Chemical Bonds and forming Compounds

How is a Compound formed? A compound is formed when two or more elements combine. In order to combine, they must share, take or give away electrons in order to form a bond. The compound formed will have new properties. For example, sodium ( shiny soft metal) and chlorine (a poisonous gas) combine to form NaCl – Table salt!

Chemical Formulas A chemical formula, such as NaCl, H 2 O, or CO 2 tells the elements involved and the exact number of each atom needed to make the compound stable. NOTE: The “2” written after the H in H 2 O is called a subscript. It tells you that in every atom of water there are two hydrogens and one oxygen.

Why do atoms form compounds? Well, opposites attract, right? Some atoms, like hydrogen, have a positive charge and others, like oxygen, have a negative charge:

Chemical stability The Noble gases do not form bonds as they are already stable. They have full outer energy levels, so they are not actively “seeking” to get or give electrons.

Electron Dot Diagrams An image like this tells you how many electrons an element has in its outer energy level. Oxygen has 6 electrons, but needs 8. What can it do?

Trends in the Periodic Table….again

Here’s an example:

Ions An ion is a charged particle that has lost or gained electrons. It no longer has a balance of electrons and protons. If an electron has been lost, it has a positive charge and if an electron has been gained, it has a negative charge.

For example… When K and I combine to form potassium iodide, the K donates its one outer electron to Iodine, which lacks one electron to complete its outer shell. The potassium now has a 1+ charge and the iodide ion has a 1- charge. However, when they form a compound, there is no charge because the + and – charges cancel each other out. Just think of adding +1 and -1. What do you get?

Ionic Bonds – a transfer or electrons

The resulting compound needs to have zero net charge How does this work? Let’s look at MgCl 2. Why is the “2” there? It means that for every one atom of Mg, two atoms of Cl are needed to make this compound. Mg gives one electron to each of two chlorine atoms.

Which elements form ionic bonds? These are usually formed between metals and nonmetals, which are across the table from each other.

Covalent Bonds This is the bond that forms when atoms share electrons. The neutral particle is called a molecule. This type of bond tends to occur in atoms that have four electrons to gain or lose, like those in Group 14. That takes too much energy!

Single Covalent Bonds This is made up of two shared electrons. H 2 O has two single covalent bonds.

Multiple Bonds Covalent bonds can contain more than one pair of electrons. For example, Nitrogen does this to form N 2. It s a diatomic molecule. It has five electrons in its outer shell, so its forms three covalent bonds with another nitrogen.

Unequal sharing Just like in a tug of war, the larger atom in a covalent bond pulls a bit harder on the “shared” electrons.

Polar vs. Nonpolar Polar – has a slightly positive end and a slightly negative end on the molecule; tends to be attracted to other substances and molecules Nonpolar – has no charge; not attracted to other molecules