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Kerala Plus One Chemistry Notes Chapter 3 Classification of Elements and Periodicity in Properties
Introduction
The systematic classification of elements made the study of elements easy. In this unit, we will study the historical development of the periodic table and also learn how elements are classified.
Genesis Of Periodic Classification
While Dobereiner initiated the study of periodic relationship, it was Mendeleev who was responsible for publishing the Periodic Law for the first time. It states as follows:
The properties of the elements are a periodic function of their atomic weights.
Mendeleev arranged elements in horizontal rows and vertical columns of a table in order of their increasing ‘ atomic weights. Elements with similar properties occupied the same vertical column or group. He realized that some of the elements did not fit in with his scheme of classification if the order of atomic weight was strictly followed. He ignored the order of atomic weights, thinking that the atomic measurements might be incorrect, and placed the elements with similar properties together.
At the same time, keeping his primary aim of arranging the elements of similar properties in the same group, he proposed that some of the elements were still undiscovered and, therefore, left several gaps in the table. He left the gap under aluminium and a gap under silicon, and called these elements Eka-Aluminium and Eka-Silicon. Mendeleev predicted the existence of gallium and germanium, and their general physical properties. These elements were discovered later.
Modern Periodic Law And The Present Form Of The Periodic Table
Modem periodic law states that “The physical and chemical properties of the elements are periodic functions of their atomic numbers”. Atomic number is equal to the nuclear charge and the elements are arranged in the increasing order of atomic number.
The period number correspond to the highest principal quantum number (n) of the elements.
Nomenclature Of Elements With Atomic Number Greater Than 100
The names (IUPAC) are derived directly form the atomic number using numerical roots for zero and numbers 1 to 9. The roots are linked together in the order of digits and ‘ium’ is added at the end. The roots for 0,1, 2 9 are nil, un, bi, tri, quad, pent, hex, sept, oct and enn respectively. For example, the element with atomic number 110 will have the name Ununnilium (Un+ un+nil + ium), The element with atomic number 114 has the name Ununquadium (un + un + quad + ium) and the element with atomic number 120 will be Unbinilium (un + bi + nil + ium).
Electronic Configurations And Types Of Elements: s, p, d, f- Blocks
The s-Block Elements
The elements of Group 1 (alkali metals) and Group 2 (alkaline earth metals) which have ns1 and ns2 outermost electronic configuration belong to the s-Block Elements. They are all reactive metals with low ionization enthalpies.
They lose the outermost electron(s) readily to form 1+ ion (in the case of alkali metals) or 2+ ion (in the case of alkaline earth metals). The metallic character and the reactivity increase as we go down the group. Because of high reactivity they are never found pure in nature.
The compounds of the s-block elements, with the exception of those of lithium and beryllium are predominantly ionic.
The p-Block Elements
The p-Block Elements comprise those belonging to group 13 to 18 and these together with the s-Btock Elements are called the Representative Elements or Main Group Elements. The outermost electronic configuration varies from ns2np1 to ns2np6in each period. At the end of each period is a noble gas element with a closed valence shell ns2np6 configuration. All the orbitals in the valence shell of the noble gases are completely filled by electrons and it is very difficult to alter this stable arrangement by the addition or removal of electrons. The noble gases thus exhibit very low chemical reactivity. Preceding the noble gas family are two chemically important groups of non-metals. They are the halogens (Group 17) and the chalcogens (Group 16). These two groups of elements have high negative electron gain enthalpies and readily add one or two electrons respectively to attain the stable noble gas configuration. The non-metallic character increases as we move from left to right across a period and metallic character increases as we go down the group.
The d-Block Elements (Transition Elements)
These are the elements of group 3 to 12 in the centre of the Periodic Table. These are characterised by the filling of inner d orbitals by electrons and are therefore referred to as d-Block Elements. These elements have the general outer electronic configuration (n-1) d1-10ns^2. They are all metals. They mostly form coloured ions, exhibit variable valence (oxidation states), paramagnetism and oftenly used as catalysts. However, Zn, Cd and Hg which have the electronic configuration, (n-1) d10ns2 do not show most of the properties of transition elements. In a way, transition metals form a bridge between the chemically active metals of s-block elements and the less active elements of groups 13 and 14 and thus take their familiar name “Transition Elements”.
The f-Block Elements (Inner-Transition Elements)
The two rows of elements at the bottom of the Periodic Table, called the Lanthanoids, Ce(Z = 58) -Lu(Z = 71) and actinoids, Th(Z = 90)-Lr(Z= 103) are characterised by the outer electronic configuration (n-2)f1-14 (n-1 )d°-1ns2. The last electron added to each element is filled in f- orbital. These two series of ‘ elements are hence called the Inner Transition Elements (f-Block Elements). They are all metals. Within each series, the properties of the elements are quite similar. The elements after Uranium are called Transuranium Elements.
Metals, Non-metals and Metalloids. In addition to displaying the classification of elements into s, p, d and f-blocks, they can be divided into Metals and Non-Metals. Metals usually have high melting and boiling points. They are good conductors of heat and electricity. They are malleable (can be flattened into thin sheets by hammering) and ductile (can be drawn into wires). In contrast, non-metals are located at the top right hand side of the Periodic Table.
In fact, in a horizontal row, the property of elements change from metallic on the left to non-metallic on the right. Non-metals are usually solids or gases at room temperature with low melting and boiling points (boron and carbon are exceptions). They are poor conductors of heat and electricity. Most nonmetallic solids are brittle and are neither malleable nor ductile. The elements become more metallic as we go down a group; the nonmetallic character increases as one goes from left to right across the Periodic Table. The elements (e.g., silicon, germanium, arsenic, antimony and tellurium) running diagonally across the Periodic Table show properties that are characteristic of both metals and nonmetals. These elements are called Semi-metals or Metalloids.
Periodic Trends In Properties Of Elements
Most of the properties such as atomic radius, ionic radius, Ionisation enthalpy, electron gain enthalpy and electron negativity are directly related to electronic configuration of their atoms. They show variation with change in atomic number within a period or a group.
Trends In Physical Properties
1. Atomic Radius :
lt is defined as the distance from the centre of the nucleus of an atom to the outermost shell of electrons. Electron cloud surrounding the atom does not have a sharp boundary since, the determination of the atomic size cannot be precise. Hence it is expressed in terms of different types of radii. Some of these are covalent radius and metallic radius. Covalent radius is defined as one half of the distance between the centres of nuclei of two similar atoms bonded by a single covalent bond. Metallic radius may be defined as half of the internuclear distance between two adjacent atoms in the metallic crystal.
2. Ionic Radius:
The removal of an electron from an atom results in the formation of a cation, whereas gain of an electron leads to an anion. The ionic radii can be estimated by measuring the distances between cations and anions in ionic crystals. When we find some atoms and ions which contain the same number of electrons, we call them isoelectronic species. For example, O2-, F~, Na+ and Mg2+ have the same number of electrons (10). Their radii would be different because of their different nuclear charges.
3. Ionization Enthalpy:
A quantitative measure of the tendency of an element to lose electron is given by its Ionization Enthalpy. It represents the energy required to remove an electron from an isolated gaseous atom (X) in its ground state. To understand the trends in ionization enthalpy, we have to consider two factors: (i) the attraction of electrons towards the nucleus, and (ii) the repulsion of electrons from each other. The effective nuclear charge experienced by a valence electron in an atom will be less than the actual charge on the nucleus because of “shielding” or “screening” of the valence electron from the nucleus by the intervening core electrons.
The first ionization enthalpy of boron (Z = 5) is slightly less than that of beryllium (Z = 4) even though the former has a greater nuclear charge. It is because, s-electron is attracted to the nucleus more than a p-electron. In beryllium, the electron removed during the ionization is an s-electron whereas the electron removed during ionization of boron is a p-electron. The penetration of a 2s-electron to the nucleus is more than that of a 2p-electron; hence the 2p electron of boron is more shielded from the nucleus by the inner core of electrons than the 2s electrons of beryllium. Therefore, it is easier to remove the 2p-electron from boron Compared to the removal of a 2s-electron from beryllium.
Thus, boron has a smaller first ionization enthalpy than beryllium. Another “anomaly” is the smaller first ionization enthalpy of oxygen compared to nitrogen. This arises because in the nitrogen atom, three 2p-electrons reside in different atomic orbitals (Hund’s rule) whereas, in the oxygen atom, two of the four 2p-electrons must occupy the same 2p-orbital resulting in an increased electron-electron repulsion. Consequently, it is easier to remove the fourth 2p-electron from oxygen than it is, to remove one of the three 2p-electrons from nitrogen.
4. Electron Gain Enthalpy :
When an electron is added to a neutral gaseous atom (X) to convert it into a negative ion, the enthalpy change accom-panying the process is defined as the Electron Gain Enthalpy (∆eg H).
5. Electronegativity:
A qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself is called electronegativity. Unlike ionization enthalpy and electron gain enthalpy, it is not a measurable quantity. However, a number of numerical scales of electronegativity of elements viz., Pauling scale, Mulliken-Jaffe scale, Allred-Rochow scale have been developed.
Trends In Chemical Properties
1. Oxidation State :
The atomic property, valency is better explained in terms of oxidation state. It is the charge which an atom of element has or appears to have when present in the combined state. Electronegative elements generally acquire negative oxidation states while electropositive elements acquire positive oxidation states.
2. Anomalous properties of second-period elements:
The first element of each group in s and p block differs in many respects from the remaining members of the respective groups. This is due to their small size, high charge/ radius ratio, high electronegativity and availability of less valence orbitals. The first member has only 4 valence orbitals (2s, 2p) whereas the second member of the same group will have nine valence orbitals (3s, 3p, 3d) for bonding. B can form only (BF4)– while Al forms (AlF6)3-
In group 1 only Li forms covalent compounds and in many respects, Li resembles Mg of group 2. Similarly, Be resembles Al of group 13. This type of similarity in properties is known as diagonal relationship.
Chemical Reactivity
Across a period ionisation enthalpy increases and electron gain enthalpy becomes more negative. Thus elements at the extreme left show lower ionisation enthalpies (more electropositive nature) and those at the right (excluding nobel gases) show larger negative electron gain enthalpies (more electronegative). Therefore high chemical reactivity is found with elements at the two extremes compared to those at the centre. Electropositivity leads to metallic behaviour and electronegativity leads to non-metallic behaviour.