Which of the following is a reasonable ground-state electron configuration? 1s^(2)2s^(2)2p^(6)3s^(2)
1s^(2)2s^(2)2p^(6)3s^(2)3d^(4)
1s^(2)2s^(2)2d^(10)2p^(3)
1s^(2)2s^(2)2p^(3)2d^(6)

2nd number is the rt one

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements

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.