Which of the following statements is true?
a. elements in the same group have the same number of electron shells.
b. elements in the same period have the same number of valence electrons.
c. elements in the same period have similar characteristics, like reactivity and valence electrons.
d. elements in the same group, or family, have similar characteristics, like reactivity and valence electrons.

Asheet of steel 1.5 mm thick has nitrogen atmospheres on both sides at 1200oc and is permitted to achieve a steady-state diffusion condition. the diffusion coefficient for nitrogen in steel at this temperature is 6 x 10- 11 m2 /s, and the diffusion flux is found to be 1.2 x 10-7 kg/m2 -s. also, it is known that the concentration of nitrogen in the steel at the high-pressure surface is 4 kg/m3 . how far into the sheet from this high-pressure side will the concentration be 2.0 kg/m3 ? assume a linear concentration profile.