Comparison between carbon and silicon

Carbon	Silicon
(1) elements of sub-group IVA and have s2p2 electron in the outermost level,                              C (6) – 1s22s22p2	(1) elements of sub-group IVA and have s2p2 electron in the outermost level,                        Si (14) – 1s22s22p63s23p2
(2) found in nature -in animal and plant kingdoms, as minerals	(2) abundant in silicate minerals and as silica and rocks
(3) common oxide is CO2,	(3) common oxide is SiO2
(4) form hydrides, halides and other compounds-                                              CCl4, CH4, C2H6	(4) form hydrides, halides and other compounds-                            SiCl4, SiH4, Si2H6
(5) exhibit isomerism,	(5) not so pronounced
(6) maximum covalency is 4	(6) it can expand to 6
(7) compounds of C are quite stable	(7) most silica compounds are unstable except silicates and polymers.

Comparison between the alkali metals and the coinage metals

Alkali metals	Coinage metals
(1) Electronic structures are different although they have s1 electron in the outermost level. Li (3) 2s1, Na (11) 3s1, K (19) 4s1, Pb (37) 5s1, Cs (55) 6s1, Fr (87) 7s1	(1) Electronic structures are different although they have s1 electron in the outermost level. Cu (29) 3d104s1, Ag (47) 4d105s1, Au (79) 5d106s1
(2) most reactive elements,	(2) unreactive and difficult to oxidize
(3) never occur in nature in free state,	(3) found abundantly in free state
(4) have only one oxidation state, +1	(4) have +1, +2 and +3 oxidation state
(5) melting points range 1800C(Li) → 2840C(Cs),	(5) near about 10000C
(6) form strong bases because of small charge (+) and large sizes,	(6) do not give strongly basic oxides or hydroxides
(7) can not be handled easily and are kept in kerosene.	(7) can be worked and used as metals because these are heavy, ductile and malleable
(8) most of them are soluble in water,	(8) are not soluble in water
(9) colourless,	(9) coloured
(10) are recent discovery and their uses are only scientific and technical,	(10) used by primitive men for ornaments and for coinage purposes.

Usefulness of the periodic table

1. Classification of the elements,
2. Prediction of undiscovered elements,
3. Correction of atomic weight,
4. Periodic table in industrial research,

Limitations of the periodic table:

1. Position of hydrogen,
2. Anomalies in Mendeleev’s table,
3. The position of rare earths,
4. The position of Actinides,
5. Oxidation states and the periodic table,
6. Properties which are not periodic functions,
7. Diagonal relationship

Summary of Periodic Trends

Serial	Properties within Periods and Groups	Variation
		Moving left → right	Moving top → bottom
1	Metallic character	decreases	increases
2	Atomic size or atomic radius	decreases	increases
3	Ionic radii	decreases	increases
4	Ionization energy	increases	decreases
5	Electron affinities	higher	lower
6	Electro negativities	increases	decreases
7	Oxidizing power	higher	lower
8	Reducing power	lower	higher

VARIATION OF PROPERTIES WITHIN PERIODS AND GROUPS

The following variations of properties within periods and groups are observed:

(1)   Variation of metallic character,

(2)   Variation in atomic size,

(3)   Variation in ionic radii,

(4)   Variation in ionization potentials,

(5)   Variation in electron affinities,

(6)   Variation in electro negativities,

(7)   Variation in stability of oxidation states,

(8)   Variation in oxidizing and reducing powers,

(9)   Variation in basic properties,

(10) Variation in physical properties.

Some outlines involving variations of properties

q       Metallic character is a rough and qualitative combination of (i) electrical and thermal conductivity, (ii) metallic luster (brightness) and (iii) reducing properties,

q       (a) As the size increases, valence electron are readily lost, elements become more metallic, because valence electrons are held less strongly due to the increasing size of the atom; (b) As the size decreases, valence electron are less readily lost, elements become less metallic, because valence electrons are held more strongly due to the decreasing size of the atom.

q       The greater the nuclear charge, the smaller is the ionic radius in a series (Na⁺, mg⁺², Al⁺³, Si⁺⁴).

q       Metals generally have small ionization energy; nonmetals have large values.

q       Metals have small electron affinity, nonmetals have large values [good oxidizing agents].

q       Higher oxidation states become more stable near the bottom of each transition metal.

Reasons of Variations

(b)   the nuclear charge and the number of electrons surrounding the nucleus,

(ii) the total number of electrons, particularly the valence electrons,

(iii) the size of the atoms, i.e. the volume occupied by the electron in various

      energy levels.

CLASSIFICATION OF ELEMENTS DEPENDING ON ELECTRONIC CONFIGURATION

The physical and chemical properties of the elements are largely determined by their electronic structures:

1. The Inert Gases (elements of ‘0’ group)
2. The Representative Elements (‘s’ and ‘p’ block elements)
3. The Transition Elements (‘d’ block elements)
4. The Inner Transition Elements (‘f’ block elements)

1. The Inert Gases

(a)    s²p⁶ arrangements in the outermost level,

(b)   are very stable,

(c)    He and Ne have complete energy level,

(d)   very low reactivity,

(e)    colourless gases.

**Why noble gases are chemically inactive?

Ans: Inactivity of the npble gases is due to- a) complete pairing of all electrons present, b) absence of any molecular orbital, c) stable energy state, d) very high ionization potential and e) negligible electron affinities.

2. The Representative Elements

(a)    outermost energy level incomplete, just after s² and s²p⁶ arrangements;

(b)   chemical behaviour depends on valence electron,

(c)    alkali metals, alkaline earth metals have valence electron in the ‘s’ orbital.

(d)   group IIIA (under boron) has single electron in ‘p’ orbital,

(e)    group IIIA-VIIA vertically occupy ‘p’ orbital,

(f)     generally form colourless compounds.

3. The Transition Elements

(a)    heavy metals of sub-group B, high melting and electropositive,

(b)   contain two incomplete energy levels (‘s’ & ‘d’ orbitals),

(c)    chemical properties depends upon the electron of the two outermost levels (‘s’ & ‘d’ orbitals),

(d)   have almost the same atomic and ionic sizes,

(e)    are effective catalytic agents,

(f)     form quite a large number of complex compounds,

(g)    show positive oxidation states of +2 and +3 generally and form mostly ionic compounds,

(h)    generally form coloured compounds.

4. The Inner Transition Elements

(a)    these elements have three incomplete outer levels (‘s’, ‘d’ & ‘f’ orbitals)

(b)   ‘f’ orbitals are being completed,

(c)    lanthanides (rare earths) and actinides (trans-uranium)

(d)   the properties are similar to as in the case of transition elements.

CHARACTERISTICS OF SOME CLASSIFIED ELEMENTS

THE ALKALI METALS (Li, Na, K, Rb, Cs, Fr ) IA

1. Shiny, soft, low melting metals.
2. They react rapidly and often violently with water to form products that are highly alkaline or basic.
3. They are never found in nature in the pure state, but only in combination with other elements.

THE ALKALINE EARTH METALS (Be, Mg, Ca, Sr, Ba, Ra) IIA

1. Lustrous, silvery metals.
2. Less reactive than alkali metals.
3. They are never found in nature in the pure state, but only in combination with other elements.

THE HALOGENS (F, Cl, Br, I ) VIIA

1. Corrosive and Non-Metallic.
2. All are found in nature, but only in combination with other elements.
3. Halogen comes from the Greek word halos meaning salt. Halogens commonly form salts.

THE NOBLE GASES (He, Ne, Ar, Kr, Xe, Rn) VIIIA

1. Very low reactivity.
2. He, Ne, Ar don't combine with any other elements, Kr and Xe combine with very few.

TRANSITION METALS / METALS

1. All except Hg are solid at room temperature.
2. Most have a silvery shine.
3. Can be easily shaped (malleable) and drawn (ductile).
4. Are good conductors of heat and electricity.

NON-METALS (C, N, O, P, S, Se; F, Cl, Br, I; He, Ne, Ar, Kr, Xe, Rn)

1. 17 are gases, 1 is liquid, 5 are solid at room temperature.
2. The solid non-metals are brittle.
3. They are generally poor conductors of heat and electricity.

SEMI-METALS (B, Si, Ge, As, Sb, Te, At)

1. Properties between those of metals and non-metals.
2. Most are silvery in appearance.

They are brittle and tend to be poor conductors of heat and electricity

MODERN PERIODIC TABLE

Short History

•       In 1863, a 44 year old French geologist, A. E. Béguyer de Chancourtois created a list of the elements arranged by increasing atomic weight. The list was wrapped around a cylinder so that several sets of similar elements lined up, creating the first geometric representation of the periodic law.
•       In England, 32 year old analytical chemist John A. R. Newlands was also wrapping the elements, noting that chemical groups repeated every eight elements. He named this the octave rule, and compared it to a musical scale. Some less observant members of the English Chemical Society considered this absurd, so his work was ignored for years.
•        Chemists Dmitrii I. Mendeleev, a Russian, and German Lothar Meyer were working independently in 1868 and 1869 on the arrangement of elements into seven columns, corresponding to various chemical and physical properties. Their tables were similar - they acknowledged each other's work - the differences are subtle but important: Meyer's table was an accurate (for the time) accounting of the known facts about each element, such as melting point and atomic volume. The table clearly showed the existence of periodic chemical families. In 1870 Meyer's table and description of the periodic law was published in Liebig's Annalen.
•       A year earlier however, Mendeleev presented a much bolder and scientifically useful table.

Mendeleev's Statement of the Law of Periodicity

"The properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence on, or (expressing ourselves algebraically) form a periodic function of, the atomic weights of the elements."

Modern Statement of the Law of Periodicity (Moseley)

"The properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence on, or (expressing ourselves algebraically) form a periodic function of, the atomic number of the elements."

 Patterns in the Periodic Table: Main Features

•        There are 8 groups and 7 periods.
•        The columns are called groups.
•        The rows are called periods (hence periodic table).
•        Group 1 is called The Alkali Metals.
•       Group 2 is called The Alkaline Earth Metals.
•        The transition metals are in the middle. They have no group number.
•        The inner transition metals are at the bottom- lanthanides and actinides.
•        Group 7 is called The Halogens.
•        Group 8 is called The Noble Gases/ Inert Gases.

Hydrogen occupies a unique position at the top of the periodic table. It does not fit naturally into any Group.
All the members of a Group have the same valence configuration but different principal quantum numbers.
The number of valence electrons equals the Group number.
The period number equals the principal quantum number of the valence shell.

The periodic table is divided into BLOCKS.

1.      The s-block elements have valence configuration s¹ or s².

2.      The p-block elements have valence configuration s²p¹ to s²p⁶.

3.      The d-block elements have valence configurations in which d-subshells are being filled.

4.      The f-block elements have valence configurations in which f-subshells are being filled.