The Periodic Table

1. Element X has an atomic number of 17.

a) Which group of the periodic table will this element belong to?

Answer: The atomic number is 17, then there will be 17 protons and 17 electrons in the atoms. 17 electrons will be arranged as 2-8-7, so with 7 electrons in the outer shell, the element is in Group 7.


b) Give the atomic number of the element that would be directly above element X in the periodic table.

Answer: Atomic number = 9 (moving up the Group we lose one shell of electrons, so the electron arrangement must be 2-7).


2. Why did Mendeleev leave gaps in his version of the periodic table?

Answer: He believed that there were still elements to be discovered, so left gaps for them.


3. How did Mendeleev’s version of the periodic table benefit from the predictions he made about undiscovered elements?

Answer: When Mendeleev’s predictions were proven correct his model of the periodic table gained more support from other chemists of the time.


4. An element reacts by losing its outer-shell electrons to form positively charged ions. Is this element a metal or a non-metal?

Answer: A metal.


5. What do the Group 0 noble gases all have in common?

Answer: All Group 0 elements have a full outer shell of electrons/they are all unreactive.


6. Describe the trend in reactivity of the Group 1 metals.

Answer: Reactivity increases down the Group.


7. Describe the trend in boiling points of the Group 7 halogens.

Answer: Boiling point increases down the Group.


8. The position of an element in the periodic table is determined by the element’s atomic number and electron structure. Carbon has an atomic number of 6 and silicon has an atomic number of 14. Both elements are in Group 4 of the periodic table.

a) Explain why carbon is positioned in Group 4 of the periodic table.

Answer: Carbon atoms have four electrons in their outer shell.


b) The figure below shows some electron structures. Choose which diagram, A–D, represents a silicon atom.


Answer: A (2-8-4)


c) Carbon reacts with hydrogen to form the compound methane, CH4 . Predict the formula of the compound formed when silicon reacts with hydrogen. Explain your answer in terms of electron structure.

Answer: SiH4 . The electron structures of carbon and silicon are 2-4 and 2-8-4 respectively / they both have 4 electrons in their outer shells so they have the same chemical properties and react similarly.


9. The halogens are in Group 7. Astatine is the fifth halogen, but is difficult to study because it is highly radioactive. Its properties can be predicted from the properties of the first four halogens. The table below shows the appearances of the first four halogens.


a) Predict the appearance of astatine at room temperature.

Answer: A dark/black coloured solid.


b) The figure below shows a graph of the boiling points of the halogens against their relative molecular mass. The figure below shows a graph of the boiling points of the halogens against their relative molecular mass. Use this graph to describe the trend in the boiling points of the halogens and to predict the boiling point of astatine.


Answer: As the relative molecular mass of the halogens increases, the boiling point increases. The boiling point of astatine is 260 °C to 270 °C.


c) The reactivity of the halogens decreases down the group. Predict the products formed when a solution of sodium astatide is mixed with a solution of chlorine.

Answer: sodium chloride and astatine


d) During the reaction between sodium astatide and bromine, astatide ions, At, would lose electrons to form astatine molecules, At2. Show this change by writing the half equation, including electrons.

Answer: 2At  →  At2 + 2e


e) Explain why chlorine would be more reactive than astatine.

Answer: When halogens react they gain one electron to fill their outer shell of electrons. Chlorine atoms will have fewer electron shells than astatine atoms, so the gained electron is closer to the nucleus in chlorine. So the electron is more strongly attracted to the chlorine nucleus than in astatine, and is easier to gain. This makes chlorine more reactive.


10. Lithium, an alkali metal in Group 1, reacts with chlorine, Cl2 , to form lithium chloride, LiCl.

a) Write a balanced symbol equation for the reaction between lithium and chlorine.

Answer: 2Li + Cl2  →  2LiC


b) During this reaction lithium atoms lose their outer-shell electron to form lithium ions. Show this change by writing and balancing the half equation, including electrons.

Answer: Li  →  Li+ + e


c) Potassium reacts with chlorine in a similar way to lithium, but the reaction releases more energy and occurs more rapidly. Explain why potassium reacts in a similar way to lithium, but is more reactive. Include the electronic arrangements of lithium and potassium in your explanation.

Answer: Potassium and lithium both have one outer-shell electron in their atoms so they have similar chemical properties. When Group 1 metals react they lose their one outer-shell electron. Potassium has more electron shells than lithium so the outer-shell electron is further from the nucleus and less strongly attracted to it. This makes it easier for the electron to be lost, so potassium is more reactive.


11. The table below shows how the boiling points of noble gases change with the relative masses of their atom.


a) Describe the trend in the boiling points of the noble gases.

Answer: As the relative mass of the noble gas atoms increases, boiling point increases.


b) Use the data to predict the boiling point of kryptons.

Answer: –160 °C  to  –140 °C


c) The noble gases and the Group 7 halogens are both non-metal elements. Use your own knowledge to compare the difference in reactivity of the noble gases and halogen elements. Explain the difference in terms of the electronic arrangements of the two groups of elements.

Answer: Noble gases are unreactive/inert elements whereas the halogens are very reactive elements. Noble gases are unreactive because they all have a stable electron arrangement/full outer shell of electrons so they do not easily react. Halogens are reactive because they all have seven electrons in their outer shells. They react by gaining one electron to complete their outer shell. The halogens get less reactive the further down Group 7 they are.