1.1. Figure 1 below shows the periodic table Mendeleev produced in 1869.

His periodic table was more widely accepted than previous versions

Figure 1 

The atomic weight of iodine (I) is 127 and that of tellurium (Te) is 128.

Why did Mendeleev reverse the order of these two elements?

1.2. Why did Mendeleev leave gaps for undiscovered elements?

1.3. Name the particle that allowed the elements to be arranged in order of their atomic number in the modern Periodic Table.

1.4. Mendeleev's periodic table shows lithium, sodium and potassium all in the same column. 

These elements are in the same column, Group 1, of the modern periodic table. 

Explain the reactivity of these elements going down Group 1.