Covalent and Ionic Bonds
There are two basic types of compounds. They are distinguished by the manner in which the atoms bind to one another in the compound. These two types are called molecular compounds and salts (or equivalently ionic compounds).
Recall, a molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties.
Molecular Compound - These compounds are made up of molecules whose atoms bind to one another through covalent bonds. Covalent Bond - The electrons are shared between atoms.
An ion is an atom or group of atoms that carries a positive or negative charge as a result of having lost or gained one or more electrons. Ionic bonds occur due to the mutual attraction between atoms with positive and negative charges.
Ionic Compound - The atoms in salts are held together with ionic bonds. Unlike molecules, salts always form solids in a regular array called a crystalline solid. So, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. To form an ionic compound, there needs to be at least one metal and one non-metal. The metal element is usually the positive charge and the non-metal element is a negative charge. Ions can be single atoms, as in common table salt sodium chloride, or more complex groups such as calcium carbonate
NaCl (Sodium chloride)
· The chloride ion is formed when the element chlorine picks up one electron to form the anion (negatively charged ion) Cl−. The salts of hydrochloric acid contain chloride ions and are also called chlorides. An example is table salt, which is sodium chloride with the chemical formula NaCl. In water, it dissolves into Na+ and Cl− ions.
Molecular Formula for Molecular Compounds
As we know, molecules consist of two or more atoms bonded to one another through covalent bonds.
The identification of these molecules is through their molecular formulas.
The Empirical Formula expresses the most simple ratio of atoms in the molecule. For example, benzene has six carbon and six hydrogen atoms. Therefore, the ratio of carbon to hydrogen atoms is 6: 6 or simplified to 1:1. The empirical formula for benzene is CH.
Molecular Formulas express the correct ratio AND the correct number of atoms in the molecule. For benzene, the molecular formula would be C6H6.
The Structural Formula has the correct number of atoms AND illustrates the bonding structure of the molecule.
Finally, there are 3-D Structural Models and Space Filling Formula. This is an illustration of the Space Filling Formula for benzene.
Molecular Formula for Ionic Compounds
Ionic compounds have atoms or molecules that bond to one another through their mutual attraction between positive and negative charges.
Positively charged atoms or molecules are called cations and negatively charged atoms or molecules are called anions.
As suggested, cations and anions attract one another. Conversely, cations repel other cations, as do anions and anions.
Electrostatic attraction is indiscriminate. That is, a cation can attract more than one anion and visa versa. The result is that cation-anion attractions form a large array called an ionic compound or salt. The bonds holding these ions together are called ionic bonds. However, this array has a very specific composition completely dictated by the charges on the cations and anions.
The composition of ionic compounds is determined by the requirement that the compounds must be electrically neutral. That is, the overall charges of the cations and anions must be zero.
· Na+ and Cl- Na is +1 charge and Cl is -1 charge. Thus, one Na cation cancels one Cl anion resulting in the formula NaCl. This formula is called the formula unit since it represents only one unit of the vast NaCl array or lattice.
An ionic compounds made up of single atom is a monatomic ion
Cation | Anion | Compound |
Ca2+ | Cl- | CaCl2 |
Ba2+ | O2- | |
K+ | S2- | |
Fe3+ | Br- | |
Cr3+ | O2- |
Molecules can also be ions - polyatomic ions. Most polyatomic ions are anions with one notable exception - the ammoniun cation (NH4+). The composition of salts with polyatomic ions is determined by the same rule as with monatomic ions.
Cation | Anion | Compound |
Ca2+ | SO32- | CaSO3 |
Ba2+ | PO42- | |
NH4+ | S2- | |
Fe3+ | SO32- | |
NH4+ | CO22- | |
H3O+ | PO43- | |
Li+ | SO42- | |
NH4+ | HPO42- | |
K+ | CrO42- | |
Be2+ | NO3- |
Teacher Copy – Answer Sheet
Ionic compounds have atoms or molecules that bond to one another through their mutual attraction between positive and negative charges:
Positively charged atoms or molecules are called cations and negatively charged atoms or molecules are called anions.
As suggested, cations and anions attract one another. Conversely, cations repel other cations, as do anions and anions.
Electrostatic attraction is indiscriminate. That is, a cation can attract more than one anion and visa versa. The result is that cation-anion attractions form a large array called an ionic compound or salt. The bonds holding these ions together are called ionic bonds. However, this array has a very specific composition completely dictated by the charges on the cations and anions.
The composition of ionic compounds is determined by the requirement that the compounds must be electrically neutral. That is, the overall charges of the cations and anions must be zero.
· Na+ and Cl- Na is +1 charge and Cl is -1 charge. Thus, one Na cation cancels one Cl anion resulting in the formula NaCl. This formula is called the formula unit since it represents only one unit of the vast NaCl array or lattice.
An ionic compounds made up of single atom is a monatomic ion
Cation | Anion | Compound |
Ca2+ | Cl- | CaCl2 |
Ba2+ | O2- | BaO |
K+ | S2- | K2S |
Fe3+ | Br- | FeBr3 |
Cr3+ | O2- | Cr2O3 |
Molecules can also be ions - polyatomic ions. Most polyatomic ions are anions with one notable exception - the ammoniun cation (NH4+). The composition of salts with polyatomic ions is determined by the same rule as with monatomic ions.
Cation | Anion | Compound |
Ca2+ | SO32- | CaSO3 |
Ba2+ | PO42- | Ba3(PO4)2 |
NH4+ | S2- | (NH4)2S |
Fe3+ | SO32- | Fe2(SO3)3 |
NH4+ | CO22- | (NH4)2CO2 |
H3O+ | PO43- | |
Li+ | SO42- | |
NH4+ | HPO42- | |
K+ | CrO42- | |
Be2+ | NO3- |
Ionic Compounds
Ionic compounds are compounds where two or more ions are held next to each other by electrical attraction. One of the ions has a positive charge (cation) and the other has a negative charge (anion). Cations are usually metal atoms, and anions are either nonmetals or polyatomic ions (ions with more than one atom).
What are the main properties of salts (ionic compounds)?
· All ionic compounds form crystals. Salts form crystals because the electrical positive and negative charges stick together in a “stack-like” formation. The arrangement is called the "unit cell". There are ten or so different general shapes.
· Ionic compounds tend to have high melting and boiling points. The melting point is so high that you cannot melt them with a Bunsen burner. Why are these temperatures so high? Recall, ionic compounds form crystals of positive and negative charges. To break the unit cell apart requires a lot off energy.
· Ionic compounds are very hard and very brittle. Again, this is because of the way that they are held together. The shape makes them hard. Their structure does not allow them to move or bend explaining the brittleness of ionic compounds. If whacked with a hammer, the energy to break the crystal shatters it.
· Ionic compounds conduct electricity when they dissolve in water. If we take a salt and dissolve it in water, the water molecules pull the positive and negative ions apart. The ions spread throughout the water. Thinking back to electricity, it is the movement of electrons. The positive and negative ions in the water allow electrons to flow.
Question: Why does electricity flow effectively through salt water but not as freely through salt crystals? The ions in the solid are in a crystal structure. The electricity does not move freely in the structure.
Question: Does water without salt in it conduct electricity? Fresh water does not conduct electricity very well. The reason you are electrocuted when you drop a hairdryer in the bathtub is because the dirt washed off the human body is ionic and the ions conduct electricity. If you all the salt (dirt) off yourself and then drop a hairdryer in the bathtub, would you be safe? Theoretically, yes. In real life, you would be electrocuted because tap water has ionic compounds dissolved in it.
0 comments:
Post a Comment