Imagine you are walking down the street with your best friend. A market has a basket of apples for sale. A sign says that they cost 40 cents each. You both want an apple, but your friend only has 35 cents while you have 45 cents. What can you do? It doesn’t take you long to figure out that if you give your friend a nickel, you can each buy an apple. Transferring the nickel to your friend gets both of you what you want. You actions model, in a simple way, what can happen between atoms.
Electron Transfer
Like your friend with not quite enough money to buy an apple, an atom with five, six, or seven valence electrons has not quite enough to total the more stable number of eight. On the other hand, an atom with one, two, or three valence electrons can lose a few and become stable. When atoms have fewer than four valence electrons, they can transfer these to other atoms that have more than four. In this way, atoms either gain or lose electrons, becoming more stable.
An ion is an atom or group of atoms that has become electrically charged. When an atom loses an electron, it loses a negative charge and becomes a positive ion. When an atom gains an electron, it gains a negative charge and becomes a negative ion.
Forming An Ionic Bond
Consider what happens to the magnesium and oxygen atoms on the right. Note how many valence electrons each has. Suppose magnesium’s valence electrons are transferred to oxygen. The transfer changes both atoms into ions. The magnesium atom becomes a positive ion (Mg ²+). The oxygen atom becomes a negative ion (O ²¯). Negative and positive charges attract each other, so the oppositely charged Mg ²+ and O ²¯come together. They form an ionic compound called magnesium oxide.
An ionic bond is the attraction between two oppositely charged ions. This attraction is similar to the attraction between the opposite poles of two magnets. When the two poles come together, the opposite charges cancel out and an electrically neutral compound is formed.. Every magnesium atom (with a charge written as 2+ ) is balanced by an oxide atom (with a charge written as 2-). The formula for magnesium oxide,MgO, shows you this ratio.
Naming Ionic Compounds
Sodium Chloride and potassium iodide are ionic compounds – where do these names come from? For an ionic compound, the name of the positive ion comes first, followed by the name of the negative ion. The name of the positive ion is usually a metal, since most metals have few valence electrons. Only a small amount of energy is needed to pull electrons from metals. The energy needed to take electrons from metals comes from negative ions.
Nonmetals typically form negative ions. Since only a few electrons are needed to fill the outer level of a nonmetal atom, atoms of nonmetals tend to gain electrons from other atoms and form negative ions.
Covalent Bonds
Remember the market with apples selling for 40 cents each? On another day, the apples are put on sale at two for 70 cents. You and your friend check your pocket and find 35 cents each. What can you do? You could give your friend a nickel to make enough money for an apple. Then you would have only 30 cents, not enough to get one for yourself. But if you share your money, together you can buy two apples.
Electron Sharing
Just as you and you and your friend can buy apples by sharing money, atoms can become more stable by sharing valence electrons. A chemical bond formed when two atoms share electrons is called a covalent bond. Most things around you, such as water, sugar, oxygen, and wood are held together by covalent bonds.
Covalent Bonds
A covalent bond forms when atoms share one or more pairs of electrons. When two atoms of nonmetals bond, a large amount of energy is needed for an atom to lose an electron. So, two nonmetals do not transfer electrons to fill the outermost of energy levels of their atoms. Instead, two nonmetal atoms bond by sharing electrons with each other as shown in the model on the right. When you count the number of electrons on one atom, you count the shared pair each time. By sharing, both atoms have eight valence electrons. In a covalent bond, both atoms attract the shared electrons at the same time.
Covalent bonds in a water molecule
USING ELECTRON DOT DIAGRAMS TO SHOW COVALENT BONDING
One way to represent atoms and molecules in covalent bonding is to use electron-dor diagrams. An electron-dot diagram is a model that shows only the valence electron sin a diagram. Electron-dot diagrams can help you predict how atoms ight bond. To draw an electron dot diagram,, write the symbol of the element and place one dot around the symbol for every valence electron in the atom, as shown below. Place the first four dots alone on each side, and then pair up any remaining dots.
COVALENT BONDING
When atoms combine chemically, rather than gain or lose electrons, the atoms form a chemical bond by “sharing” electrons. This ty[e of bond is called a covalent bond.
Problem: How does a chlorine atom bond with another chlorine atom? For each set of atoms, complete the diagram showing the covalent bond or bonds.
SADDLE BACK
Forming an Ionic Bond
An ionic bond is a bond that forms when electrons are transferred from one atom to another atom. During ionic bonding, one or more valence electrons are transferred from one atom to another. Like all chemical bonds, ionic bonds form so that the outer most energy levels of the atoms in the bonds are filled.
Charged Particles
An atom is neutral because the number of electrons in an atom equals the number of protons. So the charges of the electrons and protons cancel each other. A transfer of electrons between atoms changes the number of electrons in each atom. But the number of protons stays the same in each atom. The negative charges and the positive charges no longer cancel out, and the atoms become ions. Ions are charged particles that can form when atoms gain or lose electrons. An atom normally cannot gain electrons without another atom nearby to gain them.
Forming Positive Ions
Ionic bonds form during chemical changes when atoms pull electrons away from other atoms. A chemical change occurs when one or more substances change into entirely new substances with different properties). The atoms that lose electrons form ions that have fewer electrons than protons. Because the positive charges outnumber the negative charges, these ions form a positive charge.
Metals and the Loss of Electrons
Atoms of most metals have few valence electrons. Metal atoms tend to lose these valence electrons and form positive ions.Energy is needed to pull electrons away from atoms. Only a small amount of energy is needed to take electrons away from metal atom, as such, metals tend to be very reactive. The energy needed to take electrons away comes from the formation of negative ions.
Consider what happens to the magnesium and oxygen atoms on the right. Note how many valence electrons each has. Suppose magnesium’s valence electrons are transferred to oxygen. The transfer changes both atoms into ions. The magnesium atom becomes a positive ion (Mg ²+). The oxygen atom becomes a negative ion (O ²¯). The negative and positive charges attract each other, so the oppositely charged Mg ²+ and O ²¯come together. They form magnesium oxide. When ions come together, the opposite charges cancel out. Every magnesium ion (with a charge written as 2+ ) is balanced by an oxide ion, forming a neutral compound. The chemical formula for magnesium oxide is MgO.
Forming Negative ions