whta is (n+l)
rule explain with example
Answers
Explanation:
So the (n + l) rule is a way to account for the two main factors that affect the relative energies of atomic orbitals: the size of the orbital (depends on n) and the number of planar nodes (= l). In cases where (n + l) is the same for two orbitals (e.g., 2p and 3s), the (n + l) rule says that the orbital with lower n has lower energy. In other words, the size of the orbital has a larger effect on orbital energy than the number of planar nodes.
Like all Models, Push Aufbau (n + l) Far Enough and it Fails.
The (n + l) rule is a model. And, as we tell our students, all models have limits. The (n + l) rule works quite well up to Z = 20, calcium (Z is the atomic number). What does “works well” mean? It successfully predicts two things:
the relative energies of the orbitals
the order in which the orbitals are occupied
It may not be obvious that these two things are different. But they are, and the differences start to matter at Z = 21, scandium – the beginning of the transition metals.
For Z = 20, calcium, the (n + l) rule says:
the 4s orbital is lower energy than the 3d orbital
the 4s orbital is occupied and the 3d orbitals are not (1s2 2s2 2p6 3s2 3p6 4s2).
These are both correct!
For Z = 21, scandium, the (n + l) rule says:
the 4s orbital is lower energy than the 3d orbital
the 4s orbital is occupied and one 3d orbitals is occupied (1s2 2s2 2p6 3s2 3p6 4s2 3d1).
Here’s where the (n + l) rule first fails. #2 (the occupation) is correct, but #1 is incorrect. For transitions metals, 3d is lower in energy than 4s! Figure 5 shows the relationship between orbital energy and atomic number (Z). Notice that the curves of the 4s and 3d orbital energies cross at Z = 21.