EVERYDAY SCIENCE

Detailed explanation-1: -Mendeleev left gaps in his table to place elements not known at the time. By looking at the chemical properties and physical properties of the elements next to a gap, he could also predict the properties of these undiscovered elements.

Detailed explanation-2: -He left blank spaces for the undiscovered elements (atomic masses, element: 44, scandium; 68, gallium; 72, germanium; & 100, technetium) so that certain elements can be grouped together. However, Mendeleev had not predicted the noble gases, so no spots were left for them.

Detailed explanation-3: -Chlorine, oxygen and silicon were included in Mendeleev’s periodic table. Germanium was discovered later which fit into the empty spaces left by Mendeleev and matched to the expected properties of “Eka-silicon".

Detailed explanation-4: -(a) Gaps were left for undiscovered elements in the periodic table. (b) (i) Position of hydrogen was not justified. (ii) Increasing order of atomic mass could not be maintained. (iii) Isotopes have similar chemical properties but different atomic masses, they cannot be given separate places.

Detailed explanation-5: -While arranging the elements according to their atomic weight, if he found that they did not fit into the group he would rearrange them. Mendeleev predicted the properties of some undiscovered elements and gave them names such as “eka-aluminium” for an element with properties similar to aluminium.