Trend of ionization energy

Ionization energy

Variation Within a Group:

The factors upon which the ionization energy of an atom mainly depends are magnitude Of nuclear charge, size of the atom, and the “shielding effect”. The shielding effect is actually the Repulsion due to electrons in between the nucleus And the outermost shell. This effect increases, as the Size of the atom increases due to addition of an extra Shell successively in each period hence more Number of electrons shields the nucleus. 

Going Down in a group, the nuclear charge increases but as The size of the atom and the number of electrons Causing the shielding effect also increases therefore Ionization energy decreases from top to bottom. That Is why in alkali metals, for example, it is easier to Remove an electron from caesium atom than from Lithium atom. The change in ionization energies of IA elements is shown in Fig. 1.3.

Variation Across a Period:

Generally, smaller the atom with greater Nuclear charge, more strongly the electrons are Bound to the nucleus and hence higher the Ionization energy of the atom. By moving from left To right in a period, the outer shell remains the Same, while the nuclear charge increases Effectively that makes the removal of an electron Difficult and hence the value of ionization energy Increases.

Although, the number of electrons also Increases in this case but the shielding is not very Effective within the same shell. The trend of Ionization energies of short periods is shown in Fig.1.4. The figure also reveals that inert gases have The highest values of ionization energy because due To complete outermost shell in them, the removal of Electron is extremely difficult.

Electron Affinity (E.A.)

 The electron affinity is the energy released or absorbed, when an electron is added to a Gaseous atom to form a negative ion. Energy is usually released when electronegative elements absorb the first electron and E.A. in such cases is expressed in negative figures, as in the case of halogens. When a second Electron is added to a ion, the incoming electron is repelled by the already present Negative charge and energy is absorbed in this process.

The absorbed energy is expressed as the electron affinity in positive figures. Electron Affinity depends upon size of the atom, nuclear charge and vacancies in the outermost shell  Relatively smaller atoms with one or two vacancies in the outermost shell show large values of Electron affinity. Electron affinity generally increases with increasing atomic number within a Period and decreases from lighter to heavier elements in a given group of the periodic table.

Knowledge of electron affinities can be combined with the knowledge of ionization energies to Predict which atoms can easily lose electrons and which can accept electrons more readily.

Metallic and Non-Metallic Character

It has already been discussed in this chapter that elements of periodic table can be Divided into metals, non-metals and metalloids. Chemically all the elements which have a Tendency to form positive ions by losing electrons are considered metals. All metals are good Conductor of heat and electricity. A characteristic property of metals is that they form basic Oxides which give bases when dissolved in water.

As it becomes easier to remove the electron of an atom bigger in size, therefore metallic Character increases from top to bottom in a given group of elements. On the contrary, it decreases From left to right across a period. The elements of group VIIA (the halogens) are least metallic in Nature.The elements which gain electrons and form negative ions are called non-metals. All the Gases are non-metals. The non-metals are normally poor conductor of heat and electricity.

 Non- Metals form acidic oxides which yield acids on dissolving in water. Non-metallic character of an element, decreases as the atomic size increases. Therefore In a group of non-metals like halogens, the non-metallic character decreases from top to bottom. The member at the top, fluorine, is the most non-metallic element of the periodic table. This trend Can also be verified in the elements of groups VA and VIA. Nitrogen and oxygen are pure non- Metals and usually exist in gaseous state while bismuth and polonium, the members at the bottom Of these groups, are fairly metallic in nature.

Melting and Boiling Points

Melting and boiling points of elements tell us something about how strong the atoms or Molecules in them are bound together.

Variation in a Period

Across the short periods, the melting And boiling points of elements increase with the Number of valence electrons upto group IVA and Then decrease upto the noble gases. The melting Points of group IA elements are low because Each atom in them provides only one electron to Form a bond with other atom. Melting points of Group IIA elements are considerably higher than Those of group IA elements because each atom inThem provides two binding electrons.

The Since carbon has the maximum number of binding electrons, thus it has a very high Melting point in diamond in which each carbon is bound to four other carbon atoms. In general, The elements which exist as giant covalent structures have very high melting points, Fig. 1.5. An important change occurs when we move from group IVA to groups VA, VIA, VIIA as

The lighter elements of these groups exist as small, covalent molecules, rather than as three Dimensional lattices. For instance, nitrogen, oxygen and fluorine exist as individual molecules Which have very weak intermolecular forces between them. Consequently, their melting and Boiling points are extremely low.

Variation in a Group

The melting and boiling points of IA and IIA group elements decrease from top to bottom Due to the increase in their atomic sizes. The Binding forces present between large sized atoms Are relatively weaker as compared to those Between smaller atoms, Fig. 1.6.(b)  For elements of group VIIA, which exist In the form of molecules, the melting and boiling Points increase down the group, Fig. 1.7. This is Because large molecules exert stronger force of Attraction due to their higher polarizabilities.

Oxidation state

The oxidation state of an atom in a compound is defined as the apparent charge (with the Sign), which it would carry in the compound. In ionic Compounds, it is usually the number of electrons Gained or lost by the atom. As in the case of sodium Chloride, the oxidation states of sodium and chlorine Are +1 and -1, respectively. In covalent compounds, it Is decided on the basis of the difference in their Relative electronegativities. For example, SnCl, is a Covalent compound. The oxidation state of tin is + 4 And that of chlorine is -1. The oxidation state of an Element is zero in its free state. 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. The Elements of group IA to IVA have the same oxidation state