Types of Chemical Bonds Atoms with incomplete outer shells will try to maximize electrons in their outermost shells by doing one of the following: 1) giving away the electrons in their outer shell; this makes the full inner shell the outermost 2) picking up electrons to fill the incomplete shell 3) sharing electrons with other atoms When atoms interact with other atoms in this way they form chemical bonds.

Two types of chemical bonds are used to join atoms together: ionic bonds covalent bonds Ionic bonds form when atoms donate electrons to one another. Covalent bonds form when atoms share electrons rather than giving them away.

When this happens, an atom becomes a charged particle called an ion. Let’s first look at ionic bonds, those that are formed when atoms donate electrons to one another. If an atom either picks up or gives away an electron, the # of electrons in that atom no longer equals the # protons. When this happens, an atom becomes a charged particle called an ion. (protons = electrons) atom -------> ion

Here’s an atom with 8 protons and 8 electrons Here’s an atom with 8 protons and 8 electrons. If this atom gives away an electron what will happen? 8p

The atom would have more protons (8) than electrons (7) and would become positively charged. + ion Positively charged ions are called cations.

11p (+) 11e (-) 11p (+) 11e (-) 11p (+) 10e (-) cation Here’s an example: 11p (+) 11e (-) 11p (+) 10e (-) cation The sodium atom (Na) is now a sodium cation (Na+). Atoms with almost empty outer shells tend to give away electrons.

What if an atom picks up an electron instead of giving one away?

If this atom picks up an electron, it will have more electrons (9) than protons (8). The extra electron will make the atom negatively charged. 8p ion Negatively charged ions are called anions.

17p (+) 17e (-) 17p (+) 18e (-) 17p (+) 17e (-) anion An example: 17p (+) 17e (-) 17p (+) 18e (-) 17p (+) 17e (-) anion Here’s an example: Atoms with almost full outer shells tend to pick up electrons.

Na+ (sodium) Cl- (chloride) K+ (potassium) H+ (hydrogen) Here are examples of important cations and anions found in human body: Some ions are made up of more than one type of atom and are called complex ions: Na+ (sodium) Cl- (chloride) K+ (potassium) H+ (hydrogen) Ca++ (calcium) OH- (hydroxide ion) HCO3- (bicarbonate ion) PO4-3 (phosphate ion) Which of the ions listed on this slide are cations?

How are ionic bonds formed? Ionic bonds form in the following way: 1) one atom will give away one or more electrons to another atom 2) this produces a cation (+) and an anion (-) 3) the opposite charges on the cation and anion attract one another and pull the ions together 4) this forms an ionic bond

Atoms that like to form ionic bonds are those that have almost empty or almost full outer electron shells. Those that are almost empty give away their electrons. Those that are almost full pick up additional electrons.

A Na atom has an outer shell with only 1 electron A Na atom has an outer shell with only 1 electron. This shell is stable when it has either no electrons (empty shell) or has 8 electrons. This shell is unstable with only one electron.

An atom of chlorine, Cl, has an outer shell with 7 electrons An atom of chlorine, Cl, has an outer shell with 7 electrons. This shell is stable when it has either no electrons or 8 electrons. With 7 electrons it is unstable.

If Na gives away its one electron to Cl, its outermost shell becomes empty, while the outer shell of Cl now has 8 electrons. Na gives away its electron to becomes a cation (Na+) and Cl becomes an anion (Cl-)

The oppositely charged Na+ and Cl- are attracted to one another & the two ions are pulled together. This attractive force is an ionic bond.

To review the steps in forming an ionic bond: 1. electrons are donated from one atom to another 2. positive and negative ions form 3. attractive force between the positive and negative charges form an ionic bond

ex: NaCl ----> Na+ + Cl- Compounds formed with ionic bonds come apart or ionize or dissociate easily when put into water. ex: NaCl ----> Na+ + Cl- (put into water) ionization or dissociation

Let’s look at another, slightly more complicated, example of ionic bonding. An atom of calcium has 2 valence electrons it wants to get rid of, while an atom of chlorine needs only 1 electron to complete its outer shell. This means that for calcium and chlorine to bond, 1 calcium and 2 chlorines are required.

When calcium gives away 2 electrons, it becomes Ca++, while each chlorine, picking up 1 electron, becomes Cl-. The cation and 2 anions are attracted to one another, and an ionic bond is formed.

The second type of chemical bond that forms between atoms is called a covalent bond. In a covalent bond, atoms do not give away or pick up electrons to fill their outer shells, but rather they share their electrons with one another.

Here are two hydrogen atoms Here are two hydrogen atoms. Each H has only one electron, and that electron is in the innermost shell – the shell that can only hold a maximum of two electrons.

Each H needs only one more electron to have a full outer shell. If the two H atoms share their electrons rather than give them away, then they each will have two electrons (or a full outer shell) some of the time.

In this case the two H atoms form a covalent bond, a bond in which atoms share electrons.

H These two hydrogen atoms have bonded to form a molecule. This molecule may be denoted _ H The line between the H atoms indicates that a single covalent bond holds the two atoms together. This is the structural formula for hydrogen gas.

This same molecule may also be written as H2 The H tells you that the only element in this molecule is hydrogen. The subscript 2 tells you that this is a molecule made up of two hydrogen atoms. H2 is the molecular formula for hydrogen gas.

H H H2 = molecular formula Thus, a molecule can be illustrated in several different ways: H = electron dot structure H _ = structural formula H2 = molecular formula

Covalent bonds also form between different atoms. Here is an oxygen atom. O has 6 electrons in its outer shell, and so it needs 2 more electrons to fill this valence shell.

Here are two hydrogen atoms Here are two hydrogen atoms. Each of these atoms needs one more electron to fill its outer shell. (Remember this shell can only hold 2 electrons.)

If the two H atoms each share their one electron with O, and if the O atom shares two of its electrons, one with each H, then both H and O atoms will have full outer shells (some of the time) making them stable.

This covalent bonding forms water, H2O.

The water that was formed from two H and one O can be written H-O-H This structural formula shows you the way the atoms are held together, each hydrogen binding with the oxygen, and not with one another. The lines between the chemical symbols represent single covalent bonds. Water also can be written using a molecular formula, H2O. This formula does not tell you how the atoms are arranged, but it does tell you that water contains two hydrogen atoms and one oxygen atom.

double covalent bond Atoms can share more than one pair of electrons. Here are 2 oxygen atoms, each of which needs 2 electrons to fill its outer shell. By sharing 2 electrons, each atom has a full outer shell most of the time. This type of bond is called a:

When two atoms form a covalent bond, they both will pull on or attract the shared electrons. The term electronegativity describes the tendency for an atom to attract bonding electrons.

These atoms form a nonpolar covalent bond When two identical atoms form a covalent bond, the two atoms have equal electronegativity and pull equally on the bonding electrons. These atoms share their electrons evenly; the electrons spend equal amounts of time around each atom. Electrical charge is spread evenly throughout the molecule. These atoms form a nonpolar covalent bond

Sometimes, however, one atom in a covalent bond has a greater electronegativity and stronger pull on the electrons in the bond. This means the electrons are not shared evenly. Notice the red electrons are closer to one atom than the other. They are spending more time around the green atom than around the blue atom. Here is a diagram to show you how this might look.

Because the electrons spend more time with the green atom, it becomes slightly negative. And since the electrons spend less time with the blue atom, it becomes slightly positive. When atoms in a covalent bond do not share electrons evenly, they form a polar covalent bond.

Water is a good example of polar covalent bonding. Which atom is pulling more on the electrons? How can you tell????

The oxygen has the greater pull on the bonding electrons, so it is slightly negative, while the two hydrogens become slightly positive.

Summary: nonpolar covalent bond = even sharing of electrons polar covalent bond = uneven sharing of electrons

Another type of chemical bonding that is important in biological systems is hydrogen bonding. This type of bond forms when a H with a d+ is attracted to the d- of a nearby electronegative atom (usually O or N).

This type of bonding occurs between individual molecules of water. The positive H in one H2O is attracted to the negative O in another H2O, and a weak hydrogen bond forms.

Note that this type of bonding does not involve donating electrons or sharing electrons. It is simply an attractive force between a slightly positively charged area and a slightly negatively charged area.

Here are many water molecules together, where hydrogen bonding becomes an important force. (All of the dashed lines in this picture represent hydrogen bonds) O H

The hydrogen bonds in water account for water many of its unique properties, including cohesion and surface tension. O H hydrogen bond Hydrogen bonds are important forces that help maintain the shape of both protein molecules and DNA.