PERIODIC CLASSIFICATION OF ELEMENTS
Early attempts of classification of elements:
Dobereiners’s Triads;- Johann Wolfgang Dobereiner was a German chemist. His effort is considered as one of the earliest attempts to classify the elements into groups. He found that when elements are arranged into groups of three in the order of their increasing atomic mass, the atomic mass of the element; which comes in the middle; is the arithmetic mean of rest of the two. On this basis, he arranged three elements in one group which is known as ‘Triad’. This arrangement of elements is known as Dobereiner’s Triads.
In this table, atomic mass of sodium is equal to arithmetic mean of atomic masses of lihtium and potassium. Similarly, atomic mass of strontium is equal to arithmetic mean of atomic masses of calcium and braium.
Limitation of Dobereiner’s Triads
Dobereiner could find only three such triads (group of three elements) and he could not even put all the elements known at that time in his triads.
The rules of Dobereiner’s triads could not be applied to the elements which had very low or high atomic mass. Such as; if F, Cl and Br are put together in a triad, in increasing order of their atomic masses, the atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
After the advancement of techniques of measuring atomic mass more correctly Dobereiner’s Law became obsolete.
Newlands’ Law of Octaves:- Newlands found that every eighth element has similar physical and chemical properties when they are arranged in increasing order of their relative masses. This law is known as Newlands’ Law of Octaves which states that "any given element will exhibit analogus behaviour to the eighth element following it in the table". This means every eight element has the similar chemical and physical properties. For example; Sodium is the eighth element from Lithium and both have similar properties.
The arrangement of elements in Newlands’ Octave resembles the musical notes. In musical notes, every eighth note produces similar sound. Because of this; Newland’s classification of elements was popularly known as just Octaves.
Limitation of Newlands’ Octaves:
•Newlands’ Octaves could be valid upto calcium only; as beyond calcium, elements do not obey the rules of Octaves.
•Newlands’ Octaves was valid for lighter elements only.
•It appears that Newlands did not expect the discovery of more elements than 56 which were discovered till his time.
•More than one element had to be placed in some of the groups; in order to place the elements having similar properties in one group. But in order to do so, he also put some dissimilar elements in same group.
•Iron; which has similar property as cobalt and nickel, was placed far from them.
•Cobalt and nickel were placed in the group with chlorine and fluorine in spite of having different properties.
•In spite of above limitations; Newlands was the first scientist who arranged the elements in order of their increasing relative atomic masses.
MENDELEEV'S PERIODIC CLASSIFICATION
Dmitri Ivanovich Mendeleev, a Russian scientist arranged the elements in increasing order of their relative atomic masses. He was honoured with Noble prize in 1906 for his Periodic Table.
Mendeleev’s Periodic Law states that the properties of elements are the periodic function of their relative atomic masses.
Mendeleev arranged all 63 elements; which were discovered till his time; in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev’s Periodic Table. He divided the table in eight columns and seven rows. The columns are known as groups and rows are known as periods.
Explanation of Mendeleev’s Periodic Table:
•Elements are arranged in the periodic table in the increasing order of their relative atomic masses.
•Mendeleev divided his periodic table in eight groups and seven periods.
•Groups from I to VII are meant for normal elements and group VIII is for transition elements.
•Groups from I to VII have been divided in two sub groups, while group VIII is meant for three elements.
•Periods from 4th to 7th have been divided in two series: 1st series and 2nd series.
•Elements having similar properties have been kept in the same group. For example; lithium, potassium, rubidium, etc. are in 1st group.
Merits of Mendeleev’s Periodic Table:
Mendeleev left some blank spaces in his periodic table in order to place the elements having similar properties in the same group.
For example; titanium has been placed in IVth group, leaving a blank space adjacent to it in IIIrd group. Similarly, arsenic has been placed in Vth group; leaving two adjacent spaces blank. These spaces have been occupied by scandium, gallium and germanium after their subsequent discovery.
Prediction for the elements to be discovered in future:
Mendeleev predicted the discovery of some elements and named them as eka-boron, eka-aluminium and eka-silicon. He gave the name of these elements prefixing the word ‘eka’ to the name of the preceding elements.
Scandium, Gallium and Germanium have been discovered later and took the place of eka-carbon, eka-aluminium and eka-silicon, respectively in the gap left in the Mendeleev’s Periodic table; as their properties were exactly similar to the predicted elements.
Position of Noble gases which were discovered later:
Position of Noble gases which were discovered later:
Noble gases were discovered much later after Mendeleev. After the discovery of noble gas, they were placed in a separate group called Zero Group, after VIII group, without making any disturbance to the arrangement of any elements in the Mendeleev’s Periodic Table
Modern Periodic Classification
Law of Modern Periodic Table states that properties of elements are the periodic function of their atomic numbers. In the modern periodic table, elements are arranged in order of their increasing atomic numbers.
Explanation of Modern Periodic Table:
•Elements are arranged in order of their increasing atomic numbers.
•The vertical columns are known as groups and horizontal columns are known as periods; in the modern periodic table.
•There are 18 groups and 7 periods in the modern periodic table.
Elements having same number of valence electrons are placed in the same group. For example; elements having valence electrons equal to 1 are placed in the 1st group, elements having valence electrons equal to 3 are placed in the 13th group, elements having valence electrons equal to 2 are placed in 2nd group except helium which is placed in 18th group, since it is an inert gas.
Elements having same number of shells are placed in the same period.
Trends in Modern Periodic Table:
Valence electrons:
•Number of valence electron remains the same while moving from top to bottom in a group.
•Number of valence electron increases while moving from left to right in a period.
Valency:
•Valency remains the same on moving from top to bottom in a group.
•Valency first increases upto 4 on moving from left to right in a group and then decreases upto zero.
Atomic size:
Atomic size increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
Atomic radii: Atomic radius increases on moving from top to bottom in a group and decreases on moving from left to right in a period.
Metallic character of elements:
Metallic character decreases on moving from left to right in a period.
Tendency to lose electron: Tendency to lose electrons decreases on moving from left to right in a period and tendency to lose electrons increases on moving from top to bottom in a group.
Tendency to lose electron is also known as electropositive character, thus electropositive character decreases on moving from left to right in a period and increases on moving from top to bottom in a group.
Tendency to gain electron: Tendency to gain electrons increases on moving from left to right in a period. Tendency to gain electron is called electro-negativity, thus electro-negativity of elements increases on moving from left to right in a period.
TEXTUAL QUESTIONS
1. Did Dobereiner's triads also exist in the columns of Newlands' Octaves? Compare and find out.
Answer-Yes, Dobereiner's triads also exist in the columns of Newlands' Octaves. One such column is Li, K, Na.
2. What were the limitations of Dobereiner's classification?
Answer-Limitation of Dobereiner's classification:
All known elements could not be classified into groups of triads on the basis of their properties.
3. What were the limitations of Newlands' Law of Octaves?
Answer-Limitations of Newlands' law of octaves:
→ It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.
→ Those elements that were discovered after Newlands' octaves did not follow the law of octaves.
→ The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
→ Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
1. Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba.
Answer-K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)
Answer-Scandium and germanium.
3. What were the criteria used by Mendeleev in creating his Periodic Table?
Answer-Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.
4. Why do you think the noble gases are placed in a separate group?
Answer-Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.
Answer-Yes, Dobereiner's triads also exist in the columns of Newlands' Octaves. One such column is Li, K, Na.
2. What were the limitations of Dobereiner's classification?
Answer-Limitation of Dobereiner's classification:
All known elements could not be classified into groups of triads on the basis of their properties.
3. What were the limitations of Newlands' Law of Octaves?
Answer-Limitations of Newlands' law of octaves:
→ It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.
→ Those elements that were discovered after Newlands' octaves did not follow the law of octaves.
→ The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.
→ Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.
1. Use Mendeleev's Periodic Table to predict the formulae for the oxides of the following elements:
K, C, Al, Si, Ba.
Answer-K is in group 1. Therefore, the oxide will be K2O.
C is in group 4. Therefore, the oxide will be CO2.
Al is in group 3. Therefore, the oxide will be Al2O3.
Si is in group 4. Therefore, the oxide will be SiO2.
Ba is in group 2. Therefore, the oxide will be BaO.
2. Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)
Answer-Scandium and germanium.
3. What were the criteria used by Mendeleev in creating his Periodic Table?
Answer-Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.
4. Why do you think the noble gases are placed in a separate group?
Answer-Noble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.
1. How could the Modern Periodic Table remove various anomalies of Mendeleev's Periodic Table?
Answer-Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:
Answer-Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:
→ Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
→ In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.
→ Elements having same valence electron are kept in same group.
→ Elements having same number of shells were put under the same period.
→ Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer-Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements and since chemical properties are due to valence electrons, they show same chemical reactions.
3.Name
→ Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.
2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice?
Answer-Calcium (Ca) and strontium (Sr) are expected to show chemical reactions similar to magnesium (Mg). This is because the number of valence electrons (2) is same in all these three elements and since chemical properties are due to valence electrons, they show same chemical reactions.
3.Name
(a) three elements that have a single electron in their outermost shells.
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Answer-(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer-(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
(b) two elements that have two electrons in their outermost shells.
(c) three elements with filled outermost shells.
Answer-(a) Lithium (Li), sodium (Na), and potassium (K) have a single electron in their outermost shells.
(b) Magnesium (Mg) and calcium (Ca) have two electrons in their outermost shells.
(c) Neon (Ne), argon (Ar), and xenon (Xe) have filled outermost shells.
4. (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?
(b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common?
Answer-(a) Yes. The atoms of all the three elements lithium, sodium, and potassium have one electron in their outermost shells.
(b) Both helium (He) and neon (Ne) have filled outermost shells. Helium has a duplet in its K shell, while neon has an octet in its L shell.
5. In the Modern Periodic Table, which are the metals among the first ten elements?
Group 16
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Group 17
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−
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−
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−
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A
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−
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−
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B
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C
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Answer-Among the first ten elements, lithium (Li) and beryllium (Be) are metals.
6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Answer-Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.
EXCERCISE
4. (a) What property do all elements in the same column of the Periodic Table as boron have in common?
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Answer-(a) Valency equal to 3. (b) Valency equal to 1.
5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)
Answer-(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.
6. The position of three elements A, B and C in the Periodic Table are shown below -
6. By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic?
Answer-Since Be lies to the extreme left hand side of the periodic table, Be is the most metallic among the given elements.
EXCERCISE
4. (a) What property do all elements in the same column of the Periodic Table as boron have in common?
(b) What property do all elements in the same column of the Periodic Table as fluorine have in common?
Answer-(a) Valency equal to 3. (b) Valency equal to 1.
5. An atom has electronic configuration 2, 8, 7.
(a) What is the atomic number of this element?
(b) To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
N(7) F(9) P(15) Ar(18)
Answer-(a) The atomic number of this element is 17.
(b) It would be chemically similar to F(9) with configuration as 2, 7.
6. The position of three elements A, B and C in the Periodic Table are shown below -
(a) State whether A is a metal or non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer-(a) A is a non-metal.
(b) State whether C is more reactive or less reactive than A.
(c) Will C be larger or smaller in size than B?
(d) Which type of ion, cation or anion, will be formed by element A?
Answer-(a) A is a non-metal.
(b) C is less reactive than A, as reactivity decreases down the group in halogens.
(c) C will be smaller in size than B as moving across a period, the nuclear charge increases and therefore, electrons come closer to the nucleus.
(d) A will form an anion as it accepts an electron to complete its octet.
7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why?
Answer-Nitrogen (7): 2, 5
Answer-Nitrogen (7): 2, 5
Phosphorus (15): 2, 8, 5
Since, electronegativity decreases with moving from top to bottom in a group, thus Nitrogen will be more electronegative.
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer-In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.
9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium?
Answer-The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
10. Compare and contrast the arrangement of elements in Mendeleev's periodic Table and the Modern Periodic Table.
Answer
8. How does the electronic configuration of an atom relate to its position in the Modern Periodic Table?
Answer-In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same.
Elements across a period show an increase in the number of valence electrons.
9. In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium?
Answer-The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2).
10. Compare and contrast the arrangement of elements in Mendeleev's periodic Table and the Modern Periodic Table.
Answer
Mendeleev's periodic table
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Modern periodic table
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Elements are arranged in the increasing order of their atomic masses.
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Elements are arranged in the increasing order of their atomic numbers.
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There are 8 Groups.
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There are 18 Groups.
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Each groups are subdivided into sub group 'a' and 'b'
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Groups are not subdivided into sub-groups.
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Groups for Noble gas was not present as noble gases were not discovered by that time.
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A seprate group is meant for noble gases.
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There was no place for isotopes.
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This problem has been rectified as slots are determined according to atomic number.
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Multiple choice questions
1. Which of the following statements is not a correct statement about the trends when going from left to right across the periods of periodic Table.
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
► (c) The atoms lose their electrons more easily.
2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na
(a) The elements become less metallic in nature.
(b) The number of valence electrons increases.
(c) The atoms lose their electrons more easily.
(d) The oxides become more acidic.
► (c) The atoms lose their electrons more easily.
2. Element X forms a chloride with the formula XCl2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as
(a) Na
(b) Mg
(c) Al
(d) Si
► (b) Mg
3. Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Answer-(a) Neon
(b) Magnesium
(c) Silicon
(d) Boron
(e) Carbon
3. Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?
Answer-(a) Neon
(b) Magnesium
(c) Silicon
(d) Boron
(e) Carbon
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