Elements and Chemical Bonds

1. Elements and Chemical Bonds Chapter 4

2. Electrons and Energy Levels • Vocabulary • Chemical bond • A force that holds two or more atoms together • Valence electron • Outermost electron of an atom that participates in chemical bonding • Electron dot diagram • A model that represents valence electrons in an atom as dots around the element’s chemical symbol

3. Electron Number and Arrangement • In a neutral atom the number of protons = the number of electrons • Exact location of an electron cannot be determined but we can predict the area the electron exists within. • Electrons move around the nucleus at a distance that corresponds to the electron’s energy. The farther away the more energy.

4. Electron Number and Arrangement • The outermost electrons are weakly attracted to their nucleus and can be attracted to nuclei of other atoms. • This is how chemical bonds are formed.

5. Valence Electrons • Have the most energy of all the electrons in an atom. • The number of valence electrons in each atom can help determine the bonds that atom can make. • All the elements in a group have the same number of valence electrons, hence their similarity in reactivity.

6. Valence Electrons • For groups 1,2, 13-18 the number of valence electrons in an atom = the number in the ones digit of the group number. • Helium is the exception to the rule. It has 2 instead of 8 valence electrons. • For groups 3-12, the valence electron number varies. You will learn more about that in chemistry class.

7. Electron Dot Diagrams • Help you predict how an atom will bond with other atoms. • Dots, representing valence electrons are placed one-by-one on each side of an element’s chemical symbol until all the dots are used. • The number of unpaired dots tells you how many bonds the atom can make.

8. Noble Gases and Stable vs. Unstable Atoms • Noble gases do not easily react or form bonds. • Atoms with unpaired dots in their electron dot diagrams are unstable, or chemically reactive. • When an unstable atom forms a bond, it either gains, loses, or shares one or more valence electrons. • Unstable atoms that form bonds become stable.

9. Compounds, Chemical Formulas, and Covalent Bonds • Compounds are like cupcakes • Reactants • eggs, flour, salt, vanilla, etc. • hydrogen and oxygen • Reaction • cook it • explosion • Products • Cupcakes • Compound = water (H2O)

10. Compounds, Chemical Formulas, and Covalent Bonds • Covalent bonds • Atoms strive for empty or fullouter energy levels • To do this some atoms share electrons = covalent bonding • Single (1 shared pair), double (2 shared pairs) and triple bonds (3 shared pairs) can form • Atoms that are covalently bonded are called molecules • Water, glucose, carbon dioxide, oxygen

11. Compounds, Chemical Formulas, and Covalent Bonds • Chemical formulas • H2O • C6H12O6 • CO2 • O2

12. Understanding Ions • Ions • An atom that is no longer neutral because it has lost or gained one or more electron • Losing electrons means the atom now has more protons than electrons = positive charge • Gaining electrons means the atom now has more electrons than protons = negative charge

13. Understanding Ions • Determining ion charge • Atoms on the left side of the periodic table tend to lose electrons becoming positive ions • Atoms on the right side tend to gain electrons becoming negative ions • Calculating charge • # protons - # electrons = charge • Examples: • 10 protons – 5 electrons = +5 charge • 5 protons – 10 electrons = -5 charge

14. Ionic Bonds • This is a transfer of electrons • The attraction between positively charged ion and a negatively charged ion = ionic bond • Ionic compounds are usually: • Solid at room temp • Brittle • High melting and boiling points • Dissolve in water easily making the water a good conductor of electricity