Atoms with a complete outer shell do not react with other atoms. The outer shell is called the valence shell. Its electrons are valence electrons.

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1 Bonding and the Outer Shell Use this table for reference: Atoms with incomplete shells react with others in a way that allows it to complete the outer shell. Atoms react to give up, receive, or share electrons to produce a completed outer shell. Atoms with a complete outer shell do not react with other atoms. The outer shell is called the valence shell. Its electrons are valence electrons. Chemical bonds form when two or more atoms react to fill their outer shells with electrons. A compound is two or more elements joined together by chemical bonds. Ionic Bonding Atoms with unfilled outer shells may transfer electrons from one to another. The transfer enables the atoms to have complete outer shells. A sodium atom (Na) and a chlorine atom (Cl) are shown below. A single circle represents the nucleus (protons and neutrons) of the atoms. Dots represent the electrons. The sodium atom has a total of 11 electrons and one electron in its outer shell. Chlorine has a total of 17 electrons with seven in its outer shell. From this, we can see that the atomic number (number of protons) of sodium is 11 because the number of protons is the same as the number of electrons. Refer to the periodic table to verify that the atomic number of sodium is 11. When sodium chloride (NaCl) is formed, one electron from sodium is transferred to chlorine.

2 Atoms are neutral. When an atom transfers electrons to another, the atom that loses one or more electrons becomes positively charged and the atom that gains one or more electrons becomes negatively charged. These charged particles are called ions. Positively charged ions are cations. Negatively charged ions are anions. Ions have a charge and are written with a plus (+) or a minus (-) sign. For example, calcium loses two electrons to form a calcium ion. The chemical symbol for a calcium ion is therefore Ca ++ or Ca +2. The ions in a compound are attracted to each other due to opposite charges. The attraction, called an ionic bond, holds the ions together. A compound formed by ionic bonds is called an ionic compound or salt. A large number of sodium and chloride ions form a crystal as seen below in the photograph of table salt (sodium chloride). Ionic bonds are weak and the ions can be separated in water (discussed later).

3 Activity Draw a calcium atom. Draw a chlorine atom. Use circles to represent electrons and tell how many protons and neutrons are in the nucleus. Draw calcium chloride. Click here to view the answer. Covalent Bonds Covalent bonds form when atoms share electrons. Hydrogen atoms contain one electron and one proton. In the diagram below, two hydrogen atoms are bonded by a single covalent bond. The two atoms each share a pair of electrons. Molecules Molecules are two or more atoms that are held together by chemical bonds. They may be composed of atoms of the same element or of different elements. In the example above, two hydrogen atoms are held together to form one molecule of hydrogen. A molecule of hydrogen can be written as H2. Molecules have a fixed number of atoms. The subscripts in the formula of a molecule indicate the number of atoms in the molecule. For example one molecule of CH4 has one carbon atom and four huydrogen atoms. The number of ions in an ionic compound (discussed earlier) is not fixed. For example a large crystal of table salt (NaCl) will have more ions than a small crystal. The subscripts in the chemical formula of an ionic compound indicate the ratio of ions. For example CaCl2 has twice as many chloride ions as calcium ions.

4 Example: Methane Carbon has four electrons in its outer shell. It needs four more electrons. Hydrogen needs one. Below: CH4 The shorthand method for writing one molecule of methane is CH4. It may also be written as shown on the right side of the diagram above. The model of a methane molecule below shows that each hydrogen atom (white) is an equal distance apart. The black sphere represents a carbon atom.

5 Double and Triple Bonds The outer shells of oxygen atoms have six electrons. They need two additional electrons to become stable. This can be accomplished by sharing two pairs of electrons. In a double bond, two atoms share two pairs of electrons (4 electrons). In a triple bond, 2 atoms share 3 pairs of electrons (6 electrons).

6 A Shorthand Method for Drawing Covalent Bonds Straight lines can be used to represent a covalent bond between two atoms. A single line is used to represent a single bond, two lines are used to represent a double bond and three lines represent a triple bond. Some single, double, and triple bonds are shown below. Valence and Valence Electrons (BIO 101 only) Valence electrons are those in the outer shell (valence shell). The number of bonds that an atom can form is determined by the number of electrons in its valence shell. This number is often called its valence. Hydrogen forms 1 bond, oxygen forms 2 bonds, and carbon forms four. Some elements may form different numbers of bonds. For example, phosphorous may form 3 bonds with some atoms but 5 with others. Polar Molecules Atoms vary in their attraction for electrons. The strength of this attraction is an atom's electronegativity. Two covalently-bonded atoms that differ in their electronegativity will not share electrons equally and the molecule will be polar. The atom that is more electronegative will exert a stronger attraction for the electrons and will therefore have

7 a partial negative charge. The atom that is less electronegative will have a partial positive charge. In the drawing below, hydrogen shares one pair of electrons with chlorine by a single covalent bond. The electrons are not shared equally because chlorine is more electronegative; it has a stronger attraction for electrons and thus a partial negative charge. The hydrogen has a partial positive charge because it has less access to the shared electrons. The diagram below shows that water is composed of two hydrogen atoms bonded to an oxygen atom by two covalent bonds. Each hydrogen atom shares one pair of electrons with the oxygen atom. Oxygen is much more electronegative than hydrogen and the shared electrons spend more time closer to the oxygen part of the molecule than to the hydrogen part. Unequal sharing of electrons results in the oxygen having a partial negative charge and the hydrogen atoms having a partial positive charge.