Similarly, each calcium atom (group 2) can give up two electrons and transfer one to each of two chlorine atoms to form CaCl 2, which is composed of Ca 2+ and Cl − ions in the ratio of one Ca 2+ ion to two Cl − ions.Ī compound that contains ions and is held together by ionic bonds is called an ionic compound. For example, when each sodium atom in a sample of sodium metal (group 1) gives up one electron to form a sodium cation, Na +, and each chlorine atom in a sample of chlorine gas (group 17) accepts one electron to form a chloride anion, Cl −, the resulting compound, NaCl, is composed of sodium ions and chloride ions in the ratio of one Na + ion for each Cl − ion. The compound formed by this transfer is stabilized by the electrostatic attractions (ionic bonds) between the ions of opposite charge present in the compound. 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. Compounds are classified as ionic or molecular (covalent) on the basis of the bonds present in them. 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. When electrons are “shared” and molecules form, covalent bonds result. 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). When electrons are transferred and ions form, ionic bonds result. The nature of the attractive forces that hold atoms or ions together within a compound is the basis for classifying chemical bonding. This will be covered in more detail in the next section on nomenclature (i.e., chapter 5.4). It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized \ce. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. (b) A sodium cation (Na+) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign. (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. During the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions ( Figure 5.3.1).įigure 5.3.1. The transfer and sharing of electrons among atoms govern the chemistry of the elements. Electrons, however, can be added to atoms by transfer from other atoms, lost by transfer to other atoms, or shared with other atoms. In ordinary chemical reactions, the nucleus of each atom (and thus the identity of the element) remains unchanged. Determine formulas for simple ionic compounds.Predict the type of compound formed from elements based on their location within the periodic table.Define ionic and molecular (covalent) compounds.Thus, a nitrogen atom will form an anion with three more electrons than protons and a charge of 3−. A nitrogen atom must gain three electrons to have the same number of electrons as an atom of the following noble gas, neon. Nitrogen’s position in the periodic table ( group 15) reveals that it is a nonmetal. The symbol for the ion is Mg 2+, and it is called a magnesium ion. Thus, a magnesium atom will form a cation with two fewer electrons than protons and a charge of 2+. A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Magnesium’s position in the periodic table ( group 2) tells us that it is a metal. Write the symbol for each ion and name them. Predict which forms an anion, which forms a cation, and the charges of each ion. Magnesium and nitrogen react to form an ionic compound.
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