T YPES OF B ONDS

G AIN OR L OSS OF E LECTRONS Atoms lose and gain electrons to try to obtain a stable energy level. An atom that has lost or gained electrons is called an ion. It is a charged particle because it now has either more or fewer electrons than protons. The positive and negative charges are not balanced.

A B OND F ORMS When potassium and iodine come together, a neutral atom of potassium has one electron in the outer energy layer. This means it is not stable. When potassium forms a compound with iodine, potassium loses one electron from its fourth energy level, and the third level becomes a complete outer layer. However, the potassium atom is no longer neutral and is now an ion. When it lost the electron, it became positively charged since there are more protons than electrons.

A B OND F ORMS The 1+ charge is represented as a superscript written after the elements symbol, K +. The iodine atom also changes because it needed that one electron to fill its outer energy level. Since it has gained an electron, it now has a negative charge since there are more electrons than protons. It now has a 1- charge and is shown as I -. You can think of it as potassium wants to lend an electron and iodine wants to borrow one so they can both be happy. When they get together, they become a stable ion with a neutral charge because the + and – cancel each other out.

T HE I ONIC B OND In an ionic bond, a transfer of electrons takes place just like in the example of potassium and iodine. This can also happen when there is more than one electron to be borrowed. When magnesium reacts with chlorine, magnesium has two electrons to lend to chlorine, but each chlorine atom only needs one electron to fill its outer layer. Therefore it takes two chlorine atoms to take the two electrons from the magnesium ion. Magnesium becomes positively charged, Mg 2+ and chlorine becomes negatively charged, Cl -.

Z ERO N ET C HARGE The result of this bond is a neutral charge because the sum of all the charges is zero. When atoms form in ionic compound, their electrons are shifted to other atoms, but the overall number of protons and electrons of the combined atoms remains equal and unchanged. The compound is neutral. Ionic bonds usually form between metals and nonmetals. They are usually across the table from each other. Ionic compounds are often crystalline solids with high melting points.

S HARING E LECTRONS The elements in Groups 14 have 4 electrons that they would need to lose or gain to have a full outer energy level. This requires quite a bit of energy so these elements would rather share their electrons with other elements instead. The attraction that forms between atoms when they share electrons is known as a covalent bond. The neutral particle that is formed from this sharing of electrons is called a molecule.

S INGLE C OVALENT B OND A single covalent bond is made up of two shared electrons. Usually one of the shared electrons comes from one of the atoms and the other electron comes from the other element. A water molecule contains two single bonds. In each bond, hydrogen provides one of the electrons and oxygen provides the other.

M ULTIPLE B ONDS A covalent bond can also contain more than one pair of electrons. For example N 2 has 5 electrons in the outer layer and needs to gain 3 to become stable. It does this by sharing 3 of its electrons with another atom of nitrogen. This bond contains 6 electrons or three pairs of electrons. This is called a triple bond since it has three pairs. If there are only two pairs of electrons it would be called a double bond.

M ULTIPLE B ONDS Covalent bonds form between nonmetallic elements. These elements are close together in the upper right-hand corner of the periodic table. Many covalent compounds are liquids or gases at room temperatures.

U NEQUAL S HARING Electrons are not always shared equally between atoms in a covalent bond. The strength of the attraction of each atom to its electrons is related to the size of atom, the charge of the nucleus, and the total number of electrons the atom contains. It also depends on how far the shared electrons are from the nucleus. It also depends on the size of the positive charge of the nucleus. Example: In HCl, hydrochloric acid, chlorine atoms have a stronger attraction for electrons than hydrogen so the shared electrons will float closer to the chlorine atoms.

T UG - OF - W AR Think of the bond as the rope in tug-of-war and the knot in the middle is the electron. The side with the larger team or with the strongest person will bring the knot closer to their side. Similarly, the electrons being shared in a molecule are held more closely to the atoms with the stronger pull or larger nucleus.

P OLAR OR N ONPOLAR A polar molecule is one with a slightly positive end and a slightly negative end although the overall molecule is neutral. This is the case with the HCl molecule. A nonpolar molecule is one in which electrons are shared equally. This happens when the two elements that combine are exactly the same like N 2.