With two oppositely charged ions, there is an electrostatic attraction between them because opposite charges attract. The resulting combination is the compound sodium chloride. Notice that there are no leftover electrons.
The number of electrons lost by the sodium atom one equals the number of electrons gained by the chlorine atom one , so the compound is electrically neutral. In macroscopic samples of sodium chloride, there are billions and billions of sodium and chloride ions, although there is always the same number of cations and anions.
Write the formula of the resulting ion and its electron configuration. The electron configuration of Al atom is 1 s 2 2 s 2 2 p 6 3 s 2 3 p 1. The second shell has octet 2 s 2 2 p 6 while the valence shell has 3 electrons 3 s 2 3 p 1.
Mg can achieve octet by losing the 3 valence electrons. The electron configuration of O atom is 1 s 2 2 s 2 2 p 4. The second shell has six electrons 2 s 2 2 p 4 and needs two electrons to achieve octet. Oxygen will gain 2 electrons. In many cases, elements that belong to the same group vertical column on the periodic table form ions with the same charge because they have the same number of valence electrons.
Thus, the periodic table becomes a tool for remembering the charges on many ions. Note the convention of first writing the number and then the sign on a multiply charged ion. These diagrams have two advantages over the electron shell diagrams.
First, they show only valence electrons. Second, instead of having a circle around the chemical symbol to represent the electron shell, they have up to eight dots around the symbol; each dot represents a valence electron. These dots are arranged to the right and left and above and below the symbol, with no more than two dots on a side. For example, the representation for sodium is as follows:. For the above diagrams, it does not matter what sides the dots are placed on in Lewis diagrams as long as each side has a maximum of two dots.
These diagrams are called Lewis electron dot diagrams , or simply Lewis diagrams, after Gilbert N. Lewis, the American chemist who introduced them. Up to four dots are placed above, below, to the left, and to the right of the symbol in any order, as long as elements with four or fewer valence electrons have no more than one dot in each position.
The next dots, for elements with more than four valence electrons, are again distributed one at a time, each paired with one of the first four. In other words, place the dots singly on each side before pairing them. The Lewis electron dot diagram of fluorine, for example, with seven valence electrons, is constructed as follows:. For the main group elements, the number of valence electrons is the same as the group number listed at the top of the periodic table. Starting with lithium and bromine atoms, use Lewis diagrams to show the formation of the ionic compound LiBr.
From the periodic table, we see that lithium is in the same column as sodium, so it will have the same valence shell electron configuration. 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.
Metals particularly those in groups 1 and 2 tend to lose the number of electrons that would leave them with the same number of electrons as in the preceding noble gas in the periodic table. By this means, a positively charged ion is formed. Similarly, nonmetals especially those in groups 16 and 17, and, to a lesser extent, those in Group 15 can gain the number of electrons needed to provide atoms with the same number of electrons as in the next noble gas in the periodic table.
Thus, nonmetals tend to form negative ions. Positively charged ions are called cations, and negatively charge ions are called anions. Ions can be either monatomic containing only one atom or polyatomic containing more than one atom. Compounds that contain ions are called ionic compounds. Ionic compounds generally form from metals and nonmetals. Compounds that do not contain ions, but instead consist of atoms bonded tightly together in molecules uncharged groups of atoms that behave as a single unit , are called covalent compounds.
Covalent compounds usually form from two nonmetals. Notice that each column has more electrons in the outermost shell as you go to the right, and that the last row headed by He or Helium has a full outer shell. Elements with a full outer shell do not form ions.
The yellow section, labeled "Transition Elements" are elements that tend to lose electrons from shells other than the outermost shell and form positive ions. Understanding why there are different ions of iron is complex and beyond the scope of this course. However, you should be able to determine the ionic state of atoms from the other groups using a Periodic Table.
What ion would a Chlorine Cl atom form? What ion would an Aluminum Al atom form? What ion would a Magnesium Mg atom form?
What ion would a Potassium K atom form? Chorine is in the seventh column and therefore has 7 electrons in its outermost shell. It would tend to gain one electron and form a -1 ion.
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