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Chemistry Group Essay

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INTRODUCTION

GROUP 2 (THE ALKALINE EARTH METALS)
These elements are categorized as group 2 because they have two electrons in their outer shell, therefore each element in this group will lose two electrons in order to form a 2+. According to RSC (n.d) group 2 elements are all metals with silver-white color. ELEMENT | SYMBOL | No. OF ELECTRONS | Beryllium | Be | 2,2 | Magnesium | Mg | 2,8,2 | Calcium | Ca | 2,8,8,2 | Strontium | Sr | 2,8,18,8,2 | Barium | Ba | 2,8,18,18,2 |

TRENDS IN ATOMIC RADIUS The graph below shows that the atomic radius of group 2 elements increases as it descends down the group.

(Jim Clark, 2002)
“The radius of an atom is governed by the number of layers of electrons around the nucleus due to the pulling power of the nucleus to the outer electrons”(jim clark,2002). For example: Be 1s2 2s2 Mg 1s22s22p63s2
In both atoms the two outer electrons feel a net pull of 2+ from the nucleus. (Jim Clark, 2002) This is equally true for all the other atoms in group 2.
“The only factor which is going to affect the size of the atoms is therefore the number layers of electrons you have the more space they will take up-electrons repel each other” (Jim Clark, 2009). Electronic arrangement of electrons explains the reason for increase in atomic radius. Due to this the number of electron shells increases down the group. Element | Atomic radius | Be | 0.113 | Mg | 0.160 | Ca | 0.197 | Sr | 0.215 | Ba | 0.217 |
The compounds beryllium (Be) and calcium (Ca) will be discussed in this essay. Also the uses of calcium and barium compounds will be discussed.

Properties and Reactions of Elements

The metals of Group 2 are hard and dense and have high melting points. These properties are due to the presence of two valence electrons on each atom, which leads to stronger metallic bonding which occurs between the molecules.

(Jim Clark, 2002) Group 2 elements are all metals which have metallic bonding so it is expected that they will also have high melting points. “In metallic bonding, metal cations in a metal lattice are attracted to delocalized electrons” (Nigel Saunders, 2003). Going down Group 2 the number of delocalized electrons are the same with 2+ charges on each atom.
There is also an increase in ionic radius as you go down the group so the force of attraction between the delocalized electrons and the metal cations is high (Nigel Saunders).

Metallic bond is the bond that exists between metals. Metallic bonding is a 3D array of closely packed cations surrounded by a mobile sea of delocalized electrons. Moving electrons can pass freely through the lattice, carrying electric currents.

The reactivity of group 2 elements increases down the group this is due the 2+ electrons which are easily removed as the distance between the electrons and the nucleus increases. Group 2 elements react with both water and non-metals. “Beryllium does not react with water or steam even at red heat” (Jim Clark, 2002), While magnesium only reacts with steam to form magnesium oxide and hydrogen. Mg(s)+H2O--->MgO(s)+H2(g)

An ionic bond is formed between the Mg and O in the product. “Calcium, strontium and barium this all react with cold water with increasing vigor to give a metal hydroxide and hydrogen” (Jim Clark, 2002). Calcium is more reactive. “It reacts vigorously with cold water to produce an alkaline solution of calcium hydroxide and hydrogen gas is released” (S-cool, 2011). Ca(s)+H2O(i)--->Ca(OH)2(aq)+H2(g) The type of bond formed in in the product is also an ionic bond.
Also Group 2 elements also react with a non-metal such as air to form an oxide. For example both Ca and Mg react with O2 to form MgO and CaO. 2Mg(S)+O2(g)-->2MgO(s) 2Ca(s)+O2(g)--->2CaO(s)
In both reactions with water and air the bond that’s exists in both products is an ionic bond.
Also all group 2 elements have an effective nuclear charge of +2.

Properties and Reactions of Compounds.

Solubility of sulphates Solubility of group 2 elements decreases down the group. “The trend is true provided that hydrated beryllium sulphate is included in it, but not if beryllium sulphate is anhydrous” (Jim Clark, 2002). “The two factors that determine solubility of sulphates are the lattice energy and enthalpy of hydration” (S-cool, 2011).

Solubility of hydroxides Group 2 hydroxides become more soluble as you go down the group.
Magnesium hydroxide appears to be insoluble in water, however if you shake it with water, filter it and test the PH of the solution, you will find that it is slightly alkaline. “This shows that hence there are more hydroxides ions in the solution than there were in the original water, therefore some magnesium ions must have dissolved” (Jim Clark, 2002).

Solubility of carbonates Carbonates tend to become less soluble as you go down the group. None of the carbonates is anything more than very sparingly soluble (Jim Clark, 2002).

Reactions of hydroxides with acid Beryllium hydroxide is and amphoteric oxide. That means it reacts with both acids and base to form salts. Beryllium hydroxide reacts with acids forming solutions of beryllium salts. For example

Be(OH)2(s)+H2SO4------>BeSO4(aq)+2H20(i)

The other hydroxides of the group 2 metals are all basic .they react with acids to form salts and water (Jim Clark, 2002). Ca(OH)2(s)+2HCL(aq)-----> CaCl(aq)+2H2O(i) Mg(OH)2(s)+ 2HCL(aq)-----> MgCl(aq)+2H2O(i)

Thermal decomposition of carbonates and hydroxides
All the carbonates in this group undergo thermal decomposition to give a metal oxide and carbon dioxide gas (Jim Clark, 2002). The thermal stability of group 2 carbonates increases as you go down the group (S-cool, 2011). Thermal decomposition is the term used to describe the splitting up of a compound by heating it. Examples are MgCO3---->MgO+CO2(aq) CaCO3------>CaO+CO2(aq)
The thermal stability of hydroxides follows the same trend with that of carbonates, with thermal stability increasing as proton number increases (S-cool, 2011). The only difference is that thermal decompositions of hydroxides produce H2O. Examples are Mg(OH)2----->MgO+H2O Ca(OH)2---->CaO+H2O

Uses of Group 2 Metals and their Compounds
Barium sulfate is very toxic as a crystal. Ba2+ anion has a low charge density; this fact proves its high insolubility. BaSO4 is utilized in medical application like x-ray because it is insoluble in water, acid, and alkali. Not only this, but also BaSO4 is influensible in gastric juice and intestinal juice.

Calcium carbonate
Calcium carbonate is an essential building material as limestone. CaCO3 is geologically evident as limestone, marble, coral, pearls, and chalk, all derived from the shells of small marine animals. “The dissolved calcium is formed as a result of geological weathering over millions of years is used by marine organism to form their shells” (enotes, 2012).

Magnesium hydroxide It is used as milk of magnesia for upset stomach. It acts as an antacid to reduce the increase in acidity levels in the stomach. This is due to OH which increases the ph. in the stomach

Barium oxide
(enotes, 2012) “barium oxide is used to coat the filaments of vacuum tubes, which are still used in some applications”. Because these elements act to remove traces of oxygen and nitrogen, a single layer of barium or strontium atoms on a filament may increase the efficiency more than a hundred million times.

In conclusion Elements in Group 2 of the Periodic Table known as the Alkaline metals. The word “alkaline” means to neutralize and to be soluble while the word “earth metal” is derived from its inability to decompose when exposed to heat. They are called group 2 because they have two electrons in there outer shell. They tend to lose two electrons in order to form a doubly charged positive ion which has a balanced or stable electronic configuration during a reaction. Atomic radius increases as you go down the group and so does reactivity. They can be found in various places in the earth’s crust. They also have different properties either as an element or a compound. They also react with water to form hydroxides and they also react with carbon dioxide to form carbonates.

References:

eNotes (2012) [online] Alkaline Earth Metals (World of Earth Science) Available at http://www.enotes.com/alkaline-earth-metals-reference/alkaline-earth-metals-177005 [Accessed 26th January 2012]

Jim Clark (2002) [online] Understanding Chemistry Available at http://www.chemguide.co.uk/inorganic/group2menu.html#top [Accessed 26th January 2012]

Nigel Saunders (2003) [online] Trends in Physical Properties of Group 2 Elements Available at http://www.creative-chemistry.org.uk/alevel/module1/documents/N-ch1-30.pdf [Accessed 26th January 2012]

Rsc (n.d) [online] Group 2 – The Alkaline Earth Metals Available at http://www.rsc.org/chemsoc/visualelements/pages/data/intro_groupii_data.html [Accessed 26th January 2012]

S-cool (2011) [online] Group II Available at http://www.s-cool.co.uk/a-level/chemistry/group-ii-and-group-iv/revise-it/group-ii [Accessed 26th January 2012]

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