When balancing a chemical equation:

There must be the same number of atoms of each element on both sides of the equation.

Explanation: This is the only way the equations will be balanced. Adding the coefficients to see if they are equal will not guarantee that the equations are balanced becuase coefficients can be turned into subscripts.

indicate valleys, hills, steepness and gentleness of slopes.

explanation:

contour lines on a map indicate elevation, or depth. numeric notations on contour lines refer to the level of elevation, or height about sea level.

the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons