Chemical Bonding Ionic Bonding (Loosing Or Gaining Electrons)

Chemical Bonding – Ionic Bonding (Loosing or Gaining Electrons)

When atoms react with one another to form molecules, they do so in such a way as to fill the s and p orbitals in their outermost major energy level or shell with eight electrons (two for hydrogen and helium, since the only shell they have is the first shell, and it is filled with only two electrons). No one knows why this is a "desirable" state for an atom, but atoms will gain, lose, or share electrons to accomplish this. Atoms, such as helium and neon, which already have their outermost shells filled with electrons, are "satisfied," so to speak, and do not normally react with other atoms at all. It is a total mystery why atoms behave in this peculiar way, but they do. In this plate we will look at atoms that gain or lose electrons to fill their outer shells and form what are known as ionic bonds.

Color the heading Sodium Atom/Na at the top of the plate. Color title A and the related structures. We are once again ignoring the individual orbitals by diagramming each major energy level as a shell and simply indicating the total number of electrons in it.

Sodium has its first two shells filled with electrons, but it has only one electron in the third (outermost) shell. Since the orbitals that make up a shell are simply spaces occupied by electrons, a sodium atom can make its unfilled outer shell disappear if it just gives that one electron away to another atom. The second shell then becomes the outer one. Since the second shell is already filled with eight electrons, the atom has then reached the "desired" state and will remain in that state unless acted on by some very strong energy source, such as an electric current or some other atom with an even stronger tendency to get rid of electrons.

Color the heading Sodium lon/Na +. You will notice a size change because the 10 remaining electrons are attracted by 11 protons but repelled by only 9 other electrons, so they move in closer to the nucleus.

As a result of giving away an electron, the sodium atom becomes electrically charged, and in that condition it is called an ion. The name comes from the Greek word for "wanderer," since atoms that are positively or negatively charged wander around independently of one another when dissolved in water.

Since the sodium ion now has 11 protons (positively charged) in its nucleus and only 10 electrons (negatively charged) to neutralize them, the atom as a whole has a netpositive charge of one unit, which is indicated by placing a plus sign after the chemical symbol for sodium.

Color the heading Chlorine Atom/Cl, title B, and the related structure. Also color the heading Chloride Ion/Cl~.

A chlorine atom has seven electrons in its outer shell. It can most easily fill its outermost shell with eight electrons by accepting one electron from some other atom, such as sodium. It then has its outer shell filled with the necessary eight electrons, but it has a net negative charge of one unit, since it has 17 protons and 18 electrons. In that condition it is called a chloride ion, and we place a minus sign after its chemical symbol to indicate that it is negatively charged. (Note the change from "chlorine" for the uncharged atom to "chloride" for the ion. This change in ending is typical for negative ions, although no such change in ending is used for positive ions.) You will observe that the chloride ion is larger than the electrically neutral chlorine atom. Each electron is now repelled by 17 other electrons instead of 16 but still attracted by only 17 protons.

Ions are extremely important in many of the processes of life. Numerous ions must be present in just the right amounts inside your cells, in your tissue fluids, and your blood, or your body will not function. Calcium ion, for example, is essential for triggering the contraction of muscle. If your heart muscle gets too little calcium, you are in real trouble.

Color titles C and D and their related arrows. Color the heading Sodium Chloride Crystal, titles E and F, and their related structures.

As the water containing ions evaporates, the ions are attracted together to form a crystal (a process called crystallization). The ions align themselves in such a way that their charges tend to balance one another. Positive ions match with negative ions so that there is no net charge on the crystal. The force of attraction that holds a positive ion to a negative ion is called an ionic bond. It is important to remember that when we talk about atoms, a bond is not a physical object; it is a force or a relationship.

If water is re-added to the crystal, the water molecules, which are somewhat electrically charged themselves, attract each ion away from the other ions in the crystal. The sum of the weak charges on the water molecules exceeds the strength of the ionic bond. The ions separate from one another in a process called dissociation. This is illustrated in Plate 11.