Element A has 6 valence electrons. Element B has 7 valence electrons. Both atoms must follow the octet rule. In the Lewis structure of AB2, the number of lone pair electrons on the central atom A is:

the answers would be

the first one and the second one

answer: semi-infinite solid initially at a uniform temperature of 400 ◦c is cooled at its surface by convection. the cooling fluid at a constant temperature of 200 ◦c has a convective coefficient (ha) of 250 w/m2.k. the physical properties of the solid: k = 20 w/m.k and α = 4 * 10-5  m2/s. using δx = 0.04 m and δt = 10 s. write the finite difference equations describing the transient heat transfer of the first five δx and the temperature profile after 30 s.