Thank you a lot for your answer.
(b) I don't agree with your understanding of "bond"
- what do you mean by "spin"? the orientation of the wavefunction?
(hint: "spin" is reserved for something completely different)
I saw this:http://www.chemistry.mcmaster.ca/esam/Chapter_6/section_3.html
The Quantum Mechanical Explanation of Valency
Helium atoms in their ground state do not form a stable
diatomic molecule. In fact, helium does not combine with
any neutral atom. Its valency, that is, its ability to form
chemical bonds with other atoms, is zero. The electronic
configuration of the helium atom is 1s2(¬), a closed shell
configuration. When two helium atoms are in contact, each
electron on one atom encounters an electron on the other
atom with a parallel spin. Because of the Pauli principle,
neither electron on either atom can concentrate its density
in the region they have in common, the region between the
nuclei. Instead, the density is transferred to the antibinding
regions behind each nucleus where the overlap of the two atomic
density distributions is least. This is the same effect noted
earlier for the approach of two hydrogen atoms with parallel
There exist two different spin isomers of hydrogen diatomic molecules
that differ by the relative spin of their nuclei. In the
orthohydrogen form, the spins of the two protons are parallel
and form a triplet state with a molecular spin quantum number
of 1 (½+½); in the parahydrogen form the spins are antiparallel
and form a singlet with a molecular spin quantum number of 0
(½–½). At standard temperature and pressure, hydrogen gas contains
about 25% of the para form and 75% of the ortho form, also known as
the "normal form". The equilibrium ratio of orthohydrogen to
parahydrogen depends on temperature, but because the ortho form is
an excited state and has a higher energy than the para form, it is
unstable and cannot be purified. At very low temperatures,
the equilibrium state is composed almost exclusively of the para form.
classic sense might be not good enough for bonds.
Obviously there are crystals and plasma.
Substance in plasma state has no bonds.
Li has valency 1 when forms Li2 but also forms bonds with
other 6 Li atoms. If H2 is frozen and compressed it becomes metal with
probably also crystal structure where its valency is not 1 but more.
By orbital overlap I meant something different (my fault)
(it is hard to describe everything precisely and briefly at the same time)
If some atoms are forced to overlap their orbitals by conditions like heat and
pressure ( for example) they form bonds.
But in other cases it is not
necessary to force atoms to overlap each other
because their orbitals and nuclei attract each other and stick. (H + F)
In the rest of cases even if you force them to stay close orbitals will push
away each other due to different spins like it was described about He.