Which of the following accurately describes properties of valence? a. nonmetallic elements tend to have a positive valence and tend to be electron borrowers. b. metals tend to have a negative valence and tend to be electron borrowers. c. the greater the number of electrons an atom has to borrow or to lend, the greater the activity of the atom. d. the smaller the number of electrons an atom has to borrow or to lend, the greater the activity of the atom

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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.