Periodic Variation in Physical Properties of the Elements H to Ar

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Chapter 39. Periodic Variation in Physical Properties of the Elements H to Ar. 39.1 The Periodic Table 39.2 Periodic Variation in Physical Properties of Elements. Modern Periodic Table. 39.1 The Periodic Table (SB p.2). The elements are arranged in the order of atomic number.

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Chapter 39

Periodic Variation in Physical Properties of the Elements H to Ar

39.1The Periodic Table

39.2Periodic Variation in Physical Properties of Elements

Modern Periodic Table

39.1 The Periodic Table (SB p.2)

The elements are arranged in the order of atomic number

Periodic Table (early forms)

(b)

(a)

(a) A portion of one of Dimitri Mendeleev’s handwritten drafts of the Periodic Table (b) Dimitri Mendeleev’s Periodic Table in 1872

39.1 The Periodic Table (SB p.3)

• Elements were first arranged in order of increasing atomic masses by Dimitri Mendeleev (1834 - 1907)
• The elements were observed to repeat their properties periodically
d-block

transition elements

f-block

inner transition elements

39.1 The Periodic Table (SB p.3)

• Modern periodic table:
• Rows  periods Columns  groups
• Classified into 4 areas:

p-block

s-block

Representative elements

39.1 The Periodic Table (SB p.4)

s-block elements:

Group IA: alkali metals

1 e– in outermost shell

(e.g. Li, Na, K)

Group IIA: alkaline earth metals

2 e– in the outermost shell

(e.g. Be, Mg, Ca)

p-block elements:

Groups IIIA, IVA, VA, VIA, VIIA, 0

Group VIIA : halogens

Group 0 : noble gases

39.1 The Periodic Table (SB p.4)

d-block elements:

Electronic configuration : [ ](n – 1)d1ns2 to [ ](n – 1)d10ns2

(Group IIIB) (Group IIB)

Transition elements

f-block elements:

Lanthanide series and actinide series :

4f and 5f orbitals are filled up with 1 to 14 e-

inner-transition elements

Rubidium: s-block

Gold: d-block

Astatine: p-block

Uranium: f-block

39.1 The Periodic Table (SB p.5)

Check Point 39-1

To which block (s-, p-, d- or f-) in the Periodic Table do rubidium, gold, astatine and uranium belong respectively?

The following periodic variations of the elements will be discussed:

First Ionization Enthalpy

Electronegativity

Structure and Bonding

Melting Point

How can scientists measure the sizes of atoms?

Covalent radius is defined as half the internuclear distance between two covalently bonded atoms in a molecule of the element.

Metallic radius is defined as half the internuclear distance between atoms in a metallic crystal.

The atomic radius is governed by two factors:

• Attraction of the nucleus for the electrons
• More proton  greater attraction to e– small atomic radius

(2) Screening of the outer electrons from the nucleus by the inner electron shells

Repulsion between e– outermost shell e– screen by inner e–  outermost shell e– less strongly held  larger atomic radius

The atomic radii of the first 20 elements

Variation in atomic radius of the first 20 elements

Across the period, the atomic radii decrease progressively

First Ionization Enthalpy

X(g)  X+(g) + e–

The first ionization enthalpies of the first 20 elements

Four factors affecting the magnitude of ionization enthalpy:

• Electronic configuration
• Nuclear charge
• Screening effect

Variation in the first ionization enthalpy of the first 20 elements

Observations:

1.General increase across periods 2 and 3

∵ increase in nuclear charge with increasing atomic numbers

2.Irregularities with the general increase

Peaks in the general increase due to the extra stability provided by full-filled s sub-shell and half-filled p sub-shell

3. Sharp drop from one period to the next

∵ The electronic configuration of the element at the end of period is very stable(completely filled octet)

4. Decreases on going down any group in the periodic table

Increase in the number of inner electron shells provides better shielding for the outermost shell electrons

Electronegativity

• Electronegativity is the measure of the relative tendency of an atom to attract bond pair(s) of electrons towards itself in a covalent bond
• Electronegativity values on an arbitrary scale from 0 to 4

Electronegativity values of the first 20 elements

Variation in electronegativity value of the first 20 elements

• Across the period, electronegativity increases from left to right
• Down the group, electronegativity decreases

Structure and Bonding

• The structure and bonding vary from left to right
• Structure
• Giant metallic structure  giant covalent structure  simple molecular structure  atomic structure
• Bonding
• Metallic bonding  covalent bonding

Melting point

• Melting point of a substance is the temperature at which a transition from its solid phase to liquid phase
• Temperature depends on the magnitude of bonding

The melting points of the first 20 elements

Variation in melting point of the first 20 elements

Four observations:

1. Steady increase in melting point from Li to B and Na to Al

∵ metallic bond strength increase as no. of outermost shell electrons increase. B has a giant covalent structure, thus the melting point is much higher

2. Carbon and silicon correspond to the maxima in Periods 2 and 3

∵ both have giant covalent structures. Atoms are held together by strong covalent bonds

3.The melting points of elements from N to Ne and P to Ar are relatively low

∵ they exist as discrete molecules which held by weak van der Waals’ forces

4. Sharp drop in melting point from C to N and Si to P

∵ dramatic change in molecular structure(giant covalent structure to discrete molecules held by weak van der Waals’ forces)

• S has a higher melting point than P
• ∵ S exists as S8 molecules in its molecular crystal whereas P exists as P4 molecules in its molecular crystal
• As S8 has a higher molecular mass and larger surface area for contact with other molecules, the melting point of S is higher than that of P

Example 39-1

By considering the trend of atomic radius in the Periodic Table. Arrange the elements Si, N and P in order of increasing atomic radii. Explain your answer briefly.

Solution:

In the Periodic Table, N is above P in Group VA. As the atomic radii increase down a group, the atomic radius of N is smaller than that of P.

Si and P belong to the same period. Since the atomic radii decrease across a period, the atomic radius of P is smaller than that of Si.

Hence, the atomic radii increase in the order: N < P < Si

(a) Going across a period, the atomic radii progressively decrease with increasing atomic numbers. This is because an increase in atomic number by one means one more electron and one more proton in the atom. Since the additional electron goes to the same shell and is at approximately the same distance from the nucleus, it is only weakly shielded by other electrons in that shell from the attraction of the nucleus. On the other hand, as there is an additional proton added to the nucleus, the electrons will experience a greater attractive force from the nucleus. Hence, going across a period, there is an increase in effective nuclear charge and a reduction in atomic radius is resulted.

39.1 The Periodic Table (SB p.14)

• Check Point 39-2
• Describe and explain the general periodic trend of atomic radius.

(b) The first ionization enthalpy increases in the order:

Se < S < Ar

39.1 The Periodic Table (SB p.14)

Check Point 39-2

(b) With the help of the Periodic Table only, arrange the elements Se, S and Ar in an order of increasing first ionization enthalpies.