The valence shell of an atom is the outermost occupied electron shell. It is the shell that participates in chemical bonding and determines an atom's chemical properties.

To find the valence shell of an atom:

1. Look at the atomic number of the element.

2. The number of electron shells is equal to the period number.

3. The valence electrons are located on the outermost shell.

Exceptions:

For transition metals, the valence shell includes the outermost shell (n) and the next-to-outermost shell (n-1) because the d orbitals in the n-1 shell are also involved in bonding.

For example: Iron (Fe) has an atomic number of 26. It is in the 4th period. So it has four electron shells. The valence shell is the 4th shell, which contains 2 electrons. The 3rd shell also has 8 electrons, but they are not valence electrons because they are in the d orbital.

For noble gases, the valence shell is the outermost shell, but it is filled with electrons. This makes them very stable and non-reactive.

For example: Helium (He) has an atomic number of 2. It is in the 1st period. So it has one electron shell. The valence shell is the 1st shell, which contains 2 electrons. This makes helium a noble gas.

Valence electrons:

Valence electrons are the electrons in the valence shell. They are responsible for an atom's chemical properties and bonding behavior.

To find the number of valence electrons of an atom:

Look at the group number of the element on the periodic table.

The group number tells you the number of valence electrons.

Exceptions:

For elements in groups 3 to 12, the number of valence electrons is equal to the group number.

For example: Carbon (C) is in group 4. So it has 4 valence electrons.

For elements in groups 13 to 17, the number of valence electrons is equal to 10 minus the group number.

For example: Nitrogen (N) is in group 15. So it has 5 valence electrons (10 - 15 = 5).

For transition metals, the number of valence electrons is equal to the group number, but some of them may be in the d orbitals of the next-to-outermost shell.

For example: Iron (Fe) is in group 8. So it has 8 valence electrons, but 2 of them are in the d orbital of the 3rd shell.