Periodicity: Ionisation energies and Atomic Radii

Key Words:

o   Period

Is a horizontal row of elements in the Periodic Table. Elements show trends in properties across a period.

o   Group

Is a vertical column in the periodic table. Elements in a group have similar chemical properties and their atoms have the same number of outer shell electrons.

o   Periodicity

 

Is a regular periodic variation of properties of elements with atomic number and position in the Periodic table.

o   First Ionisation energy

 

   The first ionisation energy of an element is the energy required to remove one electron from each atom in one mole of gaseous atoms to form one mole of gaseous 1+ ions.

 

o   Electron shielding

 

    Is the repulsion between electrons in different inner shells. Shielding reduces the net attractive force from the positive nucleus on the outer-shell electrons.

Periodicity: Ionisation energies and atomic radii

Recap:

• In a vertical group they have similar electron configurations.  They have 1) The same Nº of electrons in the outer shell and 2) the same type of orbitals
•  á Nuclear charge, greater the attractive force on the outer electrons, á I.E
• Greater the atomic radius, Smaller the nuclear attraction experienced by the outer shells, â I.E
•  Electron Shielding= á inner shells, larger shielding effect, smaller nuclear attraction experienced by outer electrons, â I.E

Across a period

§  Ionisation energy increases

§  Most important factor= Increased nuclear charge

§  Attraction between the nucleus and outer electrons increases → More energy is needed to remove an electron

§  á Number of protons → á attraction acting on electrons

§  Electrons added to the same shell → Outer shell is drawn inwards slightly

§  Same Nº of inner shells → electron shielding hardly changes

§  â Atomic radius as increased nuclear charge pulls the electrons in towards it.

 

There is a sharp decrease in ionisation energy between one end of a period to the start of the next:

« Addition of new shell→ further from the nucleus

« Increased distance of outermost shell from the nucleus

« Increased electron shielding of the outermost shell by inner shells.

Down a group

µ First Ionisation energy decreases

µ Most important factor= Increased distance and shielding

µ Nº of shells á, distance of the outer electrons from the nucleus increases, weaker force of attraction on the outer electrons

µ More inner shells, shielding effect on the outer electrons from the nuclear charge increases, less attraction

µ Nº of protons in the nucleus á, but outweighed by the increase in distance and shielding

µ Atomic radius increases down the group, less attraction so electrons are not pulled as close to the nucleus.

Conclusion:

Ionisation energy shows a general increase across each period.                                                                       

Across each period, the number of protons increases, so there is more nuclear attraction acting on the electrons.

Electrons are added to the same shell, so the nuclear attraction draws the outer shells inwards slightly.

There is the same number of inner shells and so electron shielding will hardly change.

The increased nuclear charge is the significant factor.

______________

Ionisation energy shows a general decrease down each group.
The number of shells increases.

The distance of the outer electrons from the nucleus increases, increasing the atomic radius. There is a weaker force of attraction on the outer electrons.     

 

There are more inner shells, so the electrons are more effectively shielded from the nuclear charge. Again, there is less attraction.                                                                                             

The number of protons in the nucleus also increases, but the resulting increased attraction is far outweighed by the increase in distance and shielding.

This is a video that really helped me to grasp this topic: