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# Bonding and Molecule Formation

Question 1
Kn1 knowledge and understanding of atomic and molecular structure, chemical bonding, chemical formulas, chemical equations, mole calculations and the mole concept and the relation between elements of the Periodic Table.
Ky1 use mathematical skills appropriate to the study of science at this level, in particular: numbers and arithmetic, units, algebra and 'doing maths'.
Silicon is a solid grey coloured crystalline semi-metal, and is the second most abundant element on the Earth. Silicon is placed in the third row of the Periodic Table.
(a) (i) Write down the atomic number of silicon and give its full electronic configuration.
(ii) Naturally occurring silicon consists of three isotopes: silicon-28, silicon-29 and silicon-30, and these occur in the proportions of 92%, 5% and 3%, respectively. What do the numbers 28, 29 and 30 of the silicon isotopes represent in terms of the particles in an atom of silicon?
(iii) Calculate the average relative atomic mass of silicon from the information given regarding the proportions of the isotopes found in naturally occurring silicon. (Give your answer to 3 significant figures.)
(b) Silicon reacts with chlorine gas (Cl2) to give a volatile liquid, silicon tetrachloride SiC14. Silicon tetrachloride reacts readily with water to give silicon dioxide (SiO2), commonly known as sand, and hydrogen chloride gas is also released in the reaction.

(i) Write a fully balanced equation for both of these reactions. Make sure you include the state symbols.
(ii) Silicon tetrachloride exists as a covalent molecule. What is the valency of silicon in this molecule? Give reasons for your answer. (One or two sentences)

Draw the Lewis structure and structural formula to represent the bonding in this molecule.
Question 2
Kn1 knowledge and understanding of chemical formulas, chemical equations, mole calculations and the mole concept and the relation between elements of the Periodic Table.
C2 analysing data.
Ky1 use mathematical skills appropriate to the study of science at this level, in particular: numbers and arithmetic, units, algebra and 'doing maths'.
The noble gases are so named because of their lack of chemical reactivity. However, under certain conditions (namely high temperatures and pressures), some of the noble gases form compounds with fluorine and oxygen. Xenon gas (Xe) is one such noble gas. Xenon combines with fluorine to give a colourless crystalline solid, xenon tetrafluoride XeF4, which is stable at room temperature.
(a) (i) In terms of electronic configuration and bonding, explain why xenon is regarded as an unreactive gas. (Four or five sentences)
(ii) State which other gases you would expect to have similar unreactive chemical properties like xenon and explain why. (Two or three sentences)
(b) In a sealed vessel xenon tetrafluoride, XeF4, reacts with hydrogen gas to give xenon gas and hydrogen fluoride gas.
(i) Write a balanced chemical equation, including state symbols, for this reaction.
(ii) In the reaction 50.0 cm3 of xenon gas are liberated. How many moles of xenon gas is this? Using the molar ratios from your balanced equation, calculate the number of moles and therefore volume in cm3, of hydrogen fluoride gas that is also liberated. (You should assume that 1 mole of gas occupies 24.5 dm3.)

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