Trends in the periodic table

This experiment, through calculating the density of four samples and determining the solubility of the alkaline earth metal salts, investigated periodic trends in density and solubility. For the density experiment, each sample was weighed accurately with a balance and the weight was recorded in a table. The volume of each sample was then determined by dropping the sample in a known volume of water. The new volume was then subtracted by the initial volume and the density of each sample was then calculated by the density formula. The density which was recorded was very near to the correct density of the samples but a slight percentage error was obtained and was recorded in a table.

The density of germanium was determined by drawing a graph of the four samples and finding the density of germanium by interpolation. In the solubility experiment, about 1cm3 of 0.1 M solutions of the nitrate salts magnesium, calcium, strontium and barium was placed into four test tubes. One drop of 1 M H2SO4 was added to each tube and the observations of each were recorded on a result page. The experiment was then repeated by adding 1 M of Na2CO3 and 1 M of K2C2O4 separately into 4 new test tubes containing the nitrate salts. The smaller molecules containing the nitrate and sulfate groups were soluble whereas the bigger molecules containing the carbonate and the oxalate groups were mostly insoluble.

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Introduction

The aim of this experiment was to investigate trends in the periodic table of density and solubility. The aim of the density experiment was to see if the density of an element could be determined through interpolation by drawing a graph of the density against period number for the four samples measured in the experiment. The hypothesis of bigger molecules are less soluble in solutions was tested in the solubility experiment considering the lattice enthalpy and hydration enthalpy factors.

“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.