Periodic Trends Flashcards
What are the challenges faced when measuring radius of an atom
- The size of the atom is very small ~ 1.2 A
- The electron cloud surrounding the atom does not have a sharp boundary and hence atomic radius determination cannot be precise
Covalent radius
Calculated by measuring the distance between nuclei of atoms of covalently bonded molecule and taking half of that value
Metallic radius
Half the internuclear distance separating the cores in a metallic crystal
Atomic Radius variation
Atomic radius increases down the group and decreases across the period except noble gases, which have a large radius
Why does atomic radius increase down the group
As we go down the group, principal quantum number increases i.e., number of shells increase. These outer electrons are farther away from the nucleus due to good shielding effect of the inner electrons. Hence, radius keeps increasing
Why does atomic radius decrease across the period
As we move from left to right, number of shells stay the same but nuclear charge increases which means the electrons are pulled towards the nucleus with more force, hence making it smaller.
Discuss ionic radii variation
Ionic radii show the same variation as atomic radii
But, cations are smaller than the parent atom due to increased nuclear force due to lesser number of electrons
Anions are larger than parent atom due to increased repulsion caused by extra electron
Isoelectronic species
Species with same number of atoms and valence electrons
O2-, F-, Na+, Mg2+ (all of them have 10 e-)
Ionization enthalpy
Energy required to remove one electron from an isolated gaseous atom in its ground state
Always positive
Variation between first, second, third IE
The second IE is always more than the first because it is harder to remove an electron from a positive atom than a neutral one
Similarly, the third IE > second IE
Variation of IE in the periodic table
- Increases as we go across the period
2. Decreases as we go down the group
Explain variation of IE
- Along a period, increase in nuclear charge outweighs the shielding effect and so IE keeps increasing due to the increase in nuclear charge
- Down the group, increase in shielding effect outweighs the nuclear charge and so lesser energy is required to remove an electron as we go down the group
Exceptions in IE trend
- B(5) has a smaller enthalpy than Be(4) because s electron is more attached to the nucleus. In B, a p electron is removed while in Be, an s electron is removed (which is harder so higher IE)
- O(p4) has a lower IE than N(p3) because of extra stability of N’s half filled shell
Electron gain enthalpy
Energy required to convert isolated gaseous atom in ground state to an anion by adding an electron
Could be negative or positive depending on the element considered
Electron gain enthalpy variation
EGE becomes more negative as we go across the period and less negative as we go down the group
Explain EGE variation
As we go along a period, number of valence electrons keep increasing. Adding an electron would get this element loser to noble gas configuration and hence EGE gets more negative across the period
Down the group, the added electron would keep going farther from the nucleus and so, EGE becomes less negative
EGE Exception
O or F is less negative than that of the succeeding element because the electron added goes to the n=2 shell for these atoms and faces a lot of repulsion.
Electronegativity
Qualitative measure of the tendency to attract a shared pair of electrons towards itself
Scales to measure electronegativity
Mulliken-Jaffe scale, Pauling scale, Allred-Rochow scale
Pauling Scale
According to this scale, F (most EN element) has a value of 4
Every other value is relative to this
Electronegativity variation
EN increases across the period and decreases down the group
EN - metallic nature relation
EN can be seen as a measure of non metallic character
EN is directly proportional to non metallic character and inversely proportional to metallic character
Valence
Usually equal to the number of electrons in the outermost shell and/or eight minus the number of electrons in the outermost shell
Oxidation state
Charge acquired by its atom on the basis of electronegative consideration from other atoms in the molecule
