Science: Bohr Model and Melting Point Essay

Submitted By bethh96
Words: 1119
Pages: 5

Question 1
It took scientist almost 80 years after all other elements were discovered to discover Francium because it is one of the rarest natural elements.

Marguerite Perey discovered francium in 1939. Francium was the last element discovered in nature, rather than synthesized. [2] Outside the laboratory, francium is extremely rare, with trace amounts found in uranium and thorium ores, where theisotope francium-223 is continually formed and continually decays. Perhaps an ounce exists at any given time throughout the Earth's crust; the other isotopes are entirely synthetic. The largest amount ever collected of any isotope was a cluster of 10,000 atoms (of francium-210) created as an ultracold gas at Stony Brook in 1996 (Source 1).
Question 2 Name | Atomic No. | Melting pt. | Boiling pt. | Lithium (Li) | 3 | 180.54 °C | 1347.0°C | Sodium (Na) | 11 | 97.8°C | 552.9°C | Potassium (K) | 19 | 63.65°C | 774.0°C | Rubidium (Rb) | 37 | 38.89°C | 688.0°C | Caesium (Cs) | 55 | 28.5°C | 678.4°C |
(Source 2)

The graph and table show that the higher the atomic number the lower the melting point, but generally the boiling points stay about the same. With Francium’s atomic number being 87 which is higher than the rest the melting point with be lower but should have a similar boiling point to the rest. The line of best fit on my graph shows that Francium should have a melting point of about 24°C and a boiling point of about 680°C.

Question 3
All alkali metals react with water liberating hydrogen and forming the alkali metal hydroxide the reaction being:

2X + 2H2O -> 2XOH + H2 where X is the metal.

The reaction increases in violence going up the group. Lithium reacts steadily. Sodium melts and flies about the surface. With Potassium the hydrogen ignites with a violet flame. Rubidium and especially Caesium react explosively (Source 3)
Lithium – Lithium’s density is only about half that water so it floats on the surface gently fizzing and giving off hydrogen. It reacts and disappears which creats a colourless soloution of lithium hydroxide. The reaction generates heat too slowly and lithium’s melting point is too high for it to melt.
Sodium – sodium also floats on thesurface, but enough heat us given off to melt the sodium and it melts almost at once to form a small, silvery ball that dishes around the surface. A white trail of sodium hydroxide is seen in the water under the sodium, but this dissolves to give a colourless solution of sodium hydroxide.
Potassium – potassium behaves rather like sodium except the reaction is faster and enough heat is given off to set light to the hydrogen. This time the normal hydrogen flame is contaminated by potassium compounds and so is coloured lilac
Rubidium –
Rubidium is denser than water and so it sinks. It reacts violently and immediately, with everything spitting out of the container again. Rubidium hydroxide solution and hydrogen are formed.
Caesium – Caesium explodes on contact with water, wuite possible shattering the container. Caesium hydroxide and hydrogen are formed.
Question 4
Atomic radius is the distance between the nucleus and the outermost sub-shell of the atom.
If the atomic radius is small, it means the electron in the outermost subshell is close to the nucleus and so the force of attraction is strong and difficult to overcome when an element made of such atoms is subjected to melting. It is difficult and takes more heat energy, and hence has a high melting point. Alternately some other atom with a large atomic radius will have a comparatively lower melting point (Source 4). Lithium, tellurium, palladium and iodine, have nearly identical atomic radii (145 picometers, 140 pm, 140 pm and 140 pm respectively) yet their melting points are (respectively) 180°C, 450°C, 1555°C, and 114°C.

Another example is to compare cobalt, iridium, and tungsten. All three have an atomic…