Chapter-5, Periodic Classification of Elements

Explanation:

Dobereiner's triads and Newland's octaves are two early attempts that were made in order to classify elements keeping in mind their properties. Dobereiner's triads put elements that had similar chemical and physical properties in groups- together into sets of three, while Newland's octaves arranged elements in the order of atomic weight that increased, with similar elements that appear at every eighth element.

There are some overlaps between Dobereiner's triads and Newland's octaves, however, they are not exactly the same. Dobereiner's triads include just a few of the elements that were known at the time, while Newland's octaves consist of all the elements till calcium.

Some of the elements in Newland's octaves appear to have similar properties, almost the same as those in Dobereiner's triads. For instance, lithium, sodium, and potassium in the first octave have similar chemical properties, whereas calcium, strontium, and barium in the second octave also have similar properties. But, the patterns are not consistent throughout all of the octaves, and some elements do not exactly fit into either system.

Therefore, while there are similarities between the two classification systems, they are distinct from each other and do not perfectly align.

Explanation:

Dobereiner's classification system was one of the first attempts made for classifying elements with regard to their properties. While it was a significant step towards understanding the periodicity of elements, it had many limitations. Some of the main limitations of Dobereiner's classification are as follows-

Limited applicability-Dobereiner's classification was limited to just a few elements that were known at the time. The classification did not take into account the newly discovered elements, and hence, it could not be utilized for classifying a wider range of elements.

Incomplete triads- While Dobereiner's classification grouped elements into triads keeping in mind their similar properties, not all the known elements could be put into triads. Some elements did not fit into any of the triads, thereby making the classification incomplete.

Lack of explanation- Dobereiner's classification did not give an explanation for the reason why elements were grouped into triads. It was only later that the concept of atomic weight was introduced, which gave an explanation for the observed patterns.

Inaccurate atomic weights- The atomic weights used by Dobereiner were inaccurate, and some of the values he used were later found to be not correct. This made it a task to accurately give predictions regarding the properties of new elements on the basis of their atomic weight.

Failure to predict new elements- Dobereiner's classification system was not able to predict the discovery of new elements. For instance, when new elements like cobalt and nickel were discovered, they were not fitting into any of the existing triads, thereby raising questions regarding the validity of the classification.

In conclusion, Dobereiner's classification system was a crucial step toward the making of the periodic table, however, it had a lot of limitations that were later addressed by the development of more sophisticated classification systems, like Mendeleev's Periodic Table.

Explanation:

Newland's Law of Octaves was an early attempt at the classification of elements on the basis of their properties. While it was a significant step towards the development of the periodic table, it had many limitations. Some of the main limitations of Newland's Law of Octaves are-

Limited applicability- Newland's Law of Octaves was limited to just a few elements, which were identified at the time. The classification did not take into account the newly discovered elements, and hence, it could not be used to classify a wider range of elements.

Incomplete pattern- While Newland's Law of Octaves arranged elements in order of atomic weight that increased, alongside similar elements appearing every eighth element, the pattern was incomplete. The properties of some elements did not fit the pattern, and this made it tough to predict the properties of new elements.

Arbitrary selection of octaves- Newland's selection of elements for each octave was arbitrary and was not based on science. This made it tough to justify the selection of elements for each octave.

Disagreement with known atomic weights- Newland's classification did not agree with the known atomic weights of the elements all the time. For instance, cobalt and nickel were assigned to different octaves on the basis of their atomic weights, but they have similar chemical properties.

Lack of explanatory power- Newland's Law of Octaves did not give an explanation for the observed pattern. It was only later that the concept of atomic number was introduced, which gave an explanation for the observed patterns.

In conclusion, Newland's Law of Octaves was a significant step towards the making of the periodic table, but it had some major limitations that were later addressed by the development of more sophisticated classification systems, such as Mendeleev's Periodic Table.

Explanation:

Mendeleev's periodic table put together elements keeping in mind their atomic weight and properties, which allowed for the prediction of the properties of new elements as well as their compounds. Using Mendeleev's periodic table, we can make predictions regarding the formula for the oxides of these elements-

Potassium (K)- Potassium can be seen in group 1 of the periodic table and forms an oxide with the formula K2O.

Carbon (C)-Carbon can be seen in group 14 of the periodic table and can form two oxides-carbon monoxide (CO) and carbon dioxide (CO2).

Aluminum (Al)-Aluminum can be seen in group 13 of the periodic table and forms an oxide with the formula Al2O3

Silicon (Si)-Silicon can be seen in group 14 of the periodic table and forms an oxide with the formula SiO2.

Barium (Ba)- Barium can be seen in group 2 of the periodic table and forms an oxide with the formula BaO.

Hence, the predicted formulas for the oxides of the given elements are-

K2O

CO, CO2 (two possible oxides)

Al2O3

SiO2

BaO

Explanation:

Mendeleev's periodic table had gaps for many elements that had not yet been found in his time. Two of those elements that have since been discovered are:

Scandium (Sc)- Mendeleev predicted the existence of an element he called ekaboron (Eka-Boron), which he placed in the periodic table under boron. This element was later discovered in the year 1879 by Swedish chemist Lars Fredrik Nilson and called scandium.

Germanium (Ge)- Mendeleev predicted the existence of an element he called eka-silicon (Eka-Silicon), which he placed in the periodic table under silicon. This element was later discovered in the year 1886 by German chemist Clemens Winkler and called germanium.

Explanation:

Mendeleev's criteria for creating his periodic table were as follows- 

Atomic weight- Mendeleev organized the elements in the order of atomic weight that went up. He saw that elements with almost the same chemical as well as physical properties tended to have similar atomic weights.

Chemical and physical properties- Mendeleev also put together the elements on the basis of their chemical and physical properties. He grouped elements with similar properties into vertical columns called groups.

Valence- Mendeleev considered the valence or the number of bonds an atom could form with other atoms, of each element in the determination of its place in the periodic table.

Periodicity- Mendeleev saw a periodic pattern in the properties of elements as their atomic weight increased. He arranged the elements in such a manner that the periodicity of their properties was obvious.

Accommodation of newly discovered elements- Mendeleev left gaps in his periodic table for elements that were not yet discovered but were predicted to exist on the basis of the periodicity of the elements around them.

Overall, Mendeleev's periodic table was revolutionary because it allowed scientists to predict the properties of elements that had not yet been discovered and to organize the known elements in a logical and systematic way.

Explanation:

The noble gases (helium, neon, argon, krypton, xenon, and radon) are placed in a separate group (Group 18) in the periodic table as they have unique electronic configurations and chemical properties that differentiate them from other elements. One of the most defining characteristics of noble gases is that they have a complete outermost shell of electrons, which makes them very stable and unreactive under normal conditions. This stability comes from the fact that the outermost shell of noble gases is entirely filled with electrons, and hence, they do not need to gain, lose or share electrons in order to form chemical bonds with other elements. This is also why they are often known as "inert" gases. In addition to their electronic configuration, noble gases have other unique properties, like very low boiling points and high ionization energies, which also differentiate them from other elements. These properties have significant applications in fields like lighting, cryogenics, and anesthesia.

In conclusion, the unique electronic configuration and chemical properties of noble gases make them stand out from other elements, and hence, they are put in a different group in the periodic table.

Explanation:

The Modern Periodic Table is an improvement over Mendeleev's Periodic Table because it has removed many anomalies and inconsistencies in the original table. Some of the ways in which the Modern Periodic Table has addressed these issues are as follows-

The Modern Periodic Table is dependent on the atomic number of an element, whereas Mendeleev's Periodic Table was based on the atomic weight of an element. This led to some inconsistencies, as elements with different atomic weights but similar properties were placed in the same group. The use of atomic numbers in the Modern Periodic Table has solved this issue.

The Modern Periodic Table includes the concept of subshells and orbitals, which was not present in Mendeleev's Periodic Table. This allows for a more detailed understanding of the electronic configuration of elements and their placement in the periodic table.

The Modern Periodic Table includes the lanthanides and actinides, which are placed separately at the bottom of the table, rather than being inserted in the main body of the table as in Mendeleev's Periodic Table. This avoids the need to split a group into two parts, which was a problem in Mendeleev's table.

The Modern Periodic Table includes a separate group for noble gases, which were not known or understood at the time of Mendeleev's table. This reflects their unique electronic configuration and properties and avoids the need to group them with other elements with different properties.

Overall, the Modern Periodic Table has addressed many of the anomalies and inconsistencies of Mendeleev's Periodic Table and provided a more accurate and comprehensive representation of the elements and their properties.

Explanation:

Two elements that are expected to show chemical reactions similar to magnesium are calcium (Ca) and strontium (Sr).

The basis of this choice is that calcium and strontium are both in the same group (Group 2) as magnesium in the periodic table. These elements are also called alkaline earth metals and have similar electron configurations, with two valence electrons in their outermost shells. Due to this similarity, calcium and strontium have similar chemical properties to magnesium. They can all form ionic compounds with similar structures, like oxides and hydroxides, and react with similar acids, like sulfuric acid, in order to produce hydrogen gas. Additionally, these elements also have similar reactivities with water, although the reactivity generally increases down the group. For instance, magnesium reacts with water slowly, calcium reacts more quickly, and strontium reacts even more rapidly.

In conclusion, the similar electronic configuration and placement in the same group in the periodic table point out that calcium and strontium would exhibit chemical properties and reactions similar to magnesium.

Explanation:

a)Three elements that contain just a single electron in their outermost shells are as follows-

Lithium (Li)

Sodium (Na)

Potassium (K). 

b) Two elements which contain two electrons in their outermost shells are as follows-

Magnesium (Mg)

Calcium (Ca). 

c) Three elements that have filled the outermost shells are as follows-

Helium (He)

Neon (Ne)

Argon (Ar).

Explanation:

a)Yes, there is a similarity in the atoms of lithium, sodium, and potassium that makes them all react with water to liberate hydrogen gas.

The similarity lies in the electronic configuration of these elements. All three of these elements belong to Group 1 of the periodic table, also known as the alkali metals. They all have a single electron in their outermost shell, which makes them highly reactive.

When these metals react with water, the outermost electron is transferred to a water molecule, liberating hydrogen gas and forming a hydroxide ion. This reaction occurs because the outermost electron of these metals is relatively loosely held, and they have a strong tendency to lose this electron to form a positive ion. In conclusion, the similarity in the electronic configuration of these elements, specifically the presence of a single electron in their outermost shell, is what makes them all react with water to liberate hydrogen gas.

b) Helium and neon are both inert gases or noble gases, and their atoms have a complete outermost shell of electrons. This configuration makes them highly stable and unreactive under normal conditions. Both helium and neon have a full valence shell with 2 electrons, giving them an octet configuration. This makes them less likely to lose or gain electrons to form compounds with other elements. Therefore, they tend to exist as monatomic gases and have a very low tendency to participate in chemical reactions.

In summary, the common feature of helium and neon atoms is their stable electronic configuration. They both have a complete outermost shell of electrons, making them highly stable and unreactive under normal conditions.

Explanation:

In the Modern Periodic Table, the first ten elements are-

Hydrogen (H)

Helium (He)

Lithium (Li)

Beryllium (Be)

Boron (B)

Carbon (C)

Nitrogen (N)

Oxygen (O)

Fluorine (F)

Neon (Ne)

Among these, the metals are-

Lithium (Li)

Beryllium (Be)

Lithium and beryllium are the only metals among the first ten elements in the Modern Periodic Table. They belong to Group 1 and Group 2, respectively, and have metallic properties such as high electrical and thermal conductivity, luster, malleability, and ductility. The other elements in the list are non-metals (hydrogen, helium, carbon, nitrogen, oxygen, and fluorine) or noble gases (neon).

Explanation:

Among the given elements, beryllium (Be) would be expected to have the maximum metallic characteristic based on its position in the periodic table.

Beryllium is located in Group 2 of the periodic table, also known as the alkaline earth metal. Elements in this group are known for their strong metallic characteristics, including high electrical and thermal conductivity, luster, malleability, ductility, and tendency to form cations with a +2 charge.

In contrast, the other elements listed, gallium (Ga), germanium (Ge), arsenic (As), and selenium (Se), are located in Groups 13, 14, 15, and 16, respectively, and are not metals. They have different chemical and physical properties, such as being semiconductors, metalloids, or non-metals. Therefore, based on their position in the periodic table, beryllium would be expected to have the highest metallic characteristic among the given elements.

Explanation:

(b) "The number of valence electrons increases" is an incorrect statement about the trends when one goes from left to right across the periods in the periodic table.

As one moves from left to right across a period, the valency or the number of valence electrons stays the same for the elements until one reaches the last element of the period. Then, the valency goes up by one unit for the next element in the next period.

The correct trends when going from the left to the right across the periods of the periodic table are as follows-

(a) The elements will get less metallic in nature.

(b) The ionization energy increases.

(c) The atoms get smaller in size.

(d) The oxides tend to become more acidic.

Explanation:

The formula XCl2 suggests that element X has a +2 oxidation state in its chloride.

Option (b) Mg has a similar configuration to X and forms MgCl2, which is solid with a high melting point. Therefore, X is most likely in the same group as Mg, which is Group 2 of the periodic table.

Option (a) Na is in Group 1, and its chloride has a formula of NaCl, not NaCl2.

Option (c) AI is in Group 3, and its most common chloride has a formula of AlCl3, not AICl2.

Option (d) Si is in Group 4, and its most common chloride has a formula of SiCl4, not SiCl2.

Therefore, the answer is (b) Mg.

Explanation:

(a) The element containing two shells, two of which are fully filled with electrons is helium (He). Its electronic configuration is 2.

(b) The element with electronic configuration 2, 8, 2 is calcium (Ca).

(c) Silicon (Si) is an element that has a total of three shells, and four electrons in its valence shell. The electronic configuration of it is 2, 8, 4.

(d) Boron (B) is the element that has a total of two shells, with three electrons in its valence shell. Its electronic configuration is 2, 3.

(e) The element which has twice as many electrons in its second shell as in its first shell is beryllium (Be). Its electronic configuration is 2, 4.

Explanation:

(a) All elements in the column same as that of the periodic table as boron tends to have the same number of valence electrons, which is three. This means they have similar chemical properties, such as the tendency to form compounds by gaining or losing three electrons.

(b) All elements in the column same as that of the periodic table as fluorine tends to have the same number of valence electrons, which is seven. This means they have similar chemical properties, such as high electronegativity and the tendency to form compounds by gaining one electron to achieve a stable octet configuration. They are also highly reactive nonmetals with a strong tendency to form negative ions.

Explanation:

(a) The electronic configuration of 2, 8, 7 means that the atom has 2 electrons in its first shell, 8 electrons in its second shell, and 7 electrons in its third shell. The total number of electrons is 2 + 8 + 7 = 17. Therefore, the atomic number of this element is 17, which corresponds to chlorine (Cl).

(b) The element with atomic number 17 and electronic configuration 2, 8, 7 would be chemically similar to the element with atomic number 9 and electronic configuration 2, 7, which is fluorine (F). Both chlorine and fluorine are halogens and have similar chemical properties, such as high electronegativity, reactivity, and the tendency to form anions by gaining one electron to achieve a stable octet configuration.

Explanation:

(a) A is a nonmetal.

(b) C is more reactive than A.

(c) C will be smaller than B in size. 

(d) Element A is in group 16, which means it has 6 valence electrons. In order to achieve a stable electron configuration, it will gain 2 electrons to form an anion with a -2 charge, making it a nonmetal anion.

Explanation:

The electronic configurations of nitrogen and phosphorus are as follows-

Nitrogen: 1s2 2s2  2p3

Phosphorus: 1s2 2s2  2p6 3s2 3p3

Nitrogen has 5 valence electrons (2s2 2p3), while phosphorus has 5 valence electrons in the 3p orbital and also has 2 valence electrons in the 3s orbital. Both of these elements can gain 3 electrons to achieve a stable octet configuration.

Of the two elements, nitrogen is more electronegative because it is smaller in size and has a higher effective nuclear charge, which allows it to attract electrons more strongly. Additionally, the extra electrons in the 3s and 3p orbitals of phosphorus can shield some of the nuclear charges, reducing its effective nuclear charge and lowering its electronegativity.

Explanation:

The electronic configuration of an atom relates to its position in the Modern Periodic Table because it determines the element's chemical and physical properties. The position of an element in the periodic table is determined by its atomic number, which is the number of protons in the nucleus of an atom.

Elements in the same group of the periodic table have similar electron configurations and similar valence electron arrangements. For example, all the elements in group 1 (also known as the alkali metals) have a single valence electron in their outermost shell, and their electronic configurations end with ns1. Similarly, all elements in group 18 (also known as the noble gases) have a filled valence shell and their electronic configurations end with ns2np6.

The periodic table is arranged in such a way that elements with similar electronic configurations and valence electron arrangements are placed in the same group. As one moves across a period from left to right, the number of valence electrons increases by one, and the electronic configuration changes accordingly. This results in a gradual shift in the chemical and physical properties of the elements across the period. The properties of elements also change as one moves down a group, but this is mainly due to the increasing number of electron shells and the corresponding increase in atomic size.

Explanation:

The Modern Periodic Table arranges elements based on their electronic configurations, so elements with similar properties are placed in the same group or period.

Calcium is in Group 2, which means it has two valence electrons and is an alkaline earth metal. Elements in the same group as calcium have similar chemical properties, so the element with atomic number 12, which is magnesium, is most likely to have physical and chemical properties resembling calcium.

Magnesium is also an alkaline earth metal, with a similar electronic configuration (1s2 2s2 2p6 

3s2) and similar properties to calcium. Both calcium and magnesium are reactive metals that readily form cations with a +2 charge. They have similar physical properties such as metallic luster, high melting and boiling points, and good conductivity.

The other elements surrounding calcium have different electronic configurations and belong to different groups, so they are less likely to have physical and chemical properties resembling calcium. Potassium (atomic number 19) and scandium (atomic number 21) belong to different groups and have different valence electron arrangements, while strontium (atomic number 38) is in the same group as calcium but has a larger atomic size and different chemical properties. Therefore, magnesium is the element that most closely resembles calcium in terms of its physical and chemical properties.

Explanation:

Mendeleev's Periodic Table was the first periodic table to be developed, whereas the Modern Periodic Table is the current version that is widely accepted by scientists. Both tables are organized based on the periodic law, which states that the chemical and physical properties of elements are periodic functions of their atomic number.

However, there are several differences between Mendeleev's Periodic Table and the Modern Periodic Table. Here, some of the main differences are listed-

Basis of organization- Mendeleev's Periodic Table was organized based on the atomic mass of elements, while the Modern Periodic Table is organized based on the atomic number of elements.

Number of elements: Mendeleev's Periodic Table included only 63 elements, while the Modern Periodic Table includes all the known elements up to atomic number 118.

Groups and periods: Mendeleev's Periodic Table had eight groups and six periods, while the Modern Periodic Table had 18 groups and seven periods.

Elements in the same group: In Mendeleev's Periodic Table, elements in the same group had similar chemical properties but did not necessarily have the same number of valence electrons. In the Modern Periodic Table, elements in the same group have the same number of valence electrons and therefore have similar chemical properties.

Predictions- Mendeleev's Periodic Table was used to predict the properties of undiscovered elements, while the Modern Periodic Table is used to predict the properties of elements based on their position in the table and their electronic configuration.

Overall, the Modern Periodic Table is more accurate and comprehensive than Mendeleev's Periodic Table. It provides a more detailed and systematic arrangement of elements based on their atomic number, and it allows for more accurate predictions of the properties of elements.