PERIODIC TABLE

SS 2 - PERIODIC TABLE


The periodic table of elements, usually shortened to just the periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties, whose structure shows periodic trends.

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Classification of elements into groups

The six noble gases—helium, neon, argon, krypton, xenon, and radon—occur at the ends of the six completed periods and constitute the Group 18 (0) group of the periodic system. It is customary to refer to horizontal series of elements in the table as periods and vertical series as groups. 

The seven elements lithium to fluorine and the seven corresponding elements sodium to chlorine are placed,  in the seven groups, 1 (Ia), 2 (IIa), 13 (IIIa), 14 (IVa), 15 (Va), 16 (VIa), and 17 (VIIa), respectively. The 17 elements of the fourth period, from potassium, 19, to bromine, 35, are distinct in their properties and are considered to constitute Groups 1–17 (Ia–VIIa) of the periodic system.

GROUP 1 

The alkali metals are so reactive that they are never found in nature in elemental form. Although some of their ores are abundant, isolating them from their ores is somewhat difficult. For these reasons, the group 1 elements were unknown until the early 19th century, when Sir Humphry Davy first prepared sodium (Na) and potassium (K) by passing an electric current through molten alkalis. 

Preparation of the Alkali Metals

1.   Electrolytic Process

   *  Pure lithium and sodium for example, are typically prepared by the electrolytic reduction        of molten chlorides:

                            2LiCl ----------> 2Li +Cl2

      In practice, CaCl2 is mixed with LiCl to lower the melting point of the lithium salt

   * Potassium is produced commercially from the reduction of KCl by Na, followed by the            fractional distillation of K(g).

   * Although rubidium and cesium can also be produced by electrolysis, they are usually           obtained by        reacting their hydroxide salts with a reductant such as Mg:

                    RbOH  + Mg  ----------->  Rb    + Mg(OH)2

General Properties of the Alkali Metals

1.   The atomic and ionic radii increase smoothly from Li to Cs

2.   The first ionization energies decrease as the atoms become larger. 

3.   As a result of their low first ionization energies, the alkali metals 
      have an overwhelming tendency to form ionic compounds where 
      they have a +1 charge. 

4.   All the alkali metals have relatively high electron affinities because the 
      addition of an electron    produces an anion (M−) with an ns2 electron 
      configuration. 

5.   The densities of the elements generally increase from Li to Cs
                     
 Group list >>> Lithium     Sodium     Potassium Rubidium  Cesium   Fransium 

*The values cited are for four-coordinate ions except for Rb+ and Cs+, whose values are given for the six-coordinate ion.
atomic symbol	Li	Na	K	Rb	Cs	Fr
atomic number	3	11	19	37	55	87
atomic mass	6.94	22.99	39.10	85.47	132.91	223
valence electron configuration	2s1	3s1	4s1	5s1	6s1	7s1
melting point/boiling point (°C)	180.5/1342	97.8/883	63.5/759	39.3/688	28.5/671	27/—
density (g/cm3) at 25°C	0.534	0.97	0.89	1.53	1.93	—
atomic radius (pm)	167	190	243	265	298	—
first ionization energy (kJ/mol)	520	496	419	403	376	393
most common oxidation state	+1	+1	+1	+1	+1	+1
ionic radius (pm)*	76	102	138	152	167	—
electron affinity (kJ/mol)	−60	−53	−48	−47	−46	—
electronegativity	1.0	0.9	0.8	0.8	0.8	0.7
standard electrode potential (E°, V)	−3.04	−2.71	−2.93	−2.98	−3.03	—
product of reaction with O2	Li2O	Na2O2	KO2	RbO2	CsO2	—
type of oxide	basic	basic	basic	basic	basic	—
product of reaction with N2	Li3N	none	none	none	none	—
product of reaction with X2	LiX	NaX	KX	RbX	CsX	—
product of reaction with H2	LiH	NaH	KH	RbH	CsH	—

Reactions and Compounds of the Alkali Metals

All alkali metals are electropositive elements with an ns1 valence electron configuration, forming the monocation (M+) by losing the single valence electron. Because removing a second electron would require breaking into the (n − 1) closed shell, which is energetically prohibitive, the chemistry of the alkali metals is largely that of ionic compounds that contain M+ ions. However, as we discuss later, the lighter group 1 elements also form a series of organometallic compounds that contain polar covalent M–C bonds.

1.    All the alkali metals react vigorously with the halogens (group 17) to form the                         corresponding ionic        halides, where \(X\) is a halogen:

                         2Na+ Cl2 ---------> 2NaCl

       Similarly, the alkali metals react with the heavier chalcogens (sulfur, selenium, and                 tellurium in group 16) to produce metal chalcogenides, where Y is S, Se, or Te:

Assignment

write a balanced  equation on this point , alkali metals react with chacogen. 

To be submitted latest tomorrow

chacogen compound

2.   The alkali metals react with all group 14 elements, but the compositions and properties          of the    products vary significantly. For example, reaction with the heavier group 14              elements gives materials that contain polyatomic anions and three-dimensional cage            structures, such as K4Si4 whose structure is shown here.

3.   All the alkali metals react directly with gaseous hydrogen at elevated temperatures to            produce ionic hydrides (M+H−):

                               2Na + H2 -------> 2NaH

All are also capable of reducing water to produce hydrogen gas:

Na  +  H2O   ---------> NaOH   +   H2

Uses of the Alkali Metals

Because sodium remains liquid over a wide temperature range (97.8–883°C), it is used as a coolant in specialized high-temperature applications, such as nuclear reactors and the exhaust valves in high-performance sports car engines. Cesium, because of its low ionization energy, is used in photosensors in automatic doors, toilets, burglar alarms, and other electronic devices. In these devices, cesium is ionized by a beam of visible light, thereby producing a small electric current; blocking the light interrupts the electric current and triggers a response.

Compounds of sodium and potassium are produced on a huge scale in industry. Each year, the top 50 industrial compounds include NaOH, used in a wide variety of industrial processes; Na2CO3, used in the manufacture of glass; K2O, used in porcelain glazes; and Na4SiO4, used in detergents.

Several other alkali metal compounds are also important. For example, Li2CO3 is one of the most effective treatments available for manic depression or bipolar disorder. It appears to modulate or dampen the effect on the brain of changes in the level of neurotransmitters, which are biochemical substances responsible for transmitting nerve impulses between neurons. Consequently, patients who take “lithium” do not exhibit the extreme mood swings that characterize this disorder.

UPDATE 

The following names for specific groups in the periodic table are in common use:

• Group 1: alkali metals.
• Group 2: alkaline earth metals.
• Group 11: coinage metals (not an IUPAC approved name)
• Group 15: pnictogens (not an IUPAC approved name)
• Group 16: chalcogens.
• Group 17: halogens.
• Group 18: noble gases. 

The group number is an identifier used to describe the column of the standard periodic table in which the element appears.

Groups 1-2 termed s-block elements.

Groups 1-2 (except hydrogen) and 13-18 are termed main group elements.

Groups 3-12 are termed d-block elements.

Groups 3-11 are termed transition elements. Transition elements are those whose atoms have an incomplete d-subshell or whose cations have an incomplete d-subshell.

Groups 13-18 termed p-block elements.

Main group elements in the first two rows of the table are called typical elements.

The first row of the f-block elements are called lanthanoids (or, less desirably, lanthanides. The second row of the f -block elements are called actanoids (or, less desirably, actanides.

ASSIGNMENT  

1. distinguished between group vii and other non-metals

2. state why group 0 is non reactive

make sure you write your note

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