Recall that in discussing periodicity we found that the Noble
Gases were very unreactive; this is a result of completely filled
outermost electron energy levels
All of the Noble Gases have eight electrons in their outmost shell
(except He which only has two electrons); this is the origin of the
octet rule of stability
It is an observed fact that elements are more stable when they have
eight electrons in their outermost energy level (remember that we
are only talking about Main Group Elements: Groups 1, 2, and 13-18;
the transition metals are another story altogether because their
outermost energy levels are d orbitals, which hold more than
8 electrons)
Other Main Group elements gain or lose electrons to be ‘more
like’ the Noble Gases; in doing so, they become more stable
In order to gain or lose electrons and remain stable, elements
undergo chemical reactions; for this reason almost no elements
except the noble gases are found uncombined in nature
This is the origin of reactivity
Take K (potassium) as an example: it has one more electron in
its outermost energy levels (a 4s electron) than Ar
(argon)
By losing the 4s electron, K can achieve the electron
configuration of the noble gas Ar
The process of removing an electron is (you recall) called
ionization and it requires an input of energy; the process is
favorable because of the increased stability of the product
The driving force in the ionization process is the release of
energy that comes about because of the formation of a chemical
bond; the formation of bonds is an exothermic process
Chemicals react in ways that increase their stability; measuring
stability is difficult
Instead of trying to measure stability, chemists measure the
changes in energy that accompany a chemical reaction; energy is
easier to measure
Remember that objects with a high potential energy tend to lose
that energy: rocks fall off of the top of cliffs onto our heads,
not the other way around
The same is true on the atomic level: when potassium loses an
electron it releases chemical potential energy; if you try to
reverse a reaction such as the reaction of potassium with water it
requires that you add energy
For this reason, different compounds are thought of as being at
different energy levels
It takes energy to break chemical bonds
Bonds that require a lot of energy to break are low energy
bonds because they are more stable
Bonds that require a small amount of energy to break are high
energy bonds because they are less stable
Electron Transfers
When you subtract an electron, an atom becomes positively charged; this is because the number of protons (the number of positive charges) remains the same while the number of negative charges is reduced; an atom or group of atoms with a positive charge is called a cation
When you add an electron, an atom becomes negatively charged; this is because the number of protons (the number of positive charges) remains the same while the number of negative charges is increased; an atom or group of atoms with a negative charge is called an anion
When 11Na (sodium) loses an electron, it gains the stable octet of the element Ne (neon); it does not become neon because it still has 11 protons, not 10 (10Ne); you write the symbol for this cation as Na+
When 17Cl (chlorine) gains an electron, it gains the stable octet of Ar (argon); remember though that it still only has 17 protons! you write the symbol for this anion as Cl-
Opposite charges attract; when Na reacts with Cl they exchange electrons and form an ionic bond
Ionic compounds do not exist as individual molecules because they are more stable when they form crystals
Crystalline shapes or arrangements of atoms allow (in the case of NaCl) 6 Cl- anions to be next to each Na+ cation
Even though ionic compounds exist because of the attraction of positive for negative charges they are themselves electrically neutral; the charges must balance
The Properties of Salts
Solid salts are poor conductors of electricity
To conduct electricity a substance must have charged particles that can move around; solid salts’ charged particles cannot move at all except to vibrate in place
Molten and dissolved salts are excellent conductors of electricity because their ions can move toward the electrode that has the opposite charge
Differences in electronegativity between the ions in a salt play a large role in determining the properties of the salt; large differences in electronegativity mean stronger bonds than small differences in electronegativity
The stronger the bonds between ions in a salt, the higher the melting point and boiling point of that salt; ionic compounds are characterized by having very high melting points and boiling points
NaCl melts at 801°C and boils at 1413°C; water, which has a different kind of chemical bond, melts at 0°C and boils at 100°C
Salts are hard and brittle because it takes a lot of energy to push the ions apart and once they are pushed far enough apart that like charges end up next to one another they break abruptly
Energy and Ionic Bonding
The process of removing an electron (ionization) require the input of energy
Adding an electron to an atom (electron affinity) usually releases some energy but not usually enough to cause the ionization of another atom
Ionic compounds form as a result of a larger process than just the exchange of electrons
Energy must be added to the system to get things started; just like a spark is needed to start a fire even though the overall reaction releases energy
The formation of a crystal lattice ultimately releases more energy than was added to the system because it is so much more stable than the starting materials; the more energy is released in this process, the stronger the ionic bond
Metals exhibit crystal structure just like ionic compounds but the forces holding them together work differently; here are some points of contrast:
Electrons are not tied to the atoms but can move throughout the crystal structure
Because the electrons can move, metals are good conductors of electricity and heat
Also, because the electrons can move without disrupting the forces holding the atoms together, metals can be stretched or pounded flat without breaking