2.3. VALENCE AND CRYSTAL STRUCTURE41• The Principal Quantum Number(n) describes the basic level or shell that an electronresides in. The larger this number, the greater radius the electron cloud has from theatom’s nucleus, and the greater that electron’s energy. Principal quantum numbers arewhole numbers (positive integers).• The Angular Momentum Quantum Number(l) describes the shape of the electron cloudwithin a particular shell or level, and is often known as the “subshell.” There are as manysubshells (electron cloud shapes) in any given shell as that shell’s principal quantumnumber. Angular momentum quantum numbers are positive integers beginning at zeroand ending at one less than the principal quantum number (n-1).• The Magnetic Quantum Number(ml) describes which orientation a subshell (electroncloud shape) has. Subshells may assume as many different orientations as 2-times thesubshell number (l) plus 1, (2l+1) (E.g. for l=1, ml= -1, 0, 1) and each unique orientationis called an orbital. These numbers are integers ranging from the negative value of thesubshell number (l) through 0 to the positive value of the subshell number.• The Spin Quantum Number(ms) describes another property of an electron, and may bea value of +1/2 or -1/2.• Pauli’s Exclusion Principlesays that no two electrons in an atom may share the exactsame set of quantum numbers. Therefore, no more than two electrons may occupy eachorbital (spin=1/2 and spin=-1/2), 2l+1orbitals in every subshell, and nsubshells in everyshell, and no more.• Spectroscopic notationis a convention for denoting the electron conﬁguration of an atom.Shells are shown as whole numbers, followed by subshell letters (s,p,d,f), with super-scripted numbers totaling the number of electrons residing in each respective subshell.• An atom’s chemical behavior is solely determined by the electrons in the unﬁlled shells.Low-level shells that are completely ﬁlled have little or no effect on the chemical bondingcharacteristics of elements.• Elements with completely ﬁlled electron shells are almost entirely unreactive, and arecalled noble(formerly known as inert).2.3Valence and Crystal structureValence: The electrons in the outer most shell, or valence shell, are known as valenceelec-trons. These valence electrons are responsible for the chemical properties of the chemicalelements. It is these electrons which participate in chemical reactions with other elements. Anover simpliﬁed chemistry rule applicable to simple reactions is that atoms try to form a com-plete outer shell of 8 electrons (two for the L shell). Atoms may give away a few electrons toexpose an underlying complete shell. Atoms may accept a few electrons to complete the shell.These two processes form ions from atoms. Atoms may even share electrons among atoms inan attempt to complete the outer shell. This process forms molecular bonds. That is, atomsassociate to form a molecule.