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PERIODIC CLASSIFICATION OF ELEMENTS.
Q.1 how many elements are known to us today?
Ans.114
Q.2how many elements were known in 1800?
Ans.30
Q.3what was the first attempt of grouping elements?
Ans.The earliest attempts to classify the elements resulted in grouping the then known elements as metals and non-metals.
Q.4who and when tried to arrange the elements into groups of 3 elements? What were these elements called?
Ans.In 1817, Wolfgang Dobereiner, a German chemist, tried to arrange the elements with similar properties into groups of three elements which he called triads.
Q.5explain the attempt made by Dobereiner to group the elements.
Ans.-Dobereiner tried to make groups of three elements having similar properties. He named these groups as triads.
-He showed that when the three elements in a triad were written in the increasing order of atomic masses, the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
Q.6give an example of Dobereiner’s triads.
Ans.Lithium (Li), sodium (Na) and potassium (K) are triads with respective atomic masses 6.9, 23 and 39. The average of atomic masses of Li and K is approximately equal to 23.
Q.7 how many triads could Dobereiner form? What are they?
Ans.He could only form 3 triads:
1)Li, Na, K
2)Ca, Sr, Ba
3)Cl, Ba, I
Q.8what was the disadvantage of Dobereiner’s attempt to group elements?
Ans.Only three triads could be formed from the elements known at that time.
Q.9explain Newlands’ attempt of grouping elements.
Ans.-in 1866, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses.
-He started with hydrogen (having the lowest atomic mass) and ended at thorium which was the 56th element.
-He found that every eighth element had properties similar to that of the first.
-He compared this to the octaves found in music. So, he called it the ‘Law of Octaves’, known as ‘Newlands’ Law of Octaves’.
Q.10 state the disadvantages of Newlands’ Law of Octaves.
1)Newlands’ Law of Octaves was applicable only upto calcium.
2)It was assumed by Newlands that only 56 elements existed in nature, but later many new elements were discovered, whose properties did not fit into the Newlands’ Law of Octaves.
3)In order to fit elements into his table, Newlands adjusted two elements in the same slot, but also put unlike elements under same note. E.g., Co and Ni are placed under same column as F, Cl and Br, which have very different properties that these elements.
Q.11 who is credited for the most important contribution to the early development of Periodic Table of elements?
Ans.Dmitri Ivanoich Mendeleev, a Russian chemist.
Q.12 explain Mendeleev’s attempt to classify elements.
-Mendeleev developed a Periodic Table of elements wherein the elements were arranged on the basis of their atomic mass, and also on the similarity on chemical properties.
-Among chemical properties, he took the formulae of the hydrides and oxides formed by an element as one of the basic properties of an element for classification.
-He then took 63 cards (as 63 elements were discovered during his time) and wrote properties of each element on one card.
-He arranged them in increasing order of atomic masses, and observed periodic reoccurance of elements with similar properties.
-On this basis he formulated a Periodic Law, which states that ‘the properties of elements are the periodic function of their atomic masses’.
Q.13what are the columns and rows of Mendeleev’s and modern periodic table called?
Ans.Columns – Groups
Rows – Periods
Q.14why did Mendeleev select reaction with oxygen and hydrogen of elements to arrange them in his Periodic Table?
Ans.He selected oxygen and hydrogen as they are very reactive and formed compounds with most elements.
Q.15what were the achievements of Mendeleev’s Periodic Table?
1)There were a few instances where Mendeleev placed an element of greater atomic mass before the element of lower atomic mass so that elements with similar properties could be grouped together.
2)Mendeleev left some gaps and boldly predicted the existence of some elements that had not been discovered at that time.
3)Noble gases, which were discovered later, could be placed in a new group without disturbing the existing order.
Q. 16 what were the limitations of Mendeleev’s Periodic Table?
- No fixed position can be given to hydrogen.
- Isotopes, which were later discovered, have similar chemical properties but different atomic masses.
- It was not possible to predict how many elements could be discovered between two elements, as the atomic masses do not increase in a regular manner in going from one element to the next.
Q.17when and who first used atomic number as the criteria for grouping elements in periodic table?
Ans.Henry Moseley in 1913.
Q.18state the Modern Periodic Law.
Ans.Properties of elements are a periodic function of their atomic number.
Q.19explain how modern periodic table overcomes the limitations of Mendeleev’s Periodic table.
- Since all the isotopes have same atomic number, therefore they can be easily placed in the table.
- As the atomic number of elements increases by 1, it can be predicted that how many elements are possible between two elements.
Q.20 what is the anomaly faced by modern periodic table? Explain.
Ans.Hydrogen can be placed either in group 1 or group 17, because it can give one electron (as in group 1) or take one electron (as in group 17) to attain noble gas configuration.
Q.21describe the arrangement of elements in the Modern Periodic Table.
-There are 18 vertical columns called ‘groups’ and 7 horizontal rows called ‘periods.
-Elements are arranged in increasing order of their atomic number.
-Each period has same no. of shells of electrons.
-Each group has same no. of valence electrons.
-No. of valence electrons increases as go from left to right.
-No. of shells increases by 1 as we go from top to bottom.
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Q.22what is the formula for the maximum no. of electrons possible in a shell.
Ans.2n2 , where n stands for the number of the given shell.
Q.23can you say why Mendeleev’s choice of formulae of compounds as the basis for deciding the position of an element in his table was a good one? How would this lead to elements with similar chemical properties being placed in the same group?
Ans.- elements with same number of valence electrons form same type of bonds with oxygen and hydrogen, as they have same valency.
- So formation of similar types of compounds shows that the valency of the elements is same.
Q.24how is valency of an element determined?
Ans. Valency of an element is determined by the number of electrons present in the outermost shell of its atom.
Q.25what is atomic size?
Ans.Atomic size refers to the radius of an atom.
Q.26what is the atomic radius of hydrogen?
Ans.37 pm (picometre, 1 pm = 10-12m)
Q.27how does the atomic size increase as we move towards right along a period? Why?
Ans.Atomic size decreases in moving from left to right along a period due to an increase in nuclear charge which tends to pull the electrons closer to the nucleus, thus reducing the size of atom.
Q.28 why does the atomic size increases down the group in spite of the increase in nuclear charge?
Ans.new shells are being added after each group which increases the distance between the outermost electrons and the nucleus so that the atomic size increases in spite of the increase in nuclear charge.
Q.29what are metalloids? Where are they placed in the Modern Periodic Table? Give examples.
-Metalloids or semi-metals are elements which exhibit properties of both metals and non-metals.
-They are placed on the borderline of metals and non-metals in the Modern Periodic Table.
-E.g., boron, silicon, germanium, arsenic, antimony, tellurium and polonium.
Q.30explain whether metals are electropositive or electronegative.
Ans.Metals are electropositive as they tend to lose electrons while forming bonds.
Q.31explain how metallic character/ electropositivity changes across a period.
Ans.As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons decreases which results in decrease in metallic character across a period.
Q.32 explain how metallic character/ electropositivity changes down a group.
Ans.Down the group, the effective nuclear charge experienced by valence electrons decreases as the outermost electrons are farther away from the nucleus. So the metallic character increases down a group.
Q.33explain how non-metallic character/ electronegativity changes across a period.
Ans. As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to gain electrons increases which results in increase in non-metallic character across a period.
Q.34 explain how non-metallic character/electronegativity changes down a group.
Ans.Down the group, the effective nuclear charge experienced by valence electrons decreases as the outermost electrons are farther away from the nucleus. So the non-metallic character decreases down a group.