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Classification of Elements and Periodicity in Properties was essential since so many elements were discovered, the study of which had become extremely difficult. As a result, scientists devised various methods for classifying elements based on their properties in a periodic manner. Dimitri Mendeleev, known as the father of the periodic table, organized elements in increasing order of their atomic numbers, so that elements with comparable properties were grouped together in the same vertical column. There are 18 vertical columns known as Groups and 7 horizontal rows known as Periods in the modern periodic table. The elements that are there in the same group have the same valence electron configuration and have similar chemical properties while the elements in the same period have an increasing order of valence electrons.
Periodic Classification of Elements
Very Short Answer Questions (1 Mark Questions)
Ques. Which scientist was the first to classify elements based on their properties?
Ans. Johann Dobereiner, a German scientist, was the first to classify elements based on their qualities in the year 1829.
Ques. What is the basis for element triad formation?
Ans. Triad formation is mostly dependent on the middle element, whose atomic weight is half that of the other two elements in the triad, which are sandwiched between these two, and this relationship is also established by Dobereiner's law of triads.
Ques. Give the reason why the elements on the far left and far right the most reactive.
Ans. The maximum chemical reactivity can be found at the extreme left, among alkali metals, due to the loss of an electron, which leads to the formation of a cation due to its low ionization enthalpy, and at the extreme right, among halogens, due to the gain of an electron, which leads to the formation of an anion due to its high electron affinity.
Ques. What is an element's periodic classification?
Ans. By periodic classification of the elements, we mean the arrangement of the elements in such a way that elements with similar physical and chemical properties are grouped together, and various scientists contributed to this, but Mendeleev's contributions are of great significance, and he gave a periodic table called Mendeleev's Periodic 'Table, which was older and replaced by the long form of the periodic table.
Ques. Mendeleev predicted the existence and properties of which two elements that did not exist at the time?
Ans. Mendeleev explained the existence of gallium and germanium, as well as some of their physical properties, despite the fact that they did not exist at the time.
Ques. What is the modern definition of 'Periodic law'?
Ans. Elements are arranged according to their physical and chemical properties, and their atomic numbers increase.
Short Answer Questions (2 Marks Questions)
Ques. Is it true that elements with a high I.E. also have a high E.A.?
Ans. It is generally true that elements with a high I.E. affinity also have a high E affinity. There are, however, notable exceptions. It is observed that elements with stable electron configurations have very high I-Energies because it is difficult to remove electrons, as is the case with the 15th and 18th group elements, but electrons cannot be added easily in such cases, which is why elements of the 15th group have almost zero E.A. and elements of the 18th group have zero E.A. despite having very high Ionization energy values.
Ques. Why does the first ionization enthalpy increase from left to right across a given Periodic Table period?
Ans. The value of ionization enthalpy increases with an increasing atomic number over time. This is due to the fact that when moving from left to right across the period,
- The nuclear charge is increased by one unit on a regular basis.
- At the same level, electrons are added in a progressive manner.
- Atomic size is decreased.
Because of the gradual increase in nuclear charge and concurrent decrease in atomic size, electrons become increasingly tightly bound to the nucleus. This causes a gradual increase in ionization energy over time.
Read More: Ionization Energy Formula
Ques. How did Mendeleev arrange the elements?
Ans. Mendeleev arranged elements in the form of a table with horizontal rows and vertical columns, and the elements are arranged in these columns and rows in increasing order of their atomic weight, and the arrangement is such that elements with similar properties are in the same vertical column or group.
Ques. What is the reason for chlorine's higher electron affinity than fluorine?
Ans. Cl has a greater affinity for electrons than F. This is due to fluorine's small size. Because of its small size, the interelectronic repulsions in a chlorine atom's relatively compact 2p-subshell. When we compare chloride ion to fluoride ion, we discover that the electron density per unit volume in fluoride ion (F–) is greater than that in chloride (Cl–) ion. This means that the upcoming election has less attraction in the fluorine atom than in the chlorine atom. As a result, chlorine has a higher electron affinity than fluorine.
Ques. How do atomic radii change over the course of a period with an atomic number in the periodic table? Explain.
Ans. Atomic radii change over time: Atomic radii decrease as the atomic number in a period increases. In the second period, for example, atomic radii decrease from lithium to fluorine. Moving from left to right across the period, the nuclear charge increases by one unit at a time, but the extra electron goes to the same principal shell. As a result of the increased effective nuclear charge, the electron cloud is drawn closer to the nucleus. This results in a reduction in atomic size.
Ques. Which of the following has the greatest negative electron gain enthalpy and which has the least? P, S, Cl, and F. Explain your response.
Ans. Electron gain enthalpy tends to become more negative as we move from left to right over time. Within a group, electron gain enthalpy decreases as one moves down the group. Adding an electron to the 2p orbital, on the other hand, results in greater repulsion than adding an electron to the larger 3p orbital. As a result, chlorine has the highest negative electron gain enthalpy and phosphorus has the lowest negative electron gain enthalpy.
Long Answer Questions (3 Marks Questions)
Ques. List down the features of the seven periods.
Ans. The features of the seven periods are as follows:
- The first period is said to be the shortest because it only contains two elements: hydrogen and helium.
- Both the second and third periods contain eight elements, ranging from lithium to neon in the second period and sodium to argon in the third.
- The fourth and fifth periods, both of which are long, contain 18 elements ranging from potassium to krypton in the fourth and rubidium to xenon in the fifth.
- The sixth period, which is said to be the longest, contains 32 elements beginning with caesium and ending with radon.
- The seventh period is incomplete; it begins with francium with atomic number 87 and continues to 93, which are purely synthetic in nature and are referred to as transuranium elements.
Ques. Explain why cations have smaller radii than their parent atoms and anions have larger radii.
Ans. Since we know that a cation is formed by the loss of one or more electrons from the gaseous atom while the nuclear charge of the atom remains constant, the radius of the cation is smaller than that of its parent atom. During this process, the nuclear force holds the remaining electrons more tightly, which can be explained by an increase in effective nuclear charge per electron caused by a decrease in size.
Anion is formed when a gaseous atom gains one or more electrons, and the nuclear charge remains constant even though the number of electrons increases. The effective nuclear charge per electron reduces in the case of anion, which reduces the firmness of atoms with nuclei and increases their size.
Ques. Mention four characteristics of p-block elements.
Ans. The following are the four most important properties of p-block elements:
- P-block elements contain both metals and non-metals, although the number of nonmetals is far more than the number of metals. Furthermore, the metallic character grows from top to bottom within a group, while the non-metallic character increases from left to right during the course of a period in this block.
- They have higher ionization enthalpies than s-block elements.
- Covalent chemicals are formed when they combine.
- Compounds having numerous (varying) oxidation states are present in some of them. Their oxidizing character rises from left to right over time, while their reducing character rises from top to bottom.
Read More: Transition Elements Oxidation States
Ques. Compare and discuss the trends in ionization enthalpy of group 1 elements with those of group 17 elements.
Ans. The distance between the nucleus and the valence electron grows with the size of the atom, resulting in a decrease in the force of attraction between them. As a result, the energy needed to remove an electron is reduced.
Furthermore, as we move down the group, the shielding effect increases, and thus the force of attraction between the nucleus and the valence electrons weakens even more. The combined effect of the two results in a decrease in ionization energy as we move down a group.
As a result, as we move down in both groups 1 and 17, the group ionization energy decreases.
Ques. Define the term ionization enthalpy. Discuss the elements' ionization enthalpy and the factors that influence it.
Ans. An element's ionization enthalpy is defined as the amount of energy required to remove an e from an isolated gaseous atom in its gaseous state. The ionization enthalpy is influenced by various factors including the following factors:
- Atom size: The smaller the atomic size, the lower the ionization enthalpy value. In large atoms, the outer e– is far from the nucleus, so the force of attraction with which they are attracted by the nucleus is weaker, and they can thus be easily removed.
- Nuclear charge: As nuclear charge increases among atoms with the same number of energy shells, the ionization enthalpy increases due to the force of attraction towards the nucleus.
- Half-filled and fully-filled orbitals: Because atoms with half-filled and fully-filled orbitals are more stable, removing an electron from such atoms requires more energy. The ionization enthalpy of such an atom is slightly higher than expected. Stable electronic configuration of ionization enthalpy
- Orbital Shape: The same orbit's s-orbital is closer to the nucleus than the p-orbital. As a result, removing an electron from a p-orbital is simpler than removing an electron from an s-orbital. The orbital shape is s > p > d > f.
Read More: Energy Of Orbitals
Very Long Answer Questions (5 Marks Questions)
Ques. (a) Why does the electronegativity value increase over time and decrease over time?
(b) What is the relationship between electronegativity and non-metallic properties?
Ans. (a) The value of electronegativity generally corresponds to the attraction between outer electrons and the nucleus, which increases with the decrease in atomic radius along a period, and in this case, the value of electronegativity also increases. Similarly, as we move down a group, the electronegativity value decreases with increasing atomic radii.
(b) Electronegativity is related directly to the non-metallic character of elements, whereas it is inversely related to metallic properties. Thus, an increase in electronegativity across a period is defined by an increase in non-metallic properties of elements, whereas a decrease in electronegativity by moving down a group is defined by a decrease in non-metallic properties of elements.
Ques. (a) Explain the concept of an element's valency.
(b) How does it differ across periods and groups in the periodic table?
Ans. (a)The chemical properties of elements are determined by the number of electrons in an atom's outermost shell. These electrons are known as valence electrons, and they determine the atom's valency (or element). Valency in representative elements is generally equal to n or (8 – n), where n is the number of valence electrons in the atom. In a period, electron valence increases from 0 to 8 as one moves from left to right. The valency of an element with respect to H and Cl increases from 1 to 4 before decreasing to zero. When it comes to oxygen, however, valency rises from 1 to 7 before falling to zero in noble gases.
(b) Because the number of valence electrons in a group remains constant, all elements in a group have the same valency, for example, all elements in group I have valency one and those in group 2 have valency two. The transition elements, on the other hand, have variable valency.
Ques. In what ways is the long form of the periodic table better than Mendeleev's periodic table? Explain.
Ans. The long-form periodic table is considered better than Mendeleev's table for the following reasons:
- The long-form periodic table arranges all elements in increasing order of atomic number, whereas the table arranges elements in increasing order of atomic mass.
- The position of hydrogen in the long-form periodic table is justified, whereas Mendeleev's periodic table lacks such justification.
- The long-form periodic table takes into account the filling of electrons in s,p,d, and subshells, whereas the table takes into account the elements' atomic numbers.
- The periodic table has four blocks: s, p, d, and f, whereas the periodic table does not.
- Long-form periodic table groups are not subdivided into subgroups, whereas Mendeleev's periodic table has subgroups A and B for each group.
- The long-form periodic table is simple and easy to understand whereas Mendeleev's periodic table is more difficult.
Ques. What are Mendeleev's periodic table flaws that led to its modification?
Ans. The flaws that were present in Mendeleev’s periodic table are as follows:
- Position of Hydrogen: Hydrogen is assigned to group I. It does, however, resemble Group I (alkali metals) as well as halogens. As a result, the periodic table's position for hydrogen is incorrect.
- Anomalous pairs: Certain pairs of elements did not follow the increasing order of atomic masses. In these cases, Mendeleev arranged the elements based on similarities in their properties rather than the increasing order of their atomic masses. Argon (atomic mass 39.9), for example, comes before potassium (K, atomic mass 39.1). In the same manner, cobalt (Co, atomic mass 58.9) precedes nickel (Ni, atomic mass 58.6), and tellurium (Te, atomic mass 127.6) precedes iodine (I, atomic mass 126.9). These positions were not supported by evidence.
- Isotopes are identical atoms of the same element with different atomic masses but the same atomic number. As a result, these should be classified differently based on their atomic masses, according to Mendeleev's classification. Hydrogen isotopes with atomic masses 1, 2, and 3 should, for example, be placed in three different locations. Isotopes, on the other hand, do not have their own periodic table position.
- Several gaps in the periodic table were left because he believed that several elements had yet to be discovered, such as gallium, which had not yet been discovered at the time.
- Lanthanide (or lanthanides) and actinoids (or actinides) are classified as follows: The fourteen elements that come after lanthanum (known as lanthanoids, atomic numbers 58-71) and the fourteen elements that come after actinium (known as actinoids, atomic numbers 58-71) are not listed separately.
Read More: Position of Hydrogen in the Periodic Table
Ques. Discuss the factors influencing electron gain enthalpy and the periodic table's trend in its variation.
Ans. The following factors influence electron gain enthalpy:
- Nuclear charge: As the nuclear charge increases, so does the electron gain enthalpy. When the nuclear charge is high, the incoming electron is more attracted.
- Atomic size: As the size of the atom grows, so does the distance between the nucleus and the incoming electron, resulting in less attraction. As a result, as the size of the element's atom grows larger, the electron gain enthalpy decreases.
- Electronic Configuration: Elements with stable electronic configurations of half-filled and completely filled valence subshells have a very low tendency to accept additional electrons, resulting in lower negative electron gain enthalpies.
Periodic Table Variations in Electron Gain Enthalpies
In general, electron gain enthalpy decreases from left to right within a period and increases from top to bottom within a group.
- Downward variation inside a group: Moving down a group increases nuclear charge and size. However, because the effect of increasing atomic size is much stronger than that of increasing nuclear charge, the additional electron feels less attracted to the large atom. The electron gain enthalpy decreases as a result. The decrease in electron gain enthalpy when transitioning from chlorine to bromine and then to iodine demonstrates this.
- Periodic variation: As one move through a period, the atom's size decreases and its nuclear charge increases. Because both of these factors increase the incoming electron's attraction, electron gain enthalpy decreases from left to right. There are, however, some outliers in the overall trend. These are primarily due to the stable electronic configurations of certain atoms.
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