Month: July 2015

1. Newlands proposed the law of octaves. Mendeleev suggested eight groups for elements in his table. How do you explain these observations in terms of modern periodic classification? (AS – 1) (4 Marks)

A: According to Newland’s law of octaves every eighth element starting from a given element resembles in its properties to that of the starting element. If we start at Lithium, then the eighth element is sodium and next coming eighth is potassium and so on. These elements show similar physical and chemical properties.

According to modern periodic table elements lithium, sodium, potassium …. etc. are all placed in the same group starting that they have similar physical and chemical properties.

Mendeleev’s periodic table has eight vertical columns. These are groups. There are seven horizontal rows. These are periods. According to Mendeleev’s table elements present in a group have similar properties. What is proposed by Mendeleev was supported in modern periodic table?

For example Mendeleev gave the general formula for the first group of elements as R2O and for second group of elements as RO, the same thing was followed in modern periodic table.

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2. What are the limitations of Mendeleev’s periodic table? How could the modern periodic table over come the limitations of Mendeleev’s table? (AS – 1) (4 Marks)

A: Limitations of Mendeleev’s periodic table

1. Anomalous pair of elements: Certain elements of highest atomic weights precede those with lower atomic weights. For example, tellurium (atomic weight 127.6) precedes iodine (atomic weight 126.9).

2. Dissimilar elements placed together: Elements with dissimilar properties were placed in same group as sub-group A and sub-group B. For example, alkali metal like Li, Na, K etc., of I A group have little resemblance with coinage metals like Cu, Ag, Au of I B group. Cl is of VII A group and ‘Mn’ is of VII B, but chlorine is a non metal, where as manganese is a metal.

Modern periodic table: Over coming the above limitations:

The periodic law is changed from atomic weight concept to atomic number concept in the modern periodic law.

a) In Mendeleev’s periodic table tellurium (Te) (atomic weight 127.6) precedes iodine (atomic weight 126.9).

b) In modern periodic table this placement is made justified because the atomic number of tellurium is 52 and that of iodine is 53.

c) So in the long form of periodic table these dissimilar elements changed their place in groups and periods.

a) In the long form of periodic table ‘Cl’ falls in 3rd period and VII A group and ‘Mn’ falls in 4th period and VII B group.

b) Metals and non-metals are separated here.

The vertical columns known as groups are represented by using roman numerals I through VIII with letters A and B in traditional notation. According to latest recommendation of the IUPAC, these groups are represented by arabic numerals 1 through 18 with no A and B designations.

There are 7 periods in the modern periodic table. These periods are
represented by the arabic numerals 1 through 7. Each period starts with a new main shell. ‘s’ sub-shell and ends when the main shell is filled with respect to the ‘s’ and ‘p’ sub-shells (except the first period).

See: Synopsis of this lession – Part 1

The first period starts with K – shell. The first main shell (K) contains only one sub – shell. (1s). For this sub – shell only two types of electronic configurations are possible and they are 1s1 (H) and 1s2 (He). Therefore the first period contains only two elements.

Second period starts with the 2nd main shell (L). L – shell has two sub – shells namely, 2s and 2p. Eight types of configurations are possible in this shell (L) like 2s1 and 2s2 and 2p1 to 2p6. Hence the second period contains 8 elements.

Third period starts with third main shell (M). This shell (M) has 3 sub-shells namely 3s, 3p and 3d, but while electrons are being filled into the shell ‘3d’ gets electrons only after ‘4s’ is filled. Therefore the 3rd period contains again 8 elements, which includes 2 s – block elements and 6 p – block elements.

Fourth main shell (N). This shell (N) has 4 sub – shells namely 4s, 4p, 4d and 4f; but while electrons are being filled into the shell, electrons enter the atoms in the order 4s, 3d, and 4p. Due to this, the fourth period contains 18 elements which includes 2 s-block, 10 elements from d-block and 6 elements from p – block. There are altogether 18 elements in fourth period.

On the same lines, we can explain why there are 18 elements in the 5th period. There are 32 elements in the 6th period which includes 2 elements from s- block (6s), 14 elements from f- block (4f), 10 elements from d – block (5d) and 6 elements from p-block (6p). 4f elements are called lanthanoids or lanthanides.

7th period is incomplete and contains 2 elements from s-block (7s) and 14 elements from ‘f’ – block (5f), 10 elements from d – block (6d) and some elements from p-block (7p). The 5f elements are called Actinoids or as Actinides.

The ‘f’ block elements known as lanthanoids and actinoids are shown, separately at the bottom of the periodic table. The elements with three or less electrons in the outer shell are considered to be metals and those with five or more electrons in the outer shell are considered to be non metals.

Metalloids or semi metals are elements which have properties that are intermediate between the properties of metals and non-metals. They are generally semi-conductors. Valance (or) valency of an element was defined as the combining power of an element with respect to hydrogen, oxygen or indirectly any other element through hydrogen and oxygen.

In general the valence of an element with respect to hydrogen is its traditional group number. If the element is in the group V or above, its valency is 8-group number. For example, Chlorine valence is 8 – 7 = 1 Now a days the valence of an element is generally taken as the number of valence shell (outer most shell) electrons in its atom.

Oxidation number concept almost is the latest substitute to the valence concept in the modern literature. Atomic radius of an element may be defined as the distance from the centre of the nucleus of the atom to its outermost shell.

Atomic radius is measured in ‘pm’ (pico meter) units. 1 pm = 10-12 m. Atomic radii of elements decrease across a period from left to right. The energy required to remove an electron from the outermost orbit or shell of a neutral gaseous atom is called ionization energy.

The electron affinity of an element is defined as the energy liberated when an electron is added to its neutral gaseous atom. Electron affinity of an element is also called electron gain enthalpy of that element.

The electronegativity of an element is defined as the relative tendency of its atom to attract electrons towards itself when it is bonded to the atom of another element.

Pauling assigned the electronegativity values for elements on the basis of bond energies. He assumed that the electronegativity of hydrogen is 2.20 and calculated the values of other elements with respect to hydrogen.

Electronegativity values of elements decrease as we go down in a group and increase along a period from left to right. The most electronegative element is ‘F’ and the least electronegative stable element is ‘Cs’.

Metals generally show less electronegative character. In compounds, they generally show a tendency to remain as positive ions. This property to often termed as electropositive character. Metals are electropositive elements.