Ionic bonds are electrostatic forces of attraction, that is, the attractive forces experienced between objects of opposite electrical charge in this case, cations and anions.
Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and one or more pairs of electrons that are located between the atoms. Compounds are classified as ionic or molecular covalent on the basis of the bonds present in them. When an element composed of atoms that readily lose electrons a metal reacts with an element composed of atoms that readily gain electrons a nonmetal , a transfer of electrons usually occurs, producing ions.
The compound formed by this transfer is stabilized by the electrostatic attractions ionic bonds between the ions of opposite charge present in the compound. A compound that contains ions and is held together by ionic bonds is called an ionic compound. The periodic table can help us recognize many of the compounds that are ionic: When a metal is combined with one or more nonmetals, the compound is usually ionic.
This guideline works well for predicting ionic compound formation for most of the compounds typically encountered in an introductory chemistry course. However, it is not always true for example, aluminum chloride, AlCl 3 , is not ionic. You can often recognize ionic compounds because of their properties.
Ionic compounds are solids that typically melt at high temperatures and boil at even higher temperatures. When molten, however, it can conduct electricity because its ions are able to move freely through the liquid Figure 3. Figure 3. Many compounds do not contain ions but instead consist solely of discrete, neutral molecules. These molecular compounds covalent compounds result when atoms share, rather than transfer gain or lose , electrons. Covalent bonding is an important and extensive concept in chemistry, and it will be treated in considerable detail in a later chapter of this text.
We can often identify molecular compounds on the basis of their physical properties. Under normal conditions, molecular compounds often exist as gases, low-boiling liquids, and low-melting solids, although many important exceptions exist.
Whereas ionic compounds are usually formed when a metal and a nonmetal combine, covalent compounds are usually formed by a combination of nonmetals. Thus, the periodic table can help us recognize many of the compounds that are covalent. As the electron shells go from 1 to 7, they increase in size and average energy.
In other words, the farther the shell is from the nucleus, the larger it is, and the higher its average energy. The valence shell is the outermost electron shell of an atom. In general, the electrons in valence shells determine how the atom behaves in chemical reactions. For example, atoms with complete valence shells, the noble gases, are the least chemically reactive.
On the other hand, electrons that have only one electron in their valence shells Group 1 elements or elements that are just one electron short of having a complete shell Group 17 are the most reactive. While the atomic number, the number of protons in the nucleus, never changes, some electrons are easily lost or gained by an atom. When an atom gains or loses an electron, the atom no longer has a balanced charge. Therefore, the atom is no longer neutral. An ion is a charged atom. An atom that has gained negatively charged electrons becomes negative.
A negative ion or anion is an atom that has gained electrons. An atom that has lost negatively charged electrons becomes positive. A positive ion or cation is an atom that has lost electrons. Nonmetals tend to gain electrons and become anions. For example, in Fig. This gives it two more negative charges than positive charges and an overall charge of 2—. Metal elements tend to lose electrons and become cations. Group 18 elements, the noble gases, are very stable non-reactive.
This is because they have completely full valence electron shells. The octet rule states that regardless of how many electrons a shell can potentially hold, the valence shell can only hold eight electrons. The noble gases have eight electrons in their valence shells.
Other elements will gain or lose electrons to achieve completely full valence shells, eight electrons in the valence shell, so that they are also stable. In Fig. The periodic table can help in predicting the type of ion that an element will form based on how many electrons need to be gained or lost for it to become stable. When something loses electrons we say that it has been oxidised. When something gains electrons, we say it has been reduced.
So in ionic bonding, the metals are oxidised and the non-metals are reduced. Forming ions Ions are electrically charged particles formed when atoms lose or gain electrons. The charge is negative, since sulfur is a non-metal. Iodine is in group 7. The charge is negative, since iodine is a non-metal. Forming ions An ion is an atom or group of atoms with a positive or negative charge.
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