O level Chemistry Questions and Answers(1)
The table below shows the electronic arrangements of seven elements, A to G.
a) Give the letters for the following:
i) two elements in the same group of the periodic table. (1)
ii) two elements in the same period of the periodic table. (1)
iii) an element which forms a positive ion. (i)
iv) a noble gas (1)
v) a halogen (1)
vi) a non metal (1)
b) i) Name the type of bonding formed between elements B and G. (1)
ii) Draw a dot and cross diagram to show the arrangement of electrons in the compound formed when an atom of B combines with two atoms of hydrogen. (2)
Name the compound formed in (ii). (1)
a) i) ADG or B and E
The group number of an element is determined by the number of electrons in the outer shell of an element. As you can see from the table, elements A, D and G all have 1 electron in the outer shell which means they are all in the same group (group 1 in this case). B and E have also have 6 electrons in the outer shell which means they are also in the same group (group 6).
ii) ABC or DEF
The period of an element is the number of shells the element has A, B, and C are in the same period because they have the same number of shells (2 in each case). D, E and F are also in the same period because they also have the same number of shells (3).
iii) A or D or G
Elements with less than 4 electrons in the outer form positive ions by losing electrons in chemical reactions. This is because the energy required to lose electrons is low if the outer shell is less than half full. A, D, G all have 1 electron in the outer shell.
A noble gas has a full outer shell. Since the second shell carries a maximum of 8 electrons, C has a full outer shell and is therefore a noble gas.
Halogens are group 7 elements, which means they have 7 electrons in the outer shell. So F is a halogen.
vi) B or C or E or F.
Non metals generally have 4 or more electrons in the outer shell.
b) i) ionic bond or electrovalent bond.
An element with less than 4 electrons in the outershell forms an ionic bond with an element that has more than 4 electrons in the outershell. An ionic bond is also called an electrovalent bond because it is made up of ions held together by electrostatic forces.
For the record, if both elements had more than 4 electrons in the outershell they would have formed a covalent bond.
During covalent bonding, an element shares the number of electrons equal to the number of electrons it needs to form an full outershell. A hydrogen atom requires 1 electron to obtain a full outershell, so each hydrogen atom shares 1 electron. Element B has 6 outershell electrons, so it requires 2 electrons to obtain a full outershell. This means that element B provides 2 electrons to the bond. The crosses represent hydrogen electrons and the dots represent electrons of element B.
Looking at the periodic table, element B is oxygen. Therefore the compound is H2O, which is water.
a) Define the term relative molecular mass. (2)
b) i) Give the formulae of ammonium sulphate. (1)
ii) Hence calculate the relative molecular mass of ammonium sulphate. (1)
iii) Giving a reason for your answer state one use of ammonium sulphate. (1)
iv) Give the name and test for the gas which would be produced when ammonium sulphate is heated with an alkali. (2).
a) The relative molecular mass of a substance is the average mass of its molecule relative to that of the mass of an atom of carbon-12.
b) i) (NH4)2SO4
The relative molecular mass of a substance is calculated by adding up the relative atomic masses of its components in their respective proportions. The relative atomic masses of elements are found on the periodic table.
iii) Use : As a fertiliser
Reason : Ammonium sulphate contains nitrogen which is one of the vital elements required for plant nutrition.
iv) gas : ammonia
test : dip a damp red litmus paper in ammonia gas and the litmus paper turns blue.
Ammonium salts give off ammonia gas when heated with a alkali. The ammonia gas forms an alkaline solution when it dissolves on damp litmus. Thats why the litmus turns blue.
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