Long form of periodic table, ionization energy and electron affinity

Long form of periodic table or Modern periodic table

Modern periodic table is classified into seven periods, eighteen groups and four blocks namely s, p, d and f. The table below gives the number of elements in different periods.

The classification of elements into groups is divided into vertical columns. Group first and thesecond consist of alkali and alkaline earth metals. Group three to twelve contains transition metals. Elements in group seventeen and eighteen are called halogen and noble gasses respectively.

The classification of elements into blocks is according to which orbital the differentiating electron enters into. The elements in groups first and second are s-block, group 13 to 18 are p-block, group 3 to 12 are d-block elements and lanthanides and actinides are f-block elements.

Merits of periodic table

1. It is based on the atomic number which is more prominent property than atomic weight.
2. The position of elements that was not explained by the periodic table based on atomic weight (anomalous pairs like argon and aluminium) was explained on the basis of atomic number.
3. It explains the periodicity of the properties of the elements and relates them to the electronic configuration.
4. The separation of lanthanides and actinides at the bottom of periodic table makes it easy to study and understand.

Demerits of Long form of periodic table

1. The position hydrogen is controversial.
2. Lanthanides and actinides are placed separately like in Mendeleev's periodic table.
3. Elements like Li and Mg, Be and Al and B and SI are placed in separate group and period even though they showsimilar properties.

Periodicity of elements

The repetition of elements with similar properties after regular intervals when they are arranged in order increasing atomic numbers which are called periodicity. The main reason for the periodicity is the repetition of similar electronic configuration after certain intervals. The physical and chemical properties of the element depend on the number of valence electrons. So, the element belonging to the particular group have thesame number of valence electrons. Hence, they bear similar properties. For example, the elements belonging to thegroup first (alkali metals) have one s-electron in their valence shell. All of them have general outer electronic configuration ns and show similar properties.

Properties

Atomic size

1. Variation in a period: The atomic size decreases from left to right across a period. As atomic number increases from left to right across theperiod, the addition of an extra electron takes place with anaddition of positive charge but the number of valence shell remains same therefore nuclear charge increases so contraction takes place and as a result atomic size decreases. For example, in third period Na> Mg> Al> Si> P> S> Cl (atomic size decreases from left to right.
2. Variation in a group: The atomic size increases on moving from top to bottom along the group. This is because of anaddition of extra shell of anelectron on moving down the group from one element to another. For example, in thegroup first, theorder of atomic size among different elements, Li< Na< K <Rb< Cs (atomic size increases from top to bottom).

Ionic radii

Ionic radii vary same as the atomic size along the period as well as in thegroup.

Ionization energy

The amount of energy required to remove the most loosely bound electron from an isolated atom of an element in a gaseous state is called its Ionization energy.

In other words, ionization energy is the energy required to carry out the following reaction:

Na(g)→ Na⁺(g) + e^-

Ionization energy gives an idea about the tendency of an atom to form apositive ion. It is also called ionization potential.

Successive ionization energy

When the first valence electron is removed from the atom then it is possible to remove the next electrons one by one. The energy required for the removal of subsequent electrons are called successive ionization energies. They are denoted by IE₁(first ionization energy), IE₂, IE₃etc for second and third respectively.

M_(g)+ IE₁→ M⁺ + e^- (1^st IE)

M⁺_(g)+ IE₂→ M⁺ + e^- (2^nd IE)

M³⁺_(g)+ IE₃→ M³⁺ + e^- (3^rdIE)

In the above reactions,IE₁< IE₂<IE₃which means that successive ionization energy increases progressively because as the electron is remove from an atom it becomes positive ion and the nuclear charge increases which held the atoms more strongly and it is difficult to remove the electron in each progressive steps. Therefore, ionization energy increasesfrom IE₁<IE₂<IE₃. Another reason is when more electrons are removed from the atom consequently next electron maybe from a lower energy orbital and that would require much more energy for its removal.

Factors influencing the ionization energy of an element

1. The size of an atom, as the size increases ionization energy decreases and vice-versa.
2. The higher the nuclear charge higher is the ionization energy. As high energy is needed when electrons are tightly held by thenuclear charge.
3. Increase in shielding effect decreases the ionization energy. The shielding effect of the electron in the orbital of given shell is in the order s> p> d> f.
4. The half filled and completely filled orbitals are stable and it is difficult to remove electrons and the ionization energy increases.

Variation of ionization energy

1. Along period: Ionization energy increase increases along period as the atomic size decreases and nuclear charge increases. This results in electrons being more tightly held by thenucleus. Therefore, ionization energy increases along the period as we move forward.
2. Along group: Ionization energy decreases along the group. As the atomic size increases due to the addition of extra- orbital and nuclear charge increases and the shielding effect also increases. The increase in atomic size and shielding effect overcome the increase in nuclear charge and ultimately the ionization energy increases.

Electron affinity

Energy is released when an extra electron is added to an isolated neutral gaseous atom and forms negatively charged ion. The energy released is called electron affinity. The energy is released so the electron affinity has thenegative sign and is anexothermic process. This electron affinity is actually the first electron affinity i.e, only one electron is added.

H + e^-→ H^- — 72 kj mol^-1

When an electron is added to negative atom (H^-) energy is absorbed, since the electron is added against the electrostatic repulsion which exists between the negative charge on the atom and a negative charge on an electron. The process is endothermic and hence has a positive value.

Variation of electron affinity

1. Along a period: Electron affinity increases along a period as atomic size decreases and nuclear charge increases so the nucleus attracts the added electron.
2. Along a group: Atomic size increases, shielding effect increases, and nuclear charge also increases. But the increase in nuclear charge is overcome by the increase in atomic size and shielding effect and incoming electron is less attracted due to which electron affinity decreases.

Factors influencing the electron affinity

1. The size of an atom, as the atomic size decreases nuclear charge increases and the incoming electron is attracted more strongly. Therefore, electron affinity increases.
2. With the increase in shielding effect electron affinity decreases.
3. Half filled and completely filled atoms showlesser tendency to add an electron so electron affinity decreases.

References

Lee, J.D. Concise Inorganic Chemistry. John Wiley and sons.Inc, 2007.

answers. n.d. <http://www.answers.com/Q/What_are_the_Disadvantages_of_long_form_of_periodic_table?#slide=7>.

City collegiate. n.d. <http://www.citycollegiate.com/periodictable2.htm>.

Wikipedia. n.d. <https://en.wikipedia.org/wiki/Periodic_table>.