Now, contrast this activity to a typical activity on chemical
energy in which students may perform calculations using an
equation (e.g., ΔH = Σ bond energy bonds broken − Σ bond
energy bonds formed). In the calculation, students must only
insert numbers into the formula; the origin of the formula is not
apparent, nor is the reason for the negative sign. The NGSS
performance expectation is intended to elicit the students’
mechanistic understanding of what happens at the molecular
level when bonds break and form, and the energy changes
associated with this process. This approach to the energy
changes that take place when atoms and molecules interact is
designed to help students understand that there is a spectrum
of interactions, from intermolecular forces to bonds, and that
these interactions are manifestations of the same phenomena to
different degrees.