PHYSICAL AND CHEMICAL PROPERTIES

Physical and Chemical Properties of Group 2 Elements & Their Compounds

Physical and Chemical Properties of Group 2 Elements & Their Compounds

Physical and Chemical Properties

The elements of Group 2, the Alkaline Earth Metals, are:

symbol

electron configuration

beryllium

Be

[He]2s2

magnesium

Mg

[Ne]3s2

calcium

Ca

[Ar]4s2

strontium

Sr

[Kr]5s2

barium

Ba

[Xe]6s2

radium

Ra

[Rn]7s2

Physical Properties

The metals of Group 2 are harder and denser than sodium and potassium, and have higher melting points. These properties are due largely to the presence of two valence electrons on each atom, which leads to stronger metallic bonding than occurs in Group 1. Three of these elements give characteristic colours when heated in a flame:

Mg

brilliant white

Ca

brick-red

Sr

crimson

Ba

apple green

Atomic and ionic radii increase smoothly down the Group. The ionic radii are all much smaller than the corresponding atomic radii. This is because the atom contains two electrons in an s level relatively far from the nucleus, and it is these electrons which are removed to form the ion. Remaining electrons are thus in levels closer to the nucleus, and in addition the increased effective nuclear charge attracts the electrons towards the nucleus and decreases the size of the ion.

Chemical Properties

The chemical properties of Group 2 elements are dominated by the strong reducing power of the metals. The elements become increasingly electropositive on descending the Group. Once started, the reactions with oxygen and chlorine are vigorous:

2Mg(s) + O2(g) ® 2MgO(s)

Ca(s) + Cl2(g) ® CaCl2(s)

All the metals except beryllium form oxides in air at room temperature which dulls the surface of the metal. Barium is so reactive it is stored under oil. All the metals except beryllium reduce water and dilute acids to hydrogen:

Mg(s) + 2H+(aq) ® Mg(aq) + H2(g)

Magnesium reacts only slowly with water unless the water is boiling, but calcium reacts rapidly even at room temperature, and forms a cloudy white suspension of sparingly soluble calcium hydroxide. (Silberberg 2010 99)

Calcium, strontium and barium can reduce hydrogen gas when heated, forming the hydride:

Ca(s) + H2(g) ® CaH2(s)

The hot metals are also sufficiently strong reducing agents to reduce nitrogen gas and form nitrides:

3Mg(s) + N2(g) ® Mg3N2(s)

Magnesium can reduce, and burn in, carbon dioxide:

2Mg(s) + CO2(g) ® 2MgO(s) + C(s)

This means that magnesium fires cannot be extinguished using carbon dioxide fire extinguishers.

Uses of Calcium & Barium Compounds

Barium

Perhaps the best known use of barium compounds is in medicine. Doctors often want to know what is happening inside a patient's body. One way to find out, of course, is through surgery. But surgery is a drastic procedure. It can cause new problems for the patient. As a result, medical researchers have developed procedures that are less extreme. One such method is called radiography.

Radiography is a technique in which X rays are passed through the body. X rays are high energy light waves. They can pass through skin and tissue, but are absorbed by ...