Oxides of the periodic table

Oxides

Oxygen forms compounds, called oxides, with almost every other element in the Periodic table. Since, many of these have quite unusual properties, there is an extensive and Varied chemistry of the compounds of oxygen. Oxides can be classified in more than one ways: Based upon the type of bonding they have as well as their acidic or basic character. We shall Discuss here the classification based on their acidic or basic behaviour.

 In this chapter, you have Already studied that metal oxides are basic in character as they yield bases in water and Non-metallic oxides are acidic because they form acids in water. Basic oxides and acidic oxides There is a third type of oxides, which show both acidic and basic properties, these oxides Are called amphoteric oxides. The classification of elements which form oxides of acidic or basic And amphoteric properties is shown in Table 1.6.

The oxides of alkali and alkaline earth metals except beryllium are basic and contain 0² Ions. The O² ion has high affinity for proton and cannot exist alone in an aqueous solution. Therefore, it immediately takes proton from water and forms OH ion. Oxides of nonmetallic Elements i.e. of C, N, P and S are acidic in nature. They generally dissolve in water to produce Acidic solutions. Oxides of relatively less electropositive elements, such as BeO, Al₂O,, Bi,O,and ZnO are amphoteric and behave as acids towards strong bases and as bases towards strong acids.

In a given period, the oxides change from strongly basic through weakly basic, Amphoteric and weakly acidic to strongly acidic, e.g. Na,O, MgO, Al₂O3, P.O, SO,, Cl₂O..The Basic character of main group metal oxides increases on descending a group of the periodic table, (e.g. BeO<MgO <CaO <SrO <BaO), though the reverse trend is observed in the transition Metal oxides. The oxidation state of the metal also affects the acidic/basic character of its oxide. The acidic character increases with increasing oxidation state.

THE POSITION OF HYDROGEN

Although, it is not a metal but in most of the modern versions of periodic table, hydrogen Is placed at the top of the group IA. This is because of the fact that some of the properties of Hydrogen resemble with those of alkali metals. Like alkali metals hydrogen atom has one Electron in Is subshell, which it can lose to form H. Both hydrogen and alkali metals have a Strong tendency to combine with electronegative elements such as halogens. Similar to alkali Metals hydrogen also forms ionic compounds, which dissociate in water. However, hydrogen is Also markedly different from alkali metals. For example, hydrogen is a non-metal in true sense. It Does not lose electron as easily as most of the alkali metals do. Unlike alkali metals molecular Hydrogen exists in nature.

Hydrogen resembles halogens in certain respects and can be placed at the top of VIIA

Group in the periodic table. Hydrogen is a gas like most of the halogens and is stable in diatomic

Form such as F₂, Cl, and Br,. As required by halogens, hydrogen also needs one electron to

Complete its outermost shell. By accepting one electron hydrogen forms H (Hydride ion) similar

To F, CI and Br. Both hydrogen and halogens form stable ionic compounds with alkali metals.

However, hydrogen differs from halogens as well. By losing its only electron, hydrogen forms H

But halogens do not form positive ions. Combining with oxygen, hydrogen forms very stable

Oxides while halogens lack this property.

Some of the characteristic properties of hydrogen also resemble with those of group IVA

Elements such as C and Si, etc. For example, valence shell of hydrogen is half filled like those of

Group IVA elements. Both, hydrogen and group IV elements combine with other elements

Through covalent bonding. Like carbon, hydrogen also possesses remarkable reducing

Properties.

Hydrogen also shows marked differences with carbon and rest of the group members.

For example, carbon and silicon form long chain compounds, when their atoms combine with

Each other, while hydrogen does not form such compounds. Similarly, carbon can

Simultaneously form bonds with more than one elements, whereas hydrogen due to having only

One electron can combine with only one element at a time.

Some of the properties of hydrogen are similar to those of the elements of certain groups,

As discussed above, but it is a fact that hydrogen is a unique element whose properties do not

Match exactly with any of the groups in the periodic table. However, due to partial resemblance in

Properties with alkali metals and monovalent nature, hydrogen is usually placed at the top of

Elements in group IA.

KEY POINTS

1. Although a number of chemists attempted to classify the elements but Dmitri Mendeleev gave the most useful and comprehensive classification.

2. In Mendeleev’s periodic table the elements were arranged according to the ascending Order of their atomic masses.

3. The modern periodic law states “if the elements are arranged in ascending order of their Atomic numbers, their chemical properties repeat in a periodic manner.”

4. In modern periodic table elements with similar properties are placed in eight vertical Columns called groups.

5. Each Group Is divided into two subgroups A and B. Normal or Typical elements are placed in subgroup A and transition elements are placed in ssubgroupB

6. The seven horizontal rows of the periodic table are called “periods”. Metals of subgroups IA and IIA are called Alkali metals and Alkaline earth metals, Respectively. Members of subgroup VIIA are called halogens.

7. Due to their less reactivity the elements shown in subgroup VIII A are called noble gases.

8. Elements of periodic table can also be classified into s-block, p-block, d-block and f-block elements depending upon the valence orbital which is in the process of completion.

9. Elements of periodic table can also be divided into metals, non-metals and metalloids Depending upon their properties.

10. Atomic radii increase from top to bottom in a group and decrease along a period.

11. Positive ions are always smaller than their parent atoms while the negative ions are Usually larger than the atoms from which they are formed. 

12. Ionization energies increase along a period and decrease down the group.

13. Electron affinities generally increase with increasing atomic number within a period and Decrease from lighter to heavier elements in a given group.

14. Metallic character of elements increases down the group and decreases along a period. 

15. The oxidation state of a typical element is directly or indirectly related to the group Number to which the element belongs in the periodic table. 

16. The electrical conductance of an element depends upon the number of free or moveable Electrons.

17. There are three types of halides: ionic, polymeric and covalent. Halides of group IA are Ionic in nature, have three dimensional lattices with high melting and boiling points.There are three types of hydrides formed by the elements of periodic table: ionic, Intermediate and covalent.

18. Highly polar hydrides show hydrogen bonding in them. Oxides may be divided on the basis of their acidic, basic or amphoteric character.

19. Metallic oxides are basic in character, non-metallic oxides are acidic in character and Oxides of less electropositive elements like Zn and Pb are amphoteric.

20. Hydrogen is unique element of the periodic table. Due to similarities in properties it can Be placed at the top of group IA or IVA or VIIA.