Chemical Bonds
Why do atoms combine to form molecules? Remember in the previous chapters when we discussed electron arrangements? We stated that electrons were found in energy levels; 2 electrons in the 1st energy level, 8 in the second energy level, 18 in the 3rd energy level. Elements want their outermost energy levels full, that is they want they want the maximum number of electrons the energy level can hold. The electron arrangement of the outermost energy level of an atom determines whether or not the atom will form chemical bonds. Most of the elements on the periodic table lack full energy levels, and this makes the energy level unstable. In order to become stable, an atom will either gain or lose electrons. In other words, an atom will bond with another atom if the bonding gives both atoms complete outermost energy levels.
When we think of atoms we generally picture the atom being neutral, that is, the number of positive protons is equal to the number of negative electrons. Look at a sodium atom to the right. Notice that you have the same number of protons as electrons. Each positive charge is cancelled by a negative charge making the atom neutral. This balanced
number of positive and negative charges makes the atom neutral, but it doesn't make the atom stable. To achieve stability, the atoms
must bond in such a way that there energy levels have the complete amount of electrons. One way the atoms can become stable is by
transfering electrons. Bonding that involves a transfer of electrons is known as ionic bonding. In ionic bonding one atom will give extra (or undesired) valence electrons to another atom that needs electrons. For
example, Sodium (Na) has an electron arrangement of 2e, 8e, and 1e. If sodium could get rid of its 1 valence electron, its outer
energy level (the 2nd energy level) would have the desired arrangement of 8 electrons in the outermost energy level. Chlorine (Cl) has
an electron arrangement of 2e, 8e, and 7e. If chlorine could gain 1 valence electron, its outer energy level (the 3rd energy level)
would have the desired arrangement of 8 electrons in the outermost energy level.
The animation above schematically shows the process that takes place during the formation of an ionic bond. The individual atoms are sodium (Na) and chlorine (Cl) with only their valence electrons shown. Note that chlorine has seven valence electrons (it wants a full energy level of eight), and that sodium has one valence electron (it also wants a full energy level of eight). Once the transfer is made, both atoms become stable because they both have eight valence electrons. If you do the math,
because sodium lost an electron, only 10 are left. With 11+ and 10-, there's not enough negative charges to cancel each
positive charge so the sodium atom becomes positively charged. Also, think about the negative
electron chlorine gained. With 17+ and 18-, there's not enough positive charges to cancel each negative charge so the chlorine atom
becomes negatively charged. A charged atom is called an ion. The transfer of the electron caused the previously neutral sodium atom to become a positively charged ion (a cation), and the
previously neutral chlorine atom to become a negatively charged ion (an anion). The attraction for the cation and the anion is called the ionic bond. Ionic bonds are typically formed between metals and non-metals.
Another way atoms can form stable electron arrangements is by sharing valence electrons (known as a covalent bond).
The animation below is a schematic depiction of what happens in the formation of a covalent bond. Only the valence electrons of
hydrogen and oxygen are shown. Hydrogen atoms are represented in red and the oxygen atom is blue. Note that hydrogen needs one
valence electron to become stable and oxygen needs two. All the atoms need electrons, so in order to reach stability they must share their valence electrons. When each atom shares its unpaired electrons, all atoms are tricked into thinking each has a full
outer energy level. Notice that the individual atoms have full freedom from each other, but once the bond is formed,
energy is released, and the new molecule (H20) behaves as a single particle.
Chemist represent covalent bonds with an electron dot diagram. In such a diagram, the chemical symbol for an element
represents the nucleus and all the inner energy levels of the atom - that is, all the energy levels except the outermost energy level. Dots
surrounding the symbol represent the valence electrons. Remember that valence electrons occur in pairs, so the dots should
be paired whenever possible. Electrons are not always shared equally between atoms in a covalent bond. Atoms with more protons in the nucleus tend to pull with more force than atoms with fewer protons in the nucleus. This creates a tug-of-war between atoms. Like tug-of-war the atom that exerts the greatest tug will pull the electron a little closer to its nucleus. For the molecule created, one side becomes a little more + (positive) and the other side of the molecule becomes a little more - (negative). This type of molecule is described as being polar. The term polar means "having opposite ends." A polar molecule is one that has a slightly positive end and a slightly negative end although the overall molecule is neutral.
The combination of atoms forms molecules. A molecule is the smallest particle of a
substance that has all the properties of that substance. This means that 1 molecule of water hass all the characteristics of a glass of water. Covalently bonded substances sometimes form large atoms called
network solids. Network solids are large because the molecules continue to
bond to one another over and over. Graphite, sand, and certain glues form network solids. Certain ions are made of a group of covalently bonded
atoms. The molecules formed are called polyatomic ions. Poly is a term that means "many." A polyatomic ion is a positively or negatively charged, covalently bonded group of atoms. So the compound as a whole contains three or more elements bonded together, but whose charge does not become neutral. One or more charges was not cancelled out, making the molecule positvely or negatively charged. Although the bonds within the polyatomic ion are covalent, the polyatomic ion usually forms ionic bonds. A positive polyatomic molecule will bond with a negative polyatomic molecule. Many polyatomic substance are familiar to you, baking soda, milk of magnesia, and some fertilizers are polyatomic substances.
The oxidation number of an element indicates the number of electrons lost, gained, or shared as a result of chemical bonding.
Example:
Na + Cl2 -----> 2NaCl
The Na starts out with an oxidation number of zero (0) and ends up having an oxidation number of 1+. It has been oxidized from a sodium atom to a positive sodium ion.
The Cl2 also starts out with an oxidation number of zero (0), but it ends up with an oxidation number of 1-. It, therefore, has been reduced from chlorine atoms to negative chloride ions.
The sum of oxidation numbers must equal zero (0). Na 1+ minus Cl 1- = 0.
Magnesium has 2 valence electrons it wants to give away. How many chlorine atoms would be necessary to take magnesium's electrons? That's right 2. What would be the chemical formula for
the compound formed? MgCl2!
Think about the oxidation numbers. [Mg] 2+ minus [Cl2] 2- = 0.
When writing formulas it is important to remember that although the individual ions in a compound carry charges, the compound itself is neutral; which means that a molecule's positive ions must balance with its negative ions.
Compounds with water added are called hydrates. The term hydrate comes from a word that means "water." Think of the term dehydrated, what does it mean to you? That's right, you probably think of it when you're thirsty. Humans must remain hydrated in order to survive. When someone is dehydrated they don't have enough water in their body which can be a serious problem. Whenever you undergo strenous activities, drink plenty of fluids. Many compounds contain water and are thus said to be hydrated. You can remove all the water from a compound and form what is called an anhydrous compound. Anhydrous means "without water."
Additional assignments- print and turn in your work, or email your work to me at jburks1@cullmancats.net. You might need to install ShockWave:
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