Unit 1: Topic 8 - Valence Electrons & Ionic Compounds Flashcards
Valence Electrons and Ionic Compounds
Determine a general rule for where a pair of two elements would form an ionic compound.
The general rule is what is called an “octet rule.” The octet rule states that any atom with valence electrons is most stable when it has 8 valence electrons and forms a new “octet,” or gives up all of its valence electrons and is stabilized by a complete set of core electrons. For example, Sodium (Na) and Chlorine (Cl) form a stable ionic compound: Sodium, an alkali metal, only has one valence electron (and wants to get rid of this electron), while Chlorine has 7 (and wants a new one to form an octet). Conveniently, when sodium gives up its valence electron to chlorine, both of them reach a more stable state, and thus NaCl is an ionic compound because the ions used to form this compound are both stable.
What are valence electrons?
Valence electrons are electrons in the outer shell of an atom that make up the chemical properties of that atom.
What is Ionic bonding?
Ionic bonding is a bond between two elements that takes place when both atoms are of opposite charge.
What is covalent bonding?
Covalent Bonding is a bond between two elements that takes place when atoms share an electron pair.
What ionic compound would Aluminium and Oxygen form?
Aluminum has three valence electrons, and oxygen has six. Preferably, aluminum would like to lose three valence electrons instead of gaining five, and oxygen would like to gain two instead of losing six. Using this idea, we can see that it cannot make sense to include only one of each atom because aluminum would have an extra valence electron left over after donating two to oxygen. However, we note that if we use multiple aluminum and oxygen atoms, two aluminum can donate 6 valence electrons to complete the octets on three oxygen atoms. Therefore, aluminum and oxygen would form Al2O3.
Why do elements sharing a column lose or gain the same number of valence electrons to reach a more stable state?
Again, consider the octet rule. Ions are the most stable when their valence electrons form a complete shell, or they discard all their valence electrons and rely on stable core electrons. Consider the halogens (group 17): Each of them (fluorine, chlorine, bromine, iodine) has seven valence electrons and, by the octet rule, only needs another electron to form a complete valence shell. This is simply due to the structure of the periodic table: the noble gases are very stable (and never react), and when the halogens react, they will usually lose one electron to form a stable valence shell (such as in sodium chloride).
What is an analogous compound?
Elements in the same column of the periodic table typically have similar chemical properties. When a molecule has one element replaced with another from the same column, the two compounds are said to be analogous.
Determine the ionic compounds that oxygen and fluorine make with sodium.
Oxygen, a chalcogen, has 6 valence electrons, and fluorine, a halogen, has 7 valence electrons. Since oxygen would like to gain two more valence electrons to reach the octet, it requires two valence electrons from the sodium atoms. Each sodium atom can only contribute one electron, so there must be two sodium atoms, and the compound is Na2O. Fluorine, similar to chlorine, will form NaF as above. The main reason that these two form different compounds are due to the number of valence electrons, which could have been deduced by noticing that they are in different groups.
Examples of ionic compounds include what?
- sodium chloride: NaCl, with Na+ and Cl- ions, lithium nitride
- Li3N, with Li+ and N3- ions, magnesium oxide
- MgO, with Mg2+ and O2- ions
- calcium phosphide: Ca3P2, with Ca2+ and P3- ions
When are ionic compounds formed?
When metal cations form electrostatic bonds with non-metal anions. These compounds consist of ions.
