Chemistry General introduction, Group 13 Elements: The boron family

### Topics to be covered

=> General introduction
=> Group 13 Elements: The boron family

### Introduction

★ In color{red}(p)-block elements the last electron enters the outermost color{red}(p) orbital.

★ As we know that the number of color{red}(p) orbitals is three and, therefore, the maximum number of electrons that can be accommodated in a set of color{red}(p) orbitals is six.

★ Consequently there are six groups of color{red}(p)–block elements in the periodic table numbering from 13 to 18.

★ Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups.

★ Their valence shell electronic configuration is color{red}(ns^2n p^(1-6)) (except for color{red}(He)). The inner core of the electronic configuration may, however, differ.

★ The difference in inner core of elements greatly influences their physical properties (such as atomic and ionic radii, ionisation enthalpy, etc.) as well as chemical properties. Consequently, a lot of variation in properties of elements in a group of color{red}(p)-block is observed.

★ The maximum oxidation state shown by a color{red}(p)-block element is equal to the total number of valence electrons (i.e., the sum of the sand color{red}(p)-electrons). Clearly, the number of possible oxidation states increases towards the right of the periodic table. In addition to this so called group oxidation state, color{red}(p)-block elements may show other oxidation states which normally, but not necessarily, differ from the total number of valence electrons by unit of two. The important oxidation states exhibited by color{red}(p)-block elements are shown in Table 11.1.

★ In boron, carbon and nitrogen families the group oxidation state is the most stable state for the lighter elements in the group. However, the oxidation state two unit less than the group oxidation state becomes progressively more stable for the heavier elements in each group. The occurrence of oxidation states two unit less than the group oxidation states are sometime attributed to the ‘inert pair effect’.

★ It is interesting to note that the non-metals and metalloids exist only in the color{red}(p)-block of the periodic table.

• The non-metallic character of elements decreases down the group. In fact the heaviest element in each color{red}(p)-block group is the most metallic in nature.

• This change from nonmetallic to metallic character brings diversity in the chemistry of these elements depending on the group to which they belong.

• In general, non-metals have higher ionisation enthalpies and higher electronegativities than the metals.

• Hence, in contrast to metals which readily form cations, non-metals readily form anions.

• The compounds formed by highly reactive non-metals with highly reactive metals are generally ionic because of large differences in their electronegativities.

• On the other hand, compounds formed between non-metals themselves are largely covalent in character because of small differences in their electronegativities.

• The change of non-metallic to metallic character can be best illustrated by the nature of oxides they form.

• The non-metal oxides are acidic or neutral whereas metal oxides are basic in nature.

★ The first member of color{red}(p)-block differs from the remaining members of their corresponding group in two major respects.

• First is the size and all other properties which depend on size. Thus, the lightest color{red}(p)-block elements show the same kind of differences as the lightest color{red}(s)-block elements, lithium and beryllium.

• The second important difference, which applies only to the color{red}(p)-block elements, arises from the effect of dorbitals in the valence shell of heavier elements (starting from the third period onwards) and their lack in second period elements.

★ The second period elements of color{red}(p)-groups starting from boron are restricted to a maximum covalence of four (using color{red}(2s) and three color{red}(2p) orbitals).

★ In contrast, the third period elements of color{red}(p)-groups with the electronic configuration color{red}(3s^2 3p^n) have the vacant color{red}(3d) orbitals lying between the color{red}(3p) and the color{red}(4s) levels of energy.

★ Using these color{red}(d)-orbitals the third period elements can expand their covalence above four. For example, while boron forms only color{red}([BF_4]^–), aluminium gives color{red}([AlF_6]^(3–)) ion.

★ The presence of these color{red}(d)-orbitals influences the chemistry of the heavier elements in a number of other ways.

★ The combined effect of size and availability of color{red}(d) orbitals considerably influences the ability of these elements to form color{red}(π) bonds.

★ The first member of a group differs from the heavier members in its ability to form color{red}(pπ - pπ) multiple bonds to itself ( e.g., color{red}(C=C, C≡C, N≡N)) and to other second row elements (e.g., color{red}(C=O, C=N, C≡N, N=O)). This type of color{red}(π -) bonding is not particularly strong for the heavier color{red}(p)-block elements.

★ The heavier elements do form color{red}(π) bonds but this involves color{red}(d) orbitals color{red}((dπ – pπ or dπ –dπ )). As the color{red}(d) orbitals are of higher energy than the color{red}(p) orbitals, they contribute less to the overall stability of molecules than does color{red}(pπ - pπ) bonding of the second row elements. However, the coordination number in species of heavier elements may be higher than for the first element in the same oxidation state. For example, in +5 oxidation state both color{red}(N) and color{red}(P) form oxoanions : color{red}(NO_3^(–)) (three-coordination with color{red}(π) – bond involving one nitrogen color{red}(p)-orbital) and color{red}(PO_4^(3-)) − (four-coordination involving color{red}(s, p) and color{red}(d) orbitals contributing to the color{red}(π) – bond). In this unit we will study the chemistry of group 13 and 14 elements of the periodic table.

### GROUP 13 ELEMENTS: THE BORON FAMILY

★ This group elements show a wide variation in properties.

★ Boron is a typical non-metal, aluminium is a metal but shows many chemical similarities to boron, and gallium, indium and thallium are almost exclusively metallic in character.

★ Boron is a fairly rare element, mainly occurs as orthoboric acid, color{red}(H_3BO_3), borax, color{red}(Na_2B_4O_7 · 10H_2O), and kernite, color{red}(Na_2B_4O_7·4H_2O.)

★ In India borax occurs in Puga Valley (Ladakh) and Sambhar Lake (Rajasthan).

★ The abundance of boron in earth crust is less than color{red}(0.0001%) by mass.

★ There are two isotopic forms of boron color{red}(text()^(10)B (19%)) and color{red}(text()^(11)B (81%)).

★ Aluminium is the most abundant metal and the third most abundant element in the earth’s crust (color{red}(8.3%) by mass) after oxygen (color{red}(45.5%)) and color{red}(Si (27.7%)). Bauxite, color{red}(Al_2O_3. 2H_2O) and cryolite, color{red}(Na_3AlF_6) are the important minerals of aluminium. In India it is found as mica in Madhya Pradesh, Karnataka, Orissa and Jammu. Gallium, indium and thallium are less abundant elements in nature.