Chemical Bonding Review: Valence electrons (the outer most electrons) are responsible for the interaction between atoms when forming chemical compounds. Another way to say that is that valence electrons are the electrons that participate in chemical bonding. The Octet Rule explains that every atom seeks a full valence shell. It is the attaining or loss of valence electrons that will satisfy the octet rule. All Matter Exists as Atoms, Metals, Ions, or Molecules Nobel gases actually exist as individual atoms. Remember, it is the interactions between the valence electrons of different elements that determine how the element will react with other elements. Nobel gases have an octet, therefore, they do not need to interact with other atoms to be energetically favorable. All Matter Exists as Atoms, Metals, Ions, or Molecules We commonly work with pure metals. How is it then that only noble gases exist as atoms when pure metals are very common? Do they not exist simply as atoms? Unlike noble gases, metals do not have an octet, therefore, multiple atoms must interact to be energetically favorable. When metal atoms interact, they delocalize their electrons to attain a pseudooctet. Electron-Sea Model describes what is known as metallic bonding: Metals can be thought of as nuclei with core electrons suspended in sea of valence electrons. Attractions hold valence electrons near nucleus, but not so tightly as to impede their flow. All Matter Exists as Atoms, Metals, Ions, or Molecules We have seen how metal atoms can share delocalized electrons to achieve a pseudo-octet with delocalized valence electrons, allowing for a stable elemental form. However, metals rarely exist as pure substances. Rather, they exist as ions by completely giving away their valence electrons. To understand ions, let us investigate the electron configurations of some metals. 1
A metal atom, such as magnesium, has two valence electrons. The octet rule states that all atoms seek the electron configuration of the nearest noble gas, which is for most elements an octet. Observe: Mg 2s To obtain an octet, magnesium gives up its two valence electrons to have an octet in the next lowest energy level. 2+ Mg 2s 2 py The result is a 2 + charge on the atom Ions Ions When atoms lose or gain electrons, they become ions. Cations are positive and are formed by elements on the left side of the periodic chart. Anions are negative and are formed by elements on the right side of the periodic chart. PREDICTING ION CHARGES metals (Li)) lose electrons ---> cations nonmetals ()) gain electrons ---> anions Why do metals tend to loose electrons and nonmetals tend to gain electrons? ELECTRONEGATIVITY Electronegativity Describes the relative ability of an atom to attract electrons. A function of ionization energy and activation energy. More electronegative atoms gain electrons and less electronegative atoms loose electrons Charges on the representative elements can be predicted by observing their electronegativities and the electron configuration Magnesium becomes Mg 2+ to obtain an octet: 2+ Mg - 2s luorine however is electronegative. So, it is going to gain an electron to obtain an octet. 2 py Charges on Representative Atoms +1 +2-4 -3-2 -1 +3 You should be able to identify the charges for all representative metals and non-metals using their group numbers. 2
Transition metals can sometimes have more than one charge. This is due to the movement of electrons from lower energy levels to higher energy levels, ultimately changing the number of valence electrons for the atom. You will have to memorize the common charges on the transition metals. Naming Monatomic Ions Monatomic ions are single atoms that have gained or lost electrons. Monatomic cations, formed from metal atoms are named using the atomic name or example: Na Na + and are both called Sodium Monatomic transition metal cations are named the same way however, since transition metals may have more than one charge, the charge on the metal must be included in parenthesis using roman numerals following the name. or example: e 2+ and e 3+ are named Iron (II) and Iron (III) Monatomic anions formed from nonmetals are named using the root of the atomic name with an ide suffix or example: S 2- N 3- Cl - Are Sulfide, Nitride, and Chloride. Polyatomic Ions Not all ions are monatomic. Sometimes a group of atoms that are bonded together can gain or loose electrons collectively. The result is a group of bonded atoms with a charge, known as a polyatomic ion. The names of polyatomic ions must be memorized. However, a trend does exist. Notice, when there are more than one polyatomic ions that differ only in the number of oxygens, the ion with the fewer oxygens has an ite suffix and the ion with the greater number of oxygens has an ate suffix. However, Ions can not exist independently as ions. They must be electrically balanced by an ion(s) of opposite charge. Ionic compounds are compounds composed of metal cations and non-metal anions that are electrically neutral. The electrostatic force of attraction between ions in an ionic compound are called ionic bonds. 3
Ionic Bonds Since the sodium is +1 and the chlorine is -1, the ratio of cations to anions in sodium chloride is 1:1. But, not all ionic compounds are formed between ions of equal but opposite charge. Here we see the formation of sodium chloride from sodium and chlorine. The ionic compound is formed as the electronegative chlorine atom takes an electron from the sodium atom. The simplest whole-number ration of ions in a compound is called a formula unit. Therefore, the formula unit for sodium chloride is NaCl; or, one sodium per one chloride ion. So, what would be the formula unit for a compound created between magnesium and chlorine? Since magnesium forms a 2+ ion and chlorine forms a 1- ion, there would need to be two chlorine for every magnesium to be electrically neutral. Therefore: MgCl 2 What about ionic compounds formed from polyatomic ions? We treat the polyatomic ions as whole units by placing parenthesis around them and balancing them as they were individual atoms or Example: Magnesium and Phosphite could combine to form magnesium phosphite. Charges are Mg 2+ and (PO 3 ) 3- Therefore, the chemical formula should be Mg 3 (PO 3 ) 2 Nomenclature of Ionic Compounds 1. Write the name of the cation. 2. If the cation can have more than one possible charge, write the charge as a Roman numeral in parentheses. 3. If the anion is an element, change its ending to -ide; if the anion is a polyatomic ion, simply write the name of the polyatomic ion. Writing Chemical ormulas for Ionic Compounds Write chemical formulas for each of the ionic compounds you see here. Remember: You must identify each ion Identify the charge on each ion Balance charges 4
lets put it all together Write all possible ionic compounds that could be formed by the following pairs and name them correctly: 1. Hydrogen phosphate and gold 2. Calcium and sulfur 3. Phosphate and cobalt 4. Cyanide and potassium 5. Ammonium and fluorine All Matter Exists as Atoms, Metals, Ions, or Molecules So: Nobel gases have an octet and can exist as atoms in nature. Metal atoms can share delocalized electrons to achieve a pseudo-octet with delocalized valence electrons, allowing for a stable elemental form. Metals can also loose electrons to highly electronegative non-metals forming ions resulting in an ionic compound. But, what happens when non-metals bond with other non-metal atoms and not metal atoms? Things to consider: Since all non-metals are electronegative, no one nonmetal atom can remove electrons from another; Therefore, no ions will form. All atoms still require a full valence shell, or an octet. If the difference in electronegativities is not large enough for one atom to remove electrons from another atom (ionic bonding), and they can not delocalize their electrons to obtain an octet (metallic bonding), the atoms will have to share electrons in order to obtain an octet. The sharing of electrons in order to obtain a full valence shell in known as covalent bonding. In an ionic bond, metals loose electrons resulting in an octet and non-metals gain electrons to gain an octet. Magnesium becomes to obtain an octet: Mg 2+ - 2s 2 py Chlorine however is electronegative. So, it is going to gain an electron to obtain an octet. Non-metals must overlap their atomic orbitals in order to share electrons with other non-metals. 1 H The sharing of an electron pair in overlapping atomic orbitals known as a covalent bond. Each atom has an octet by sharing 5
Examples of Molecules Methane Aspirin Some elements can only exist in the elemental form with covalent bonds between atoms. These are called molecular elements: H 2, N 2, O 2, 2, Cl 2, Br 2, I 2, P 4, S 8. Carbon dioxide Boron triflouride Ammonia Or, H-7, P. S. Remember us! Anytime we have a molecular element, we must write them as molecules. Covalent Bonding In these bonds atoms share electrons. There are several electrostatic interactions in these bonds: Attractions between electrons and nuclei Repulsions between electrons Repulsions between nuclei Each atom seeks and octet, or a full valence shell. If an atom can not take electrons from other elements to fill its octet, it must arrange itself in such a way as to share electrons to fill its valence shell. Each pair of electrons shared by atoms constitutes a single bond. Essentially, the concentration of electron density between the two atoms sharing the electrons holds the atoms together by electrostatic attraction. The shared electrons of the covalent bond can be shown using Lewis structures. To draw elementary Lewis structures, the Lewis symbols can be used. If you know a molecule can be made from one carbon and one oxygen, Lewis symbols can be used to determine the Lewis structure. C O O Carbon needs four electrons and each oxygen needs two. The only possible arrangement is: O == C == O According to Lewis Theory, there are two types of valence electrons: Non-bonding (or unshared) pairs Bonding single (or unpaired) electrons Boron has three unpaired electrons therefore it can form three covalent bonds Bromine has three unshared pairs and one unpaired electron, therefore it can only form one covalent bond. What about nitrogen? 6
1. Use electron dot symbols (Lewis Symbols) to build covalently bonded molecules for the following sets of atoms. N and 3 H s O and O N and N C and 4 H s How can we use dot structures to determine the number of possible covalent bonds between atoms in a molecule? Because molecules do not contain ions, there are no charges to determine the formulas for the compounds. No Ions! Therefore, in order to name molecules we must use prefixes to denote the number of atoms in a molecular compound. Molecular Prefixes: 1 mono- 2 di- 3 tri- 4 tetra- 5 penta- 6 hexa- 7 hepta- 8 octa- nona- 10 deca- Naming Molecular Compounds List atoms in order of increasing electronegativity Use prefixes to denote the number of atoms in the molecule. (mono-, di-, etc ) The last atom is given an ide suffix Drop o or a from prefix for atoms beginning with a vowel Drop -mono for any single first atom Lets try naming the following molecules: 1.CCl 4 2.Molecule made from 3 sulfur, one carbon, 2 oxygen and one chlorine atoms 3.HCH 3 CO 2 4.P 2 S 4 Br 2 7