Chapter 2 - Periodic Trends

8/10/2019 Chapter 2 - Periodic Trends

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Periodic Trends


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Atomic size increase from top to down

Reason: because the number of inner shells down the

group increases

Atomic size

(i) Down the Group


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Atomic size

(ii) Across the period

Atomic size decreases from left to right

Reason : steady increase in the effective nuclear charge

cause the valence electron to be held more strongly by

nucleus


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Explanation

Elements in same period have the same principal

quantum number, n.However the nuclear charge increases as the

number of protons increases.

Effective nuclear charge the strength ofattraction between the outer most electrons andthe nuclear charge .

Element Electronconfiguration Period

Na 1s22s22p63s1 3

Si 1s22s22p63s23p2 3

Cl 1s22s2sp63s23p5 3


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Effective nuclear charge

does not change.

Effective nuclear charge Increases with the number

of inner electron remains the same

Na Mg Al Si

1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 1s22s22p63s23p2

Zeff = 11-10 Zeff = 1210 Zeff = 1310 zeff = 14 - 10

= 1 = 2 = 3 = 4

Decreases the

atomic size


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The shielding Effect in Many- Electron Atoms

-the inner electrons shield the outer electrons from the

attractive force of nucleus.-Conclusion : Shielding reduces the electrostatic attraction

between the positive charged protons in nucleus and the

outer electrons.

No changes

because the no. of

inner group shell

remains.

Increasesbecause

has more inner

shellsthan the

preceding group


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Atomic size contributes toward the trends in

1. Ionization energy2. Electron affinity3. electro negativity


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Ionisation Energy (for cation)

-Minimum energy to remove 1 mol electron from the 1 mol

ground state atom/ion in gaseous state.-The greaterthe ionisation energy, that hard to remove electron

from an atom.

- 1stionisation energy minimumenergy required to remove an

electron from the ground state atom in gaseous state.

M(g) - 1e M+(g) H = First ionisation energy

-Second ionisation energy is the energy required to remove the

second electronfrom the gaseous positive ionin its ground state.

M+(g)1e M 2+(g) H = First ionisation energy


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The first Ionisation energy increases from left to right

Reason: atomic radii decrease, the outer electron are more tightly

held to the nucleus, more energy need to supply to remove first

electron.

Decrease moving down the group

Reason: distance from nucleus decrease, weaker attraction from

nucleus easy to remove the electron, thus the metallic character

increase from top to down.


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Electron affinity

Ability to accept 1 or more electrons.

Definition :energy change that occurs when an

electron is accepted by an atom in its gaseous state.

More negative the electron affinity the greater the

tendency of the atom to accept an electron.

Example : O (g) + eO-(g) H= -142 kJ

O- (g) + eO2- (g) H = +745 kJ

More energy hard to accept electron cause the net

charge (-1) repelled, so more energy need to force

the electron.


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Electron affinity more negative

Atomic radii smallerattractive forceof nucleus increase

tendency to accept electron increase

Electron affinity less negative,

atomic radii increasenucleus

attraction force weakerless

tendency to accept electron


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-Ionisation energy and electron affinity only apply

to isolated gaseous not directly atoms in

molecules.

Noble gases

-Monoatomic species

-Very unattractive

-No tendency to join with others

-Completely filled outer shell

-The highest ionisation energy, no tendency to

accept extra electron.


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Electronegativity

Tendency of an atom to attract bondingelectron

to itself when it is in a molecule.

related to electron affinity with different

concepts. Both refer to tendency to attract

electron.Electron affinityrefer to an isolated atoms

attraction for an additional electron.

Experimentally measured.

Electronegativitythe attraction of an atom in

a chemical bond(with other atom) for the shared

electrons. Relative number, which is not

measured.


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Electronegativity increase

Reason: Atomic radii decrease, the pulling effect

from nuclear charge (positive) on electron

(negative) is bigger

Electronegativity decreaseReason : atomic number increaseatomic radii increase

forces of nucleusdecrease attractive


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less electronegativity high metallic character

High electronegativity less metallic character

Electronegativity difference and bond type

(i) Wide different ionic bond

Less electronegativity gives up its electrons

High electronegativity accept electrons

(ii) Small difference

-Polar covalent bonds (small shift in electron density takes place)

-Same electronegativity

-Pure covalent bond or non polar bond

Electronegativity > 2 normally ionic bond (but not all)

Chapter 2 - Periodic Trends

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