Which statement is true about an atom and an element?
an atom can be broken down into different substances and an element is formed when two or more atoms combine.
an atom is made of identical elements and an element retains all of the characteristics of a particular atom.
an atom is the smallest unit of matter and an element is a pure substance that is made of identical atoms.

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.