Sodium has atomic number $11$ chlorine has atomic number $17$. Compare the structure of a sodium atom with that of a chlorine atom. 

Neon is in Group $18$ of the Periodic Table. Argon is below neon in the group. Which of these statements is true? 

Read the following information about three Group $17$ elements. 

At room temperature: 

• bromine is a brown liquid 

• iodine is a purple-black solid 

• chlorine is a green gas. 

Write down in order of increasing atomic number these three Group $17$ elements. Give a reason for your answer. 

Magnesium and calcium are both in Group $2$ of the Periodic Table. Magnesium is above calcium. When magnesium is added to hydrochloric acid you can see bubbles of hydrogen gas. Predict what you will observe when calcium is added to some hydrochloric acid. 

Explain why elements in the same group of the Periodic Table have similar chemical properties. 

This is Mendeleev's Periodic Table from 1869. 

Explain why Mendeleev left gaps in his version of the Periodic Table.