Combining Atoms and Molecules

1. Combining Atoms and Molecules 5-1: How Atoms Form Compounds

2. Vocabulary • compound • chemical formula • molecule • ionic bond • chemical bond • Lewis dot diagram • valence • covalent bond • organic compound

3. What is a Compound? • A compound is a pure substance that contains two or more elements. • Compounds are chemical combinations of elements with properties that are different from the elements that formed them. • Table salt is formed when the elements sodium and chlorine combine (NaCl) – sodium is a soft metal, chlorine is a greenish yellow gas.

4. What is a Compound? • A chemical formula is an ingredient list for a compound that uses atomic symbols and subscripts. • The chemical formula for sucrose (sugar) is C12H22O11 • A neutral particle that forms as a result of electron sharing is a molecule.

5. Ionic Bonds and Ionic Compounds • The force that holds atoms together in a compound is called a chemical bond. • An ionic bond is an electrical attraction between positively and negatively charged ions in an ionic compound. • An ionic compound is one in which two or more elements or compounds gain or lose electrons and form ionic bonds.

6. Ionic Bonds and Ionic Compounds (cont’d) • The positive ion is usually a metal (like lithium) • The negative ion is usually a non-metal (like fluorine)

7. Ionic Bonds and Ionic Compounds (cont’d) • Lithium loses its outer electron and becomes a positively charged ion. • Fluorine gains the electron from Lithium and becomes negatively charged.

8. Ionic Bonds and Ionic Compounds (cont’d) • Because the Lithium and Fluoride ions now have an equally opposite charge (1+ and 1-) they are attracted to each other and form an ionic compound, Lithium fluoride (LiF) • Lithium fluoride is the simplest type of compound made only of two elements and known as a binary compound.

9. Ionic Bonds and Ionic Compounds (cont’d) • Properties of Ionic Compounds include: • Usually solids at room temperature • Brittle and break apart easily • High melting and boiling points • Many dissolve in water becoming good conductors of electricity.

10. Ionic Bonds and Ionic Compounds (cont’d) • Lewis dot diagrams are one method for using atomic symbols and dots representing the valence electrons to help predict how compounds will form. • The number of electrons in an atom’s outermost energy level are its valence electrons.

11. Ionic Bonds and Ionic Compounds (cont’d) • Chlorine has 7 valence electrons and needs to gain one more to become more stable. It will then have a 1- charge. • Argon is stable with a full valence shell and will neither gain or lose electrons.

12. Concepts in Motion- Electron Transfer and Sharing

13. Covalent Bonds- Sharing Electrons • A covalent bond is a chemical bond formed when atoms share electrons. • All organic compounds are covalent compounds based on carbon atoms. • Covalent compounds (molecular compounds) can be solids, liquids, or gases at room temperature.

14. Covalent Bonds- Sharing Electrons (cont’d) • Atoms that have unpaired electrons can form compounds if they share electrons, as it requires too much energy to gain or lose a large number of electrons. • Carbon has four unpaired electrons, and can therefore form four covalent bonds.

15. Concepts in Motion – Electron Sharing

16. Covalent Bonds- Sharing Electrons (cont’d) • Properties of Covalent Compounds include: • Can be solids, liquids, or gases at room temperature • Usually have lower melting and boiling points than ionic compounds • Do not usually separate in water • Most do not conduct electricity

17. Covalent Bonds- Sharing Electrons (cont’d) • Covalent bonds can consist of single bonds (one pair of electrons share),double bonds(two pairs of electrons shared- stronger than single bonds), and triple bonds(three pairs of electrons shared- stronger than double bonds).

18. 5-1 Lesson Review Questions Bromine is in Group 17. How many electrons does bromine need to gain or lose to obtain a noble gas structure? A gain one electron B gain two electrons C lose one electron D lose two electrons

19. 5-1 Lesson Review Questions Which element can form a negative ion? A carbon B magnesium C chlorine D lithium

20. 5-1 Lesson Review Questions What holds two elements together in an ionic bond? Acovalent bonds from shared electrons Belectron clouds combining Cunpaired electrons attracting each other Dopposite charges on negative and positive ions

21. Homework • Answer the three multiple choice lesson review questions. • Write a “Step-Up-To-Writing” style paragraph regarding the following: Suppose we were limited to the 100+ elements. “How would the world be different if atoms did not combine to form molecules of different compounds?”