Physical and Chemical characteristics of Alkali metals, Chemistry tutorial

INTRODUCTION

The gas properties of hydrogen resemble the group 1 elements in some respects and the group 17 elements in others.

The elements of group 1 and 2 are termed the S-block elements since the outermost electron(s) in such elements occupy the S-orbital. Group 1 elements consists of Li, Na, k, Rb, C and Fr. They are termed alkali metals since they foam hydroxides which are strong alkalis

Alkali metals:

Alkali metals are useful as metals also as in the form of their compounds.

Occurrence:

The alkali metals are highly reactive thus they do not take place in the free state in nature. They occur in the combined form in the earth's crust in the given relative abundance:

Sodium 2.27%, potassium 1.84% lithium, rubidium and calcium in trace amounts 1.8 x 10-3 % , 7.8 x 10 3 %  and 0.26 x 10-3 % respectively.

Sodium as sodium chloride is the most abundant metal in sea water (M1.08%). Lithium take places in alumino silicate rocks, for example spodumene, LiAI (SiO3)2 and Lepidolite (Li, Na, K)2 (F,OH)2 Sodium in rock salt, NaCl, in Chile Saltpetre NaNO3 , or in Cryolite, Na3AlF6 , Potassium in carnallite, KCI, in Mg Cl 2.6H2 0, in Saltpetre KNO3   and in Kainite, kCl, MgSO4 . 3H20.

Rubidium and calcium are unusual elements and usually take place in small quantities along with other alkali metals. For instance, carmallite have up to 0.94% rubidium chloride, Lepidolite, and containing about 0.2 to 0.7 percent of calcium expressed as calcium oxide. Francium being a radio active element with a very short ½ life period (21.8 minutes) occurs in very minute's traces in nature. 

Extraction of Alkali Metals:

Lithium and sodium are extracted via electrolysis of their fused (molten) chlorides. Potassium is attained via the reduction of its chloride with sodium vapor. This reduction through Na appears to be contrary to the normal order of reactivity, K>Na. Though, at about 1150k the given equilibrium is set up:-

Na (g) + K+ (L)  ↔ Na+ (l) + K (g)

Because potassium is more volatile, it distils off more readily displacing the equilibrium to the right and allowing the forward reaction to proceed. Rubidium and Caesium, can be prepared throughout the reduction of their chlorides with calcium metal at 1000K under calcium pressure. Rubidium or caesium salts are obtained during the recrystallisation of other naturally occurring alkali metal salts. Francium is produced consequently of α- emission (1%) during the radioactive decay of actinium.

495_Production of francium.jpg

Uses of Alkali Metals:

The alkali metals are very good conductors of warmth or electricity. Although due to their highly reactive nature they cannot be utilized for this purpose. Sodium in polyethylene enclosed cables is utilized in several underground high voltage transmission applications. Since of the high specific warmth and thermal conductivity, liquid sodium is employed as a coolant in nuclear reactors. We must have seen bright yellow lights on the streets and particularly on the highways. Such are sodium vapor lamps and the light from them can penetrate fog well.

Caesium has the distinction of being the metal from which electrons are ejected most easily on exposure to light. This phenomenon is termed as photoelectric effect. Photocells, which are a mechanism for converting light into electricity, are based on this phenomenon.

Other every day uses of alkali metal compounds enclose the following:

Lithium in the form of lithium stearate is used for the production of lubricating greases.

The hydrides of lithium and sodium viz LiA1H4 and NaBH4 are used as reducing agents in synthetic organic chemistry. Lithium or potassium compounds are utilized in picture tubes of colour televisions.

Can you visualize food without common salt? Apart from being a necessary constituent of food, sodium chloride has many other significant industrial uses like in the manufacture of NaOH, Na2CO3 Cl2, and H2 gases. Apart from sodium chloride, other compounds of sodium as well have many uses. Caustic soda and sodium hydroxide is utilized in making soaps, sodium carbonate as well recognized as washing soda, in laundering and the assemble of glass, sodium bicarbonate as baking soda in baking powder in medicine and in fire extinguishers.

Potassium compounds as well contain many uses. Potassium hydroxide is used in liquid detergents. Potassium superoxide in breathing apparatus, potassium chlorate in matches and explosives and potassium bromide (KBr) in photography. Potassium nitrate is utilized along with charcoal and sulphur in gun powder. Potassium is a major component of plant fertilizers, where it is employed in form of chloride or nitrate salts.

Physical properties:

Most of the physical properties of the alkali metals are directly relocated to atomic properties of elements. Deviation of physical properties from 1 element to the other in a group is governed through the trends of the various atomic properties. We shall now apply them to understand the group trends in the various physical properties of the group 1 elements given in table.

1632_Properties of the Group 1 metals.jpg

Table: Properties of the Group 1 metals

Atomic size:

From your studies, we will recall that the alkali metals are the largest in their analogous periods in the periodic table. The size of the atom or its ion enhances on descending the group. This is due to the addition of an extra shell of electrons as we move down the group from 1 element to the next. The addition of the extra shell of electrons out-weighs the consequences of increased nuclear charge and therefore there is a raise in size from Li - Cs. This trend is shown in fig.

112_Trend in ionic radii of Group 1 elements.jpg

Fig: Trend in ionic radii of Group 1 elements

Density:

Related to atomic size is the density of the elements. Density can be named as mass per unit volume. For solids the density is a function of atomic weight, size of the atom or the structure of the solid (for example the closeness of the packing of the atoms). There are 2 general trends examined in the densities of the elements in the periodic table. Along a period there is a general enhance in density since of the increase in the size of the atom. Therefore in a particular period the alkali metals have the lowest density, considering the solid elements only. In a group also density increases on going down the group. Since the elemental structuring is often similar within any group, the factors that determine the density are atomic mass and volume. As we can see from table density increases as we move from Li - Cs. This signifies that the raise in atomic weight from 1 element to the next in passing down the group overweighs the consequence of raise in the size of the atom. There are though several exceptions to this general trend and in this particular group of alkali metals we can see from table that the density of potassium is less than that of sodium. Therefore potassium is an exception in this trend. 

Melting Points and Boiling Points:

Such metals are soft and can be cut with a knife. It's a effect of enhance in size and repulsion of the non-bonded electrons, their cohesive energy and tendency for metallic bonding decreases down the group and thus softness increases as we go down from Li to Cs. These metals contain low melting or boiling points that as well reflect the low values of cohesive energy between the atoms. Their melting and boiling points reduces as we go down the group.

1303_Trend in the melting point of Group 1 elements.jpg

Fig: Trend in the melting point of Group 1 elements

Thermal and Electrical Conductivity:

In alkali metals, electrons of the noble gas core efficiently shield the lone valence shell electron from the nuclear charge. Thus the effective nuclear charge felt via the electron in the valence shell of an atom of an alkali metal is the least or their atoms are the largest in relevant periods. As a result, the sole valence electron is very loosely held through the nucleus. It can shift freely from one metal ion to another in the lattice. This builds the alkali metals good conductors of warm and electricity. This loosely bond electron is as well responsive for the silvery luster of the alkali metals whenever freshly cut.

Ionisation energy:

By loosing the loosely bond solitary outermost electron, such elements can obtain the electronic configuration of the preceding noble gas elements. They contain, thus, a high tendency of giving up this electron to form univalent cations. Their 1st inonization energies are the lowest in the relevant periods and thus they are the most reactive of all metals. As we go down the group, their atomic size enhances, their inonization energies reduce, resulting in a raise in their reactivity. The effective nuclear charge felt through the electrons increases after the removal of one electron from the atom of any element and therefore, their 2nd ionization energies are always higher than the first. It is even more so in the case of the alkali metals, because their charged ions (Li, +Na+   K+ etc) contain the stable noble gas configuration of the stable noble gas configuration of the proceeding group. Removal of an electron from a stable noble gas configuration is tremendously difficult. Such metals, consequently, form univalent cations only. Fig. shows the trend in the first conisation energies of the alkali metals down the group.

74_first ionization energies.jpg

Fig: Trend in first ionization energies of alkali metals

On account of their low ionization energies such elements contain a high tendency to form cations. In other words, they enclose a high electropositive or metallic character that increases as we move down in the group from Li - Cs. In fact Rb or Cs is so extremely electropositive that they release electrons even whenever exposed to light. That is they exhibit photo electric effect.

Electronegativity:

Since alkali metals have a tendency to loose electron easily rather than to gain, values of electronegativity of these elements are very small. In fact, alkali metals are the least electronegative elements in the periodic table. As expected the electronegativity decreases on moving down the group.

Ionic character of compounds:

Compounds formed through alkali metals with extremely electronegative elements similar to halogens and oxygen, are largely ionic in nature since of the large electronegativity difference.
We can see that the trends in ionic character (in given fig.) show that the ionic character increases with increase in cation size and decreases with increase in anion size. Since of the small size of Li+, it has more polarizing power and thus favours covalent bonding.

Solubility, Lattice Energy and Hydration Energy:

Alkali metal salts like halides, oxides, hydroxides, carbonates, sulphates etc exhibit several interesting trends in their solubility in water. First let us remind ourselves that lattice energy is the driving force for the formation of an ionic compound or its stability. Lattice energy is directly proportional to the charge on the ions and inversely proportional to the distance between the cation. This distance is taken as the total of radii of cation and anion (rc + ra).

Let us again remind ourselves that lattice energy is the enthalpy transform whenever one mole of crystal lattice is formed from the isolated gaseous ions and hydration energy is the enthalpy transform when one mole of solute is dissolved in water. In a group, the charge on cations remains constant. Therefore, lattice energy depends mainly on the size of the cation. Likewise hydration energy varies with the charge and size of the cation. The higher the charge and the smaller the size of the ion, the more is the hydration energy. In a group, lattice energy and hydration energy reduce as we move down. Whereas the decrease in lattice energy favous the solubility, the reduce in hydration energy makes the compound insoluble.

788_ionic character of alkali halides.jpg

Figure: Trend in ionic character of alkali halides metals 

For the salts of small anions (like F-, O2- OH- etc) the lattice energy that is inversely proportional to rc+ ra , is extremely sensitive to the change in the size of the cation, anion being very small in size has tiny contribution in the total (rc,+ ra ) and decreases sharply as we move down the group. Therefore, in these salts, reduce in lattice energy is greater than reduce in hydration energy and thus, the solubility of these salts increases as we go down the group. For instance, in the case of alkali metal fluorides, the solubility increases in the order LiF< Na F < KF <RbF < CsF.

For the salts having large anions (SO42- , I- , NO3- HCO3- , CO32-, etc) as ra>>rc, the radii of the cation has little contribution in the total (rc + ra) and decreases sharply as we move down the group. Therefore, in these salts, the decrease in lattice energy is greater than the decrease in hydration energy and thus, the solubility of these salts increases as we go down the group. For instance, in the case of alkali metal carbonates, lithium carbonate is highly soluble whereas the solubility of calcium carbonate is very little. Another significant factor contributing to the solubility of the compound is the match in the size of the cation and anion. When there is a mismatch, as example cation is small, anion is large or vice-versa, this will consequence in the enhanced solubility of the compound. Let us keep the cation constant, say calcium. If we, then transform the anion from fluoride to iodide, then the solubility of the compound will vary as CsF > CsCI > CsBr > CsI. Therefore, calcium fluoride will be most soluble or calcium iodide will be the least. In the same way, from lithium fluoride to lithium iodide: the solubility will increase in the order: LiF < LiCl <LiBr < LiI.

1783_properties of alkali metalskk.jpg

Table: The main treads in the properties of alkali metals

  • The density of K is less than that of Na:

There can be many more instances that can be described on the basis of above reasoning. Table shows above is a summary of the trends in the properties of alkali metals.

Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)

Expand your confidence, grow study skills and improve your grades.

Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.

Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with an expert at http://www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology.  Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.

Free to know our price and packages for online chemistry tutoring. Chat with us or submit request at info@tutorsglobe.com