Explain why the 4s sublevel fills before the 3d begins to fill as electrons are added.

The energy of the 4-s subshell is lower than the energy of 3-d subshell.

Explanation:

During the filling of electrons in subshells, the lower energy levels are filled before the higher energy levels. Also known as Aufbau principle.

Energy of the sublevel = (n + l)

where : n = Principal quantum number

             l = Azimuthal quantum number(s=0,p=1,d=2,f=3)

Energy of 4-s subshell= (4+0) = 4

Energy of 3-d subshell=(3+2) = 5

Energy of 4-s subshell is lower than the energy of 3-d subshell, that is why 4s orbital is filled before the 3-d subshell.

it takes on an electronical energy

the enthalpy of formation of water is given as -285.8kj/mol, which states that the enthalpy of formation is negative.

negative enthalpy of formation indicates that the heat is released during the reaction process. the heat is released from reaction to the surroundings the enthalpy is expressed with a negative sign with the energy given out in the reaction process.

the formation of water takes place by giving out heat which is -285.8 kj/mol.