Some lattice energies are given in Table 9.1 “Lattice Energies of Some Ionic Compounds.” Table 9.1 Lattice Energies of Some Ionic Compounds Compound The measured strength of ionic bonding is called the lattice energy. The smaller the ion, the stronger the ionic bond (because a smaller ion size allows the ions to get closer together). The greater the magnitude of the charge, the stronger the ionic bond. The strength of ionic bonding depends on two major characteristics: the magnitude of the charges and the size of the ion. With arrows, illustrate the transfer of electrons to form potassium sulfide from K atoms and S atoms. The oppositely charged ions attract each other to make CaCl 2. Thus we need two Cl atoms to accept the two electrons from one Ca atom. A Cl atom needs only one more to complete its octet, while Ca atoms have two electrons to lose. SolutionĪ Ca atom has two valence electrons, while a Cl atom has seven electrons. With arrows, illustrate the transfer of electrons to form calcium chloride from Ca atoms and Cl atoms. This is required by the law of conservation of matter as well. The need for the number of electrons lost being equal to the number of electrons gained explains why ionic compounds have the ratio of cations to anions that they do. These three ions attract each other to give an overall neutral-charged ionic compound, which we write as Na 2O. What we need is a second Na atom to donate a second electron to the O atom: The O atom still does not have an octet of electrons. What about when an Na atom interacts with an O atom? The O atom needs two electrons to complete its valence octet, but the Na atom supplies only one electron: Remember, in the final formula for the ionic compound, we do not write the charges on the ions. The two ions each have octets as their valence shell, and the two oppositely charged particles attract, making an ionic bond: A similar process occurs between Mg atoms and O atoms, except in this case two electrons are transferred: In electron transfer, the number of electrons lost must equal the number of electrons gained. Ionic bonds are caused by electrons transferring from one atom to another. The attraction between oppositely charged ions is called an ionic bond, and it is one of the main types of chemical bonds in chemistry. Where we have written the final formula (the formula for sodium chloride) as per the convention for ionic compounds, without listing the charges explicitly. ![]() This is what happens to the Na + and Cl − ions: From basic physics, we know that opposite charges attract. Resulting in two ions - the Na + ion and the Cl − ion:īoth species now have complete octets, and the electron shells are energetically stable. An electron transfers from the Na atom to the Cl atom: The two atoms have these Lewis electron dot diagrams and electron configurations:įor the Na atom to obtain an octet, it must lose an electron for the Cl atom to gain an octet, it must gain an electron. Now consider an Na atom in the presence of a Cl atom. ![]() The octet rule is a result of trends in energies and is useful in explaining why atoms form the ions that they do. It turns out that the Na + ion has a complete octet in its new valence shell, the n = 2 shell, which satisfies the octet rule. However, that requires much more energy than is normally available in chemical reactions, so sodium stops at a 1+ charge after losing a single electron. We could remove another electron by adding even more energy to the ion, to make the Na 2+ ion. It is rather reactive, however, and does not require a lot of energy to remove that electron to make the Na + ion. Consider sodium: in its elemental form, it has one valence electron and is stable. It is not impossible to violate the octet rule. When atoms form compounds, the octet rule is not always satisfied for all atoms at all times, but it is a very good rule of thumb for understanding the kinds of bonding arrangements that atoms can make. The trend that atoms like to have eight electrons in their valence shell is called the octet rule. For whatever reason, having eight electrons in a valence shell is a particularly energetically stable arrangement of electrons. The lower shell, now the valence shell, has eight electrons in it, so the atom becomes positively charged. Either atoms gain enough electrons to have eight electrons in the valence shell and become the appropriately charged anion, or they lose the electrons in their original valence shell. ![]() The astute reader may have noticed something: Many of the ions that form have eight electrons in their valence shell. In the section “Lewis Electron Dot Diagrams”, we saw how ions are formed by losing electrons to make cations or by gaining electrons to form anions. Demonstrate electron transfer between atoms to form ionic bonds.
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