Refer to pages 79 and 81 of the Formulae and Tables booklet when answering this question - Leaving Cert Chemistry - Question 5 - 2019

A hydrogen atom consists of one electron and one proton. The diagram below illustrates this arrangement:

  •
 ( H )

The dot represents the single electron surrounding the nucleus.

An oxygen atom has eight electrons, with six in the outer shell. The diagram below represents this:

       • •
    •  O
       • •

The dots around the oxygen represent the valence electrons.

Electronegativity is the relative measure of attraction or number expressing the giving attraction for shared electrons in a covalent bond.

The electronegativity of hydrogen is 2.20 and that of oxygen is 3.44.

Given the difference in electronegativity, the bond between hydrogen and oxygen is a polar covalent bond due to oxygen's higher electronegativity.

The water molecule (H2O) consists of one oxygen atom and two hydrogen atoms, with shared electrons shown in the diagram below:

    •
  H-O-H
    •

This diagram represents the two bonding pairs formed between the hydrogen and oxygen atoms.

Given the electronegativity values of 2.3 and 3.16, compound X would likely form ionic bonds due to the significant difference in electronegativity.

Yes, compound X is expected to have a high melting point due to the strong ionic bonds present.

Yes, compound X would be expected to be soluble in water due to its ionic nature.

Mendeleev's original periodic table of 1869 did not include any of the noble gases because they had not been discovered at that time. The noble gases are largely unreactive and were not recognized until later.