Amongst $LiCl, RbCl, BeCl_2$ and $MgCl_2$,the compounds with the greatest and least ionic character respectively are:

What is a portal system?

$\log _4 2 - \log _8 2 + \log _{16} 2 - \dots$ is equal to

The total number of atomic orbitals in the fourth energy level of an atom is .......

$A$ metal wire $PQ$ slides on parallel metallic rails having a separation of $0.25\ m$,each having negligible resistance. There is a $2\,\Omega$ resistor and a $10\ V$ battery as shown in the figure. $A$ uniform magnetic field of magnitude $0.5\ T$ is directed into the plane of the paper. $A$ force of $0.5\ N$ to the left is required to keep the wire $PQ$ moving with a constant speed to the right. With what speed is the wire $PQ$ moving? (Neglect self-inductance of the loop)

Two particles which are initially at rest,move towards each other under the action of their internal attraction. If their speeds are $v$ and $2v$ at any instant,then the speed of the centre of mass of the system will be