Understanding Chemical Bonding: Lewis Structures, Ionic and Covalent Bonds

1. Chemical Bonding Mr. Tsigaridis

2. Chemical Bonding • Of all the elements in the universe, that either occur naturally or synthetically, only the elements known as the Nobel gases exist as uncombined atoms • The rest of the elements in the universe are held together by chemical bonds in molecules or compounds • These bonds exists because of a difference in electrostatic forces in the molecules

3. Chemical Bonding • If you recall from your previous classes, the electrostatic forces are created when electrons move from one atom to another and by doing so, change the charges associated to those atoms • This is a process known as Ionization, where some atoms become cations, and others become anions

4. Chemical Bonding • The reason that bonding occurs is becasue of energy • We saw that when you take an object and give it some energy (ex. holding an apple in the air), and then allowing the apple to do as it will, it reconfigures itself in the most energy efficient way • In nature, lower energy systems are favoured over higher energy systems (they are more favourable)

5. Chemical Bonding • We have also seen this characteristic dropping in overall energy in the electrons within atoms (lowest electron configuration) • There are even shapes that are more energy efficient • Think about the shape of every planet in the universe, or indeed the shapes of the very building blocks of everything, the atoms, they are all relatively spherical because a sphere is the most energy efficient shape

6. Lewis Structures • Later in this course we will be using Lewis structures to determine the 3D conformational shape of specific molecules and compounds, but for now we will use them in an attempt to explain chemical bonding • Lewis structures, as many of you already know express pictorially what the valence shell (outer shell) of an atoms is like in terms of its electrons and their configuration

7. Lewis Structures • The reason that we want to represent the valence shell is that the valence electrons are the only ones that take place in chemical bonding • They can either be shared between atoms or they can be donated completely • When we represent Lewis structures in this course we will represent them using dots to represent the lone pairs of electrons and lines between atoms to represent bonding pairs of electrons

8. Lewis Structures

9. Lewis Structures

10. Ionic Bonding • When an ionic bond occurs, it is the attraction between oppositely charged atoms (cations and anions) that constitute an ionic bond • This occurs between elements with large differences in electronegativity • Usually a metal with a low electronegativity and a non-metal with a high electronegativity • These compounds are usually arranged in an crystalline structure much like an array which makes them form crystals

11. Ionic Bonding • As you can see from the diagram below, salt crystals are formed when you have an alternating atoms of chlorine and sodium • This alternating pattern makes the ionic bonds very strong and difficult to break apart from simply heating

12. Ionic Bonds • If we take another compound, using Lewis structures we can see how bonds start to form

13. Ionic Bonds • Similarly, if we use the electron configurations we see that after the bonding occurs, there is a reconfiguration of the electrons into the most energy favourable state 1s 2s 2p 3s Sodium Fluoride

14. Properties of Ionic Solids • Ionic Solids have the following properties • Crystals with smooth shiny surfaces • Hard • Brittle • Non-conductors of electricity and heat • High melting points

15. Covalent Bonding • The word covalent when broken down means to share the valence shell or valence electrons • Atoms will do this with other atoms in order to achieve noble gas electron configuration • When a bond is made, a pair of electrons fills the valence level for both atoms • The areas where there is overlap represent areas of increased electron density Increased Electron Density

16. Covalent Bonding • Covalent bonding involves a delicate balance of attractive and repulsive forces between electrons and the nuclei of atoms • Electrons and electrons naturally repel one another and similarly two nuclei • Electrons and the nucleus, have an attractive force between them due to the opposite charges they posses

17. Covalent Bonds • When you have studied covalent bonding, and ionic bonding for that fact, you have learned that they must follow a rule called the octet rule (noble gas configuration) and bonding at the 2p level must still obey this rule

18. Covalent Bonding • The previous example of fluorine gas shows that covalent bonds form single bonds within molecules, but they may also form double and triple bonds as well • Single Bond • Double Bond • Triple Bond