“Density refers to the mass contained within a unit volume under specified conditions” (Density of solid: periodicity, 2011). The difference in density of elements can be explained by two factors, namely, by the amount of atoms you can pack in a given volume and what the mass of the individual atoms is (Chemguide: Trends in density, 2011). The number of atoms which can be packed depends on volume and the volume of an element depends on its atomic radius. Atomic radius can be explained by the distance from the nucleus within which 90% of all the charge density is found (Inorganic Chemistry 244, 2011:56).
“ The higher the principal qauntam number of an electronic shell, the farther from the nucleus will significant electron charge density still exist ” (Inorganic Chemistry , 2011:57). In the group members of lower atomic numbers the increase in radius from the one period to the next is large whereas group members of higher atomic numbers the increase in radius is smaller because their outer-shell electrons are held more tightly by the inner-shell electrons in the d and f subshells. Thus, atomic radius increases from top to bottom within a group (Inorganic Chemistry , 2011:58).
Furthermore, atomic radius decreases from left to right across a period but does not include transition elements (Inorganic Chemistry , 2011:58). Thus explaining the trends in the periodic table of density, the density of the elements will increase down a group and decrease across a period as the atomic radius increases and decreases (table 1 & figure 1).
“Solubility is a result of an interaction between polar water molecules and the ions which make up a crystal” (About.com Chemistry, 2001). Two forces determine the extent to which a solution will occur. The force of attraction between H2O molecules and the ions of a solid tends to bring ions into solution. If this is the predominant factor, then the compound may be highly soluble in water (About.com Chemistry, 2001). The second force is the force of attraction between oppositely charged ions which is the force that tends to keep the ions in the solid state. If it is a major factor, then water solubility will be very low (About.com Chemistry, 2001).
“However, it is not easy to estimate the relative magnitudes of these two forces or to quantitatively predict water solubilities of electrolytes. Therefore, it is easier to refer to a set of generalizations, sometimes called 'solubility rules’ that are based upon experimentation.” (About.com Chemistry, 2001). All nitrates are soluble (table 2) and all carbonates are insoluble except NH +, whereas most sulfates are soluble except BaSO4, PbSO4 and SrSO4 (About.com Chemistry, 2001). Furthermore the molecules containing the carbonate and oxalate groups are very big molecules and with increasing atomic radius the molecules become less soluble due to charge density.
Methods
The methods and techniques used in the experiment were done according to the methods in the practical manual without any changes made.
Results
χ - light precipitate χχ - average precipitate (bigger, χχχ - heavy precipitate (small white formations) white formations) (heavy white formations,
pastes on the test tube)
Question 1
The density increases down a group and decreases across a period. The trend can be explained by referring to the atomic radius trend which also increases down a group and decreases across a period. Because density refers to volume and atomic radius influences the volume of an element, the density of an element will therefore increase as atomic radius increases.
Question 2
The solubility of the molecules was found that they became less soluble down a group and more soluble across a period. This can be explained by the atomic radius of an atom which increases down a group and decreases across a period. The solubility of a molecule becomes less soluble with increased radius due to the charge density of the atoms. The factors of lattice enthalpy and hydration enthalpy also play a predominant role in whether or not a molecule will be soluble.
Discussion
In the density experiment it was found that the trend in the periodic table due to density was that it relates to the atomic radius trend. As the atomic radius increases down a group and decreases across a period, the density also increases down a group and decreases across a period due to the effective nuclear charge. The results show that carbon is indeed less dense than silicon and silicon on his turn less dense than tin. The densities obtained during the experiment were slightly incorrect due to the absolute densities of the elements shown the percentage error column. The reason for that could be due to inaccurate measurements made during the experiment by determining the netto volume reading or by weighing the samples. The results can be improved by repeating the experiment several times and to work more accurately during the experiment.
In the solubility experiment it was found that all nitrates were soluble, whereas sulfates and oxalates were mostly insoluble and all carbonates insoluble. Insolubility was determined by observing a heavy white precipitate in the test tubes. The reason for the insolubility can be explained by the increasing in atomic radius. Molecules with higher atomic radius become more and more insoluble because of a bigger overall molecule. Carbonates and oxalates are very big and it was found in the results that all their molecules were insoluble with the exception of magnesium oxalate. The results shows accurate measurements but to improve the results accurate measurements must be done ensuring that exactly 1cm3 of solution must be placed into the test tubes and exactly one drop of 1 M solutions must be dropped into the test tubes.
Conclusion
The aim of the experiments was achieved by investigating the trends in the periodic table due to density and solubility. It was found that in both experiments the trend in atomic radius played a major role in determining both the density of an element and whether or not a molecule is soluble or not. Density increases down a group and decreases across a period due to the effective nuclear charge whereas solubility decreases down a group and increases across a period. Solubility is also dependent on the lattice enthalpy and hydration enthalpy factors and this will predominantly decide whether or not it will be soluble or not.
References
About.com Chemistry. 2001. [Online]. Available: .[2011, August 10].
Chemguide: Trends in density. 2011. [Online]. Available: .[2011, August 10].
Density of solid: periodicity. 2011.[Online]. Available: . [2011, August 10].
Inorganic Chemistry 244. 2011. SUN Media.
