*Chemical Bonding By Anthony Carpi, Ph. D.

Transcription

1 *Chemical Bonding By Anthony Carpi, Ph. D. Did you know that the same elements can be part of a compound that is either deadly or essential to life depending on how those elements are arranged and the bonding between them? While there are only about 118 known elements, these combine through chemical bonds to form the billions of different substances we encounter in everyday life. Though the periodic table has only 118 or so elements, there are obviously more substances in nature than 118 pure elements. This is because atoms can react with one another to form new substances called compounds. Formed when two or more atoms chemically bond together, the resulting compound is unique both chemically and physically from its parent atoms. Let's look at an example. The element sodium is a silver-colored metal that reacts so violently with water that flames are produced when sodium gets wet. The element chlorine is a greenish-colored gas that is so poisonous that it was used as a weapon in World War I. When chemically bonded together, these two dangerous substances form the compound sodium chloride, a compound so safe that we eat it every day - common table salt! In 1916, the American chemist Gilbert Newton Lewis proposed that chemical bonds are formed between atoms because electrons from the atoms interact with each other. Lewis had observed that many elements are most stable when they contain eight electrons in their valence shell. He suggested that atoms with fewer than eight valence electrons bond together to share electrons and complete their valence shells. While some of Lewis' predictions have since been proven incorrect (he suggested that electrons occupy cube-shaped orbitals), his work established the basis of what is known today about chemical bonding. We now know that there are two main types of chemical bonding; ionic bonding and covalent bonding.

2 Ionic bonding In ionic bonding, electrons are completely transferred from one atom to another. In the process of either losing or gaining negatively charged electrons, the reacting atoms form ions. The oppositely charged ions are attracted to each other by electrostatic forces, which are the basis of the ionic bond. For example, during the reaction of sodium with chlorine: sodium (on the left) loses its one valence electron to chlorine (on the right), resulting in a positively charged sodium ion (left) and a negatively charged chlorine ion (right). Notice that when sodium loses its one valence electron it gets smaller in size, while chlorine grows larger when it gains an additional valence electron. This is typical of the relative sizes of ions to atoms. Positive ions tend to be smaller than their parent atoms while negative ions tend to be larger than their parent. After the reaction takes place, the charged Na + and Cl - ions are held together by electrostatic forces, thus forming an ionic bond. Ionic compounds share many features in common: Ionic bonds form between metals and nonmetals. In naming simple ionic compounds, the metal is always first, the nonmetal second (e.g., sodium chloride). Ionic compounds dissolve easily in water and other polar solvents. In solution, ionic compounds easily conduct electricity. Ionic compounds tend to form crystalline solids with high melting temperatures.

3 This last feature, the fact that ionic compounds are solids, results from the intermolecular forces (forces between molecules) in ionic solids. If we consider a solid crystal of sodium chloride, the solid is made up of many positively charged sodium ions (pictured below as small gray spheres) and an equal number of negatively charged chlorine ions (green spheres). Due to the interaction of the charged ions, the sodium and chlorine ions are arranged in an alternating fashion as demonstrated in the schematic. Each sodium ion is attracted equally to all of its neighboring chlorine ions, and likewise for the chlorine to sodium attraction. The concept of a single molecule does not apply to ionic crystals because the solid exists as one continuous system. Ionic solids form crystals with high melting points because of the strong forces between neighboring ions. Sodium Chloride Crystal NaCl Crystal Schematic Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Cl -1 Na +1 Covalent bonding The second major type of atomic bonding occurs when atoms share electrons. As opposed to ionic bonding in which a complete transfer of electrons occurs, covalent bonding occurs when two (or more) elements share electrons. Covalent bonding occurs because the atoms in the compound have a similar tendency for electrons (generally to gain electrons). This most commonly occurs when two nonmetals bond together. Because both of the nonmetals will want to gain electrons, the elements involved will share electrons in an effort to fill their valence shells. A good example of a covalent bond is that which occurs between two hydrogen atoms. Atoms of hydrogen (H) have one valence electron in their first electron shell. Since the capacity of this shell is two electrons, each hydrogen atom will "want" to pick up a second electron. In an effort to pick up a second electron, hydrogen atoms will react with nearby hydrogen (H) atoms to form the compound H 2. Because the hydrogen compound is a combination of equally matched atoms, the atoms will share each other's single electron, forming one covalent bond. In this way, both atoms share the stability of a full valence shell.

4 Unlike ionic compounds, covalent molecules exist as true molecules. Because electrons are shared in covalent molecules, no full ionic charges are formed. Thus covalent molecules are not strongly attracted to one another. As a result, covalent molecules move about freely and tend to exist as liquids or gases at room temperature. QUIZ 1. What does Gilbert Lewis's theory of chemical bonding state? a.all of the choices b.a chemical bond is formed when an atom's electrons interact with each other c.atoms bond together to fill their valence shells d.the most stable configuration for many elements is one that contains eight valence electrons 2. What are the two main types of chemical bonds? a.nonpolar covalent bonding and covalent bonding b.polar bonding and covalent bonding c.ionic bonding and covalent bonding d.simple bonding and ionic bonding 3. Which particles play the most active role in chemical bonding? a.valence electrons b.neutrons c.electrons d.protons 4. An ionic bond is formed when electrons are: a.equally shared b.divided c.completely destroyed d.completely transferred

5 5. Due to the fact that Ionic compounds have strong intermolecular forces they are at room temperature. a.bonded covalently b.gases c.liquids d.solids 6. Ionic bonds form between which elements in the periodic table? a.metals and nonmetals b.noble gases and metalloids c.metals and metalloids d.noble gases and nonmetals 7. A covalent bond is formed when electrons are: a.shared b.transferred c.destroyed d.split 8. Covalent bonds form between which elements in the periodic table? a.two metals b.a metal and metalloid c.two metalloids d.two nonmetals

6 Bonding: Atoms and Molecules What do the rows represent? The rows in the periodic table correspond to the number of energy levels of the atoms in that row. So the atoms in the first row have one energy level, the atoms in the second row have two energy levels and so on. Understanding how electrons are arranged within the energy levels can help explain why the periodic table has as many rows and columns as it does. Let s take a closer look. Electrons and Energy Levels Every atom contains different energy levels that can hold a specific number of electrons. For a moment, let s imagine the simplest possible scenario: once all the positions are occupied within one energy level, any remaining electrons begin filling positions in the next energy level. To picture this, imagine people filling rows of chairs in an auditorium. If each person sits next to another person until one row is filled, any remaining people must begin taking their seats in the 2nd row, and so on. Not so bad, right? In general, this simple case is a helpful analogy. Electrons fill a given section until it is full, and then any more electrons move on to another unoccupied section where they continue filling there. Electrons begin filling the lowest energy level (closest to the nucleus) and then move on to higher energy levels (further form the nucleus). Energy levels Electrons surround the nucleus of an atom in regions called energy levels. Even though atoms are spherical, the energy levels in an atom are more easily shown in concentric circles. Which atom is this supposed to be? The larger dot in the center of this atom represents the nucleus, which contains both protons and neutrons. The smaller dots surrounding the nucleus represent electrons. In order to figure out which atom this represents, count up the number of electrons. There are

7 8 electrons in this atom. Because the number of electrons and protons is the same in an atom, this atom has 8 protons. Look at the chart Periodic Table, Elements The number of protons is the same as the atomic number, so this drawing represents an oxygen atom. Explain It: If you look closely at the tip of a sharpened pencil, you will see that it is made of graphite. Going deeper, graphite is made of carbon atoms. Deeper still, each carbon atom is made of protons, neutrons, and electrons. 1. Label the nucleus (protons, neutrons) and electrons in the drawing of a carbon atom to the right. 2. Draw a line between the subatomic particle and its charge. proton electron neutron no charge positive charge negative charge 3. What is the basic difference between covalent and ionic bonding? 4. Write a short caption under each picture to describe the process of covalent bonding.

8 5. Write a short caption beside each picture to describe the process of covalent bonding:

9 6. Write a short caption beside each picture to describe the process of ionic bonding:

10 In the beginning of this packet, you saw energy level models for each atom that used concentric circles to represent energy levels and dots for electrons. These diagrams were also used to show what happens to the electrons when different atoms bond. Sometimes electrons were shared (covalent bonding) and sometimes electrons were transferred from one atom to another (ionic bonding). There is a common, shorthand way to represent bonding called Lewis dot diagrams. Dots still represent electrons, but they are drawn around the symbol for the element. And only the electrons in the outermost energy level are drawn. 7. Compare the periodic table of energy levels to the Lewis dot diagrams. Look at the dots around each symbol and the energy levels in your chart. What relationship do you notice between the dots in these two charts? 8. The number of dots near hydrogen and helium are the same as in the energy level chart. Why?