Electron Configuration of Monatomic Ions

It is possible to assign electronic structures to monatomic ions, formed from atoms by gaining or losing electrons. In general, when a monatomic ion is formed from an atom, electrons are added to or removed from sublevels in the highest principle energy level, n.

Ions with Noble –Gas Structures

Elements close to a noble gas in the periodic table form ions that have the same number of electrons as the noble-gas atom. This means that the atoms have noble-gas electron configurations. 

Example:  Nonmetal atoms achieve this by gaining electrons to form anions:

7N (1s2 2s2 2p3) + 3e- → 7N (1s2 2s2 2p6)
8O (1s2 2s2 2p4) + 2e- → 8O (1s2 2s2 2p6)
9F  (1s2 2s2 2p5) + e-   → 9F  (1s2 2s2 2p6)

Example: Metal atoms achieve this by losing electrons to form cations:

11Na (1s2 2s2 2p6 3s1) → 11Na+ (1s2 2s2 2p6) + e-
12Mg (1s2 2s2 2p6 3s2) → 12Mg2+ (1s2 2s2 2p6) + 2e-
13Al (1s2 2s2 2p6 3s2 3p1) → 13Al3+ (1s2 2s2 2p6) + 3e-

The species N3-, O2-, F-, Ne, Na+, Mg2+, and Al3+ are said to be isoelectric, that is they all have the same electron configuration.

Transition Metal Cations

Applying the principle that, in forming cations, electrons are removed from the sublevel if highest n, you can predict correctly that when transition metals atoms from positive ions, the outer s electrons are lost first.

Example:
		25Mn: [Ar] 4s2 3d5		25Mn2+: [Ar] 3d5

All the transition metals form cations by a similar process, that is, loss of outer s electrons. Only after those electrons are removed are electrons removed from the inner d sublevel. 

Example: Iron forms two different ions Fe2+ /  Fe3+
26Fe: [Ar] 4s2 3d6
To form Fe2+ the 4s electrons are lost first: 26Fe2+: [Ar] 3d6
To form Fe2+ the 4s electrons are lost first, and then one electron from the 3d subshell is lost: 26Fe3+: [Ar] 3d5

Multivalence appears to be explained by transition metal atoms:
i)	 losing the electrons first in the highest principle energy level, n, (ns2) 
ii)	losing inner d sublevel electrons after losing the electrons in the highest principle energy level (will lose electrons to obtain a d sublevel that is half filled or empty)
iii)	promotion of electrons in the highest principle energy level (ns) to the inner d sublevel ((n-1)d)so that the d sublevel will become half filled or filled

Example: copper forms two different ions Cu2+ /  Cu+
29Cu: [Ar] 4s2 3d9
To form Cu2+ the 4s electrons are lost: 29Cu2+: [Ar] 3d9
To form Cu+ one of electrons in the 4s sublevel is promoted to fill the 3d sublevel (29Cu: [Ar] 4s2 3d10)
     and then electron remaining in the 4s subshell is lost: 29Cu+: [Ar] 3d10