Chemical Bonding - we have finished our discussion of individual elements and the atoms that make them up. We are now going to begin talking about compounds, which are collections atoms (usually of different elements) chemically bound together. The focus for the next few days is on what hold the various elements together - chemical bonds.
We represent compounds by chemical formulas - shorthand notation showing the kinds and numbers of atoms in the smallest representative unit of a substance.
glucose: C6H12O6 - a compound with the smallest representative unit being 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen. In chemical formula, we use elemental symbols to indicate the types of atoms present and subscripts to indicate the number them.
Glucose is an example of a molecule- an electrically neutral group of tightly bound atoms that act a as a unit. A molecule is made up of chemically bound atoms. Glucose exists predominately as a cyclic molecule termed a glucopyranose.
Valence electrons and chemical bonding
In chemical bonding, valence electrons (outermost electrons, highest primary energy level) are particularly important. Remember that each group is characterized by a particular outer electron configuration:
e.g. the valence electron configuration for the halogens: Group 7A may be expressed as: ns2np5
The noble gases are particularly inert (nonreactive) - this is an important observation and can be used to rationalize much of chemical bonding. What makes the noble gases special is that their outer electron shells are full ns2np6. We will come back to this again and again.
in forming compounds, valence electrons can be gained, lost or shared - (often to achieve an electron configuration close to that of the noble gases)
Ionic Compounds -
Before beginning a discussion of this particular type of compound, we need to start by talking more about ions:
Ion - a charged atom. Forming an ion from an atom involves the loss or gain of electrons
A Metals typically lose electrons to form positive ions called cations (notice the t looks like a plus)
M --> M+ + e- M+ = cation
Li --> Li+ + e-
Nonmetals, typically gain electrons to form negative ions called anions
X + e- --> X- X- = anion
F + e- --> F-
How do we know ions exist? In solution they form an "electrolyte"
A solution of table salt contains Na+ and Cl- ions. Just like charged electrons, these charge ions can carry an electric current.
Demo: Lighting up a light bulb with a salt solution. Electric pickle.
We can rationalize the charges on ions of elements by their valence electron configurations (hence by their group number).
In forming ions, noble gas configuration is often achieved.
Atoms gain or lose electrons to obtain filled primary energy levels. Na+ is "isoelectronic" with [Ne] and has a filled "valence shell".
Let's look at a few groups
Group 1A - Form singly charged cations M+ (Li+, Na+, K+)
Group 2A - lose two electrons to form doubly charged cations (Ca2+, Mg2+)
Group 7A - halogens
Cl - 1s22s22p63s23p5 - How can these achieve noble gas configuration?
Gain an electron to form an anion.
F-, Cl-, Br-
Group 6A - chalcogens
O2-, S2-, Se2-, Te2-
Transition metals - lose 1-4 electrons in a somewhat unpredictable fashion and ions a various charges for a given element are often common.
Metals form cations, non-metals generally form anions. Elements on the far left of the periodic table generally relatively easy to remove electrons from while elements on the far right generally have an affinity for additional electron.
Back to Ionic compounds -
One way to think of atoms of different elements being held together is through simple electrostatic (positive attracts negative) interactions between ions.
Na+ + Cl- --> NaCl
The attraction between the negative and positive ion that holds them together is termed an ionic bond.
Ionic compounds are generally made from metals and nonmetals since cations typically form from metals and anions from non-metals.
Another example: CaCl2
Why two chlorines? Need to maintain charge neutrality - equal number of positive and negative charges
Ca2+ = +2
2Cl- =2( -1) = -2
Net charge = 0
What is the chemical formula of the compound formed from Na+ and S2-?