Experiments show that the $H_2O$ molecule has a dipole moment,while $CO_2$ does not. Which of the following structures best represents this fact?

Elements $X, Y$ and $Z$ have $4, 5$ and $7$ valence electrons respectively.
$(a)$ Write the molecular formula of the compounds formed by these elements individually with hydrogen.
$(b)$ Which of these compounds will have the highest dipole moment?

Which of the following pairs of molecules will have a permanent dipole moment?

The dipole moment of $HBr$ is $1.6 \times 10^{-30} \ C \ m$ and interatomic spacing is $1 \ \mathring{A}$. The $\%$ ionic character of $HBr$ is

$BF_3$ and $NF_3$ both are covalent compounds but $NF_3$ is polar whereas $BF_3$ is non-polar. This is because

Assertion : Molecules of larger size have higher polarizability.
Reason : Polarizability is observed only in those molecules which have a permanent dipole moment.