element X belongs to second group so element X has 2 valence electrons. Hence the formula of its chloride will be XCl₂.

Since element B belongs to 13 group and second period so element B has 3 valence electrons and 2 shells. Hence the oxide of B will be B₂O₃.

The examples of elements are:

Group 1- Sodium (Na)

Potassium (K)

Group 2- Beryllium (Be)

Magnesium (Mg)

Group 16- Oxygen (O)

Sulphur (S)

Group 17- Fluorine (F)

Chlorine (Cl)

The last element of third size means last element of third period and the last element of third period is Argon (Ar) which has 8 valence electrons.

(i) Na has bigger size than Cl because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the size of atom reduces.

(ii) Cl has bigger size than F because number of shells increases down a group. So the size of atom also increases down a group.

(i) Na has the smaller size than K because number of shells increases down a group. So the size of atom increases down a group. Hence K is bigger and Na is smaller.

(ii) C has smaller size than B because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the size of atom reduces. Hence C has smaller size.

(i) Ca is a metal because it contains 2 valence electrons. So it will lose these electrons to acquire stable noble gas electronic configuration.

Ca₂₀- 2, 8, 8, 2

(ii) Yes, Ca is more reactive than Mg because the size of Ca is bigger than Mg. Due to this bigger size, the effective nuclear charge decreases so the attraction of nucleus on valence electrons decreases. So Ca will lose its valence electrons more easily than Mg.

(iii) Ca has two valence electrons so its valency will be 2.

(iv) Ca has two valence electrons and it will lose these two electrons so the formula of its chloride will be CaCl₂.

(v) Ca is smaller than K because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of the nucleus on valence shell electrons also increases. So the size decreases.

(a) We know that metallic character decreases from left to right in a period and metallic character increases down a group. So element C is the most metallic.

(b) We know that the size of atom decreases from left to right in a period and size of atom increases down a group. So atom C is the largest.

(a) Most reactive metal is Z because size of atom increases down a group and effective nuclear charge decreases. The attraction of nucleus on valence electrons also decreases. So, the tendency of losing electrons increases. Hence reactivity increases.

(b) We know that electronegativity of non-metals increases from left to right in a period due to the increment of effective nuclear charge and decreases down a group due to the increment of size of atom. So, the most reactive non-metal is L.

(c) L, Q, R, T are the elements of the halogen family.

(d) Group 2- Magnesium (Mg)

Group 13- Boron (B)

Group 15- Nitrogen (N)

(a) (i) Elements of 18 groups are noble gases. So Neon (Ne) with atomic number 10 and Argon (Ar) with atomic number 18 are the noble gases because these have complete octet.

(ii) Elements of 17 group are halogens. So Fluorine (F) with atomic number 9 and Chlorine (Cl) with atomic number 17 are halogens.

(iii) Elements of 1 group are alkali metals except hydrogen (H). So Lithium (Li) with atomic number 3 and Sodium (Na) with atomic number 11 are alkali metals.

(iv) Elements with valency 4 are elements of group 14. So Carbon (C) with atomic number 6 and Silicon (Si) with atomic number 14 are elements with valency 4.

(b) Element with atomic number 3 is Li which has 1 valence electron and element with atomic number 17 is Cl has 7 valence electrons. Element with atomic number 17 wants one electron to complete its octet. So the formula will be LiCl.

(c) The electronic configuration of element with atomic number 10 is

₁₀Ne- 2, 8

(i) Mg.

(ii) Al.

(iii) O and S.

(iv) Most reactive non-metal of group 17 is fluorine because its electronegativity is more. Its symbol is F.

K is more reactive than Li because the size of K is bigger than Li and due to this bigger size, the effective nuclear charge decreases so the attraction of nucleus on valence electrons also decreases. So K loses its valence electrons more easily than Li.

He, Ne and Ar are called inert gases because they have complete octet and due to this complete octet they do not react to any other.

Br has larger atomic size than Cl because the atomic size increases from top to bottom in a group. From top to bottom in a group, the number of shells increases. So, the atomic size increases.

(a) The elements with atomic number 15, 16, 17 and 18 are non-metals in 3 period because silicon with atomic number 14 is metalloid and rest of the elements are metals in 3 period.

(b) We know that non-metals form negative ions. So, the elements with atomic number 15, 16, 17 and 18 are the elements in 3 period which form negative ions.

(c) We know that metals have highest melting point due to strong metallic bond. So, the elements with atomic number 11, 12 and 13 have highest melting point in 3 period.

(d) We know that metals form positive ions. So, the elements with atomic number 11, 12 and 13 are the elements in 3 period which form positive ions.

(e) The elements with atomic number 11, 12 and 13 are the metals in 3 period.

(f) We know that non-metals have low boiling point. So, the elements with atomic number 15, 16, 17 and 18 are the elements which have low boiling point.

(a) We know that every element of group 17 has 7 valence electrons. So this element also has 7 valence electrons.

(b) It is non-metal because it can only gain electron.

(c) Its name is Bromine (Br).

(d) The formula of its compound with hydrogen is HBr.

(e)

(a) Element E will form only covalent bond because it has 4 valence electrons.

(b) Element B is a non-metal with valency 2 because element B has 6 valence electrons so it can take only two electrons to complete its octet.

(c) Element D is a metal with valency 2 because it has 2 valence electrons.

(d) We know that atomic size increases from up to down in a group due to increment in number of shells. So F has largest atomic size.

(e) Noble gases is the name of family to which H, C and F belong.

(a) Element belongs to 17 group and 3 period.

(b) Atomic number of this element is 17 because atomic number is equal to total electrons present in it.

(c) It is non-metallic because it has 7 electrons in its valence shell so it can gain only one electron to complete its octet.

(d) This element is Chlorine.

(e) Bromine is chemically similar to this element because bromine and chlorine are the elements of same group.

(a) Element C is non-metal because it belongs to 17 group and it contains 7 valence electrons so it can gain only one electron. So it is non-metal.

(b) In non-metals, electronegativity increases from left to right in a period due to increase in effective nuclear charge and decreases from top to bottom in a group due to increment in size. So A is the most active element.

(c) Since C is non-metal and it has 7 valence electrons. So it can gain only one electron and anion will be formed.

(i) A will be hydrogen (H) because A, B, C, D, E, F and G all have same number of valence electrons so this means these elements belong to same group. Since element C combine to Cl and forms NaCl so these elements belong to 1 group. Only H form an acid on reaction with Cl in 1 group and its oxide is also neutral substance.

(ii) Elements A to G will belong to same group but periods will different because number of shells are different in all elements.

(iii) A is hydrogen (H) then the formula of the compound formed by the reaction of the element A with oxygen is H₂O.

(iv) Element C is Na because C combine with chlorine and forms common salt (NaCl).

(v) A is H so the compound formed by the combination of element A with carbon will be CH₄ and the ratio of combining atoms will be 4:1.

(vi) The element which has the smallest atomic radius is A.

(a) (i) G with atomic number 10 is a noble gas because it has complete octet.

(ii) F with atomic number 17 is a halogen.

(b) B with atomic number 11 has 1 valence electron and F with atomic number 17 has 7 valence electrons so the formula of the compound formed would be BF.

(c) Electronic configuration of C and E are

E₈- 2, 6

C₁₂- 2, 8, 2

(i) Electronic configuration of F is

F₁₂- 2, 8, 2

(ii) There are 2 valence electrons in the atom of F.

Explanation- Valence electrons are those electrons which present in the last shell.

(iii) There are 3 shells in the atom of F.

(iv) We know that atomic size increases from top to bottom in a group due to increment in shells. So the order will be H>G>F>E.

(v) Since F has 2 valence electrons so it will donate these two electrons to acquire noble gas electronic configuration. Hence it is a metal.

(vi) We know that atomic size increases from top to bottom in a group and decreases from left to right in a period. So B has the bigger atomic size.

(a) (i) Atomic radius decreases from left to right in a period because effective nuclear charge increases from left to right in a period. Due to this effective nuclear charge, the electronegativity increases and the attraction of nucleus on valence shell electrons also increases. So the size decreases.

(ii) Atomic radius increases down a group because number of shells increases.

(b) X₁₂- 2, 8, 2

Y₁₆- 2, 8, 6

These two elements belong to 3 period because period of an atom in periodic table is equal to the number of shells present in that atom.

These two elements will form ionic bond because X has two valence electrons so it will completely lose these two electrons to acquire noble gas electronic configuration and Y has six valence electrons and it has more electronegativity so it will gain two electrons to complete its octet. So X will completely give its two electrons to Y and ionic bond will form.

(a) Li₃- 2, 1

(b) Li belongs to 1 group.

(c) The valency of Li is 1 because it contains one valence electron.

(d) It will form positive ion because it has one valence electron so it will lose this one electron to acquire stable noble gas electronic configuration and it will form positive ion.

(e) The formula of the compound formed by it is LiX.