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8/13/2019 Periodicity 1
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Syllabus
Periodic variation in physical properties of the
elements H to Ar
Variations in first ionization enthalpies, atomic radii,electronegativitiesand melting points.
Interpretation of these variations in terms of structure andbonding.
Periodic relationship among the oxides of theelements Li to Cl
Bonding and stoichiometric composition of the oxidesofthese elements, and their behaviour with water, diluteacids and alkalis.
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The Periodic Table
The elements are arranged in the order of atomic number
Do. Q. 1b
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Elements were first arranged in order of increasing atomic
massesby Dimitri Mendeleev(1834 - 1907)
The elements were observed to repeat their properties periodically
(a) (b)
Periodic Table (early forms)
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Modern periodic table: (p.1)
Rows periods Columns groups
Classified into 4 areas:
p-blocks-block
d-block
transition elements
f-block inner transition elements
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s-block elements (p.2):
Group IA: alkali metals
1 ein outermost shell (ns1)
(e.g. Li, Na, K)
Group IIA: alkaline earth metals
2 ein the outermost shell (ns2)
(e.g. Be, Mg, Ca)
p-block elements:
Groups IIIA, IVA, VA, VIA, VIIA, 0
Group VIIA : halogens (ns2np5)
Group 0 : noble gases (ns2 np6)
d-block elements:
Electronic configuration : (n1)d1ns2 to (n1)d10ns2
(Group IIIB) (Group IIB)
Transition elements
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f-block elements:
Lanthanide series and actinide series :
4fand 5forbitals are filled up with 1 to 14 e
-
inner-transition elements
Aims of Periodic Table: (p. 4):
1. Similar elements to be grouped together as families;
2. Gradual changes in properties such as electronegativity,
ionization enthalpies.
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Covalent radius is defined as half the internuclear distance
between two covalently bonded atoms in a molecule of the
element.
Atomic Radius
(p.9) How can scientists measure the sizes of atoms?
(1) For non-metals, atomic radius refers to the covalent radius:
(2) For metals, atomic radius refers to the metallic radius:
Metallic radius is defined as half the internuclear distancebetween atoms in a metall ic crystal.
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Across the period, the atomic radii decreaseprogressively
Variation in atomic radius of the first 20 elements
Down the group, the atomic radii increaseprogressively
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(1) Screening/Shielding of electrons (repulsion between
electrons)
(2) Attraction of the nucleus (protons) for the electrons
The atomic radius is governed by two factors: (p.6, notes p.9)
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Down a group:
Increase in number of electron shells.
Increase shielding effect from inner shells electrons.
Across a period:
Electrons add to the same outermost shellnot much
increase in shielding effect
More protonsgreater attraction to eoutweighs increase
in shielding effectsmaller size
Decrease along the period of transition series is small:
Electrons are added to inner d-orbitalsscreen the
outermost electron shell.
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Melting point (notes p.11)
Melting Temperature depends on the magnitude offorcesbetween particles
Metals: Metallic Bond
Giant Covalent Crystals: Covalent Bond Molecular Crystals:
Van der Waals forces
permanent dipolepermanent dipole
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Variation in melting point of the first 20 elements
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Structure and Bonding (p.4)
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1. Steady increasein melting point from Li to Beand Na to Al
no. of outermost/delocalised electrons increases
no. of protons increases
metallic bond strength increases
- similar forces exist in liquidmelting point not very high.
2. Carbon and silicon correspond to the maximain Periods
2 and 3
both have giant covalent structures. Atoms are held
together by strong covalent bonds.
Across a Period (p.11)
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1. Metallic structures
Very strong metallic bond because of the availability of d
electrons / orbitals for metallic bonding.
Maximum reaches in the middle except manganese as it has
stable half-filled structure.
2. Carbon and silicon correspond to the maximain Periods
2 and 3
both have giant covalent structures. Atoms are held
together by strong covalent bonds.
Across the Period of transition elements (p.13)
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More on carbon: (p.7, 11)
Carbon has two allotropes: graphite and diamond.
Which of them is more stable?
The (C-C) bond distance in graphite is 1.415 A while that in
diamond is 1.54 A.
Diamond is hard while graphite can be used as lubricant?
Why?
3. The melting points of elements from N to Neand P to
Arare relatively low
they exist as discrete moleculeswhich held by weak
van der Waals forces
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S has a highermelting point than P although the atomic
size of P is larger than S. (Why?)
S exists as S8moleculesin its molecular crystal whereasP exists as P4moleculesin its molecular crystal
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1. For Group I Metals, melting point decreases down the group.
Size increases
Shielding effect of inner shell e-increases
Metallic bond strength decreases
2. For halogens and noble gases, melting point increases down
the group.
Size increases
Van der Waals forces increases
Down a group (p.13)
Do Q. 2, Q.6 on p. 32
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a. Giant covalent structure
Large amount of energy used to
break the strong covalent bonds.
b. Metallic bond.
Valence electrons & Size Metallic bond
c. Metals: metallic bonds persist in
liquid.
Non-metals: weak van der Waals
forces.
d. Van der Waals forces determine
the m.p. in non-metals.
S8largest sizestrongest van
der Waals forces.
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F irst I onization Enthalpy (p.7, notes p.16)
X(g) X+(g) + e
The first ionization enthalpies of the first 20 elements
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Electronic configuration (s, p, half-filled?)
Nuclear charge
Screening/Shielding effect
Atomic radius
Fourfactors affecting the magnitude of ionization enthalpy:
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Variation in the first ionization enthalpy of thefirst 20 elements
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1. Noble Gases have the highest I.E. (p.17)
The electronic configuration of noble gases is very
stable(completely filled octet)
2. Alkali Metals have the lowest I.E. in a period (p.18)
It has the lowest effective nuclear charge in the period
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1. General increaseacross periods 2 and 3
increase in nuclear charge outweighs the increase in
shielding effect of additional electron of the same shell.
stronger attraction to outermost electrons
2. Irregularitieswith the general increase
Peaks in the general increase due to the extra stability
provided by full-filled ssub-shell(Be) and half-filled p
sub-shell (N)
3. Across a Period: (notes p.18)
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Explain why Group III elements have a lower
first I.E. than Group II elements? (p. 19)
i. Extra stability is gained for completely
filled s orbital in Group II elements.ii. For Group III elements, electron is
removed from p-orbital which is further
away (at higher energy level) from thenucleus and shielded by the s electrons.
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Explain why Group VI elements have a lower
first I.E. than Group V elements? (p.20)
i. Extra stability is gained for half filled p
orbital in Group V elements.ii. For Group VI elements, repulsion exists
between the first paired p-electrons
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Decreasedown a group
i. outermost electrons are further away from the
nucleus
ii. Shielding effect of inner shells electrons.
weaken attraction to outermost electrons
4. Down a group: (notes p.18)
Arrange the following in increasing first I.E.:
i. N, C, B
ii. B, Be, Li
iii. S+, S, S-
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Electronegativity (notes p.22)
Electronegativityis the measure of the relative tendency
of an atom to attract bond pair(s) electrons towards
itself in a covalent bond
Electronegativity values on an arbitrary scale from 0 to 4
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Electronegativity values of the first 20 elements
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Across the period, electronegativity increasesfromleft to right
Down the group, electronegativity decreases
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2. Decrease down groups
i. increase in size
ii. the increasing number of electron shells creates agreater shielding effect.
smaller attraction to bonding electrons
Explanations:
1. General increaseacross periods 2 and 3
increase in nuclear charge outweighs the increase in
shielding effect of additional electron of the same shell.
stronger attraction to outermost electrons
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Li and Mg; Be and Al; B and Si show similarproperties: like 1stI.E. and electronegativity
Why?
Shielding effect increases down a group andnuclear charge increases across a period.
Ionization enthalpy/ electronegativity of elementsdiagonally below one another are similar
They form bond with similar strength / show similarchemical properties (will be discussed later)
However, C and P, N and S showNOdiagonalrelationship. Why?
C and N has no low lying empty d-orbital
Diagonal Relationship (notes p. 23)
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The END
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