1. Glenn V. Lo Department of Physical Sciences Nicholls State University
Ions and Molecules
Glenn V. Lo
Department of Physical Sciences
Nicholls State University

2. What atoms typically do
Most naturally-occurring atoms have a stable nucleus (non-radioactive).
For most everyday phenomena
the nuclei of atoms remain intact
atoms may lose electrons to other atoms
atoms may gain electrons from other atoms
atoms may share electrons with other atoms

3. Valence electrons
Valence electrons = outermost electrons of an atom; determines properties of atoms
“Property of atom” = behavior, types of bonds formed.
All atoms belonging to the same group (vertical column) have same valence configuration.
For main group elements (the “A” groups): number of valence electrons = group number

4. Lewis symbol Symbol for an atom surrounded by dots
Number of dots = number of valence electrons
Example:

5. Ions
Every atom is electrically neutral --- this means equal number of protons (+) and electrons (-).
If an atom loses or gains electron(s), the result is an ION.
Cation = positively-charged ion = due to loss of electron. [Pronounce: cat-eye-on]
Anion = negatively-charged ion = due to gain of electron. [Pronounce: an-eye-on]

6. Mg 2+ Ion Symbol Charge is shown in upper-right hand corner
+, 2+, 3+, etc., or
-, 2-, 3-, etc.
Charge = number of protons minus number of electrons. 2+ Mg

7. Ion Symbol In one Na+ ion, there are how many electrons and protons?
Symbols means “Na after it has lost two electrons.”
Na atom has 11 protons, Na+ ion still has 11 protons.
Na atom has 11 electrons, Na+ has only 10 electrons.

8. Ion Symbol In one F- ion, there are how many electrons and protons?
Symbols means “F after it has gained one electron.”
F atom has 9 protons, F- ion still has 9 protons.
F atom has 9 electrons, F- has one more (10 electrons).

9. Ionization energy
(First) ionization energy is a measure of how hard it is to remove an electron from an atom (in the gas phase). Example. IE of Na is the energy associated with the process Na(g)  Na+(g) + e-
Second ionization energy is a measure of how hard it is to remove a second electron, after the first one has been removed. 2nd IE of Na: Na+(g)  Na2+(g) + e-

10. Trends in ionization energy

11. Explaining IE trend
Ionization energy depends on how strongly outer electrons are pulled in:
nucleus pulls them in
electrons shield each other from pull of nuclear charge.
Effective nuclear charge increases from left to right
Going down a column,
More protons in nuclei, but also more core electrons.
Outer electrons are farther away. (Remember Coulomb’s law)

12. First 5 IE of Aluminum
In nature, aluminum ion is always found with a +3 charge. How can we explain it using ionization energies?
After removal of 3 electrons – removal of next electron drastically increases.
Why? Al has 3 valence electrons. Removal of 3 valence electrons leaves behind a stable noble-gas-like configuration.

13. Trend in Atomic Size
The general trend in atomic size is opposite the trend in ionization energy. The higher the ionization energy, the smaller the atomic size.

14. Question
Why is it very unlikely for an atom like fluorine to form a positively charged ion?
The only monatomic ion derived from fluorine that you can find in nature is F-. Why is F2- highly unlikely?

15. Typical monatomic ions-3 +3 -2 -1
Variable (+1 to +4) Most common: +2 +1 +2
Group IA: +1
Group IIA): +2
Al: +3
Transition and post-transition: variable; Zn, Cd, Hg: +2
Hydrogen: H+
Hydride: H-
Nonmetals: Group VIIA: -1
Group VIA: -2
Group VA: -3

16. Ionic and Covalent Bonding
Nonmetal atoms tend to gain electrons from metal atoms resulting in the formation of ions, which are attracted to each other by Coulombic force. Ionic bond: attractive force between a cation and anion
Nonmetals and metalloids – tend to share electrons with other nonmetals and metalloids. Atoms sharing electrons are said to be held together by a covalent bond.

17. Inert atoms
Noble gas (VIIIA) atoms tend not to form ions or molecules under ordinary conditions.

18. Molecules and Polyatomic Ions
Molecule = two or more atoms linked by covalent bonds (sharing electrons), with equal number of protons and electrons (electrically neutral).
Polyatomic ion = electrically-charged group of two or more atoms sharing electrons.

19. Molecules vs. Polyatomic Ion
Compare SO3 vs. SO32-.
SO3 is a molecule and has:
40 protons (16 from S, 8 from each O)
40 electrons (same as number of protons)
SO32- is a polyatomic anion and has
42 electrons (2- means two more electrons)

20. Lewis structures Lewis dot symbol for noble gases:
Draw eight dots (representing valence electrons)
Exception: He (two dots)
Lewis structure: a sketch showing how atoms in a molecule or polyatomic ion are sharing valence electrons
Follow octet rule: share valence electrons so that each atom appears surrounded by eight dots.
Follow duet rule for H
Atoms acquire a noble-gas-like configuration by following the octet/duet rule.
For stable, naturally occurring molecules and polyatomic ions, it is generally possible to draw a Lewis structure.

21. Interpreting Lewis Structures
Dots = valence electrons
Dash = same as a pair of dots; a shared pair of electrons
Dots or dashes drawn between atoms = shared valence electrons
Lewis structure for polyatomic ions are enclosed in brackets and charge indicated as a superscript outside the brackets

22. Terminologies
Single (covalent) bond: one shared pair between two atoms
Double (covalent) bond: two shared pairs between two atoms
Triple (covalent) bond: three shared pairs between two atoms
Lone pair: unshared pair of valence electrons
In the structure shown above, identify the single bonds, double bond, and lone pairs.

23. Diatomics
Shown below are five possible ways of drawing a Lewis structure for a diatomic molecule or ion.

24. Triatomics
Draw Lewis structures for the following: HCN, HOH, HOCl, NH2-

25. Triatomics
Draw Lewis structures for the following: CO2, OCS, Cl2O, SO2, O3, N3-

26. Resonance Structures
Two or more ways of distributing valence electrons; atoms connected same way
Example: a triatomic molecule or ion with 16 valence electrons can have the following resonance structures

27. Exceptions to Octet Rule
Some molecules have an odd number of valence electrons.
Not possible to draw a Lewis structure
Generally reactive; called “radicals”
Experimental evidence for unpaired electron: paramagnetism

28. Exceptions to Octet Rule
Molecules with B, Be, and Al. Example: BF3
Coordinate covalent bonding occurs when a lone pair of another molecule ends up being shared with the B atom.

29. Exceptions to Octet Rule
Expanded octets possible with molecules or ions of type AXn, where A (the “central atom”) is an atom from row 3 and below
additional electrons accommodated by d orbitals
Examples: PCl5, SF6, SO42-