4.1 Atomic Theory and Bonding (Part 2)

1. 18 p 22 n What element is this? argon 4.1 Atomic Theory and Bonding (Part 2) • Bohr diagrams show how many electrons appear in each electron shell around an atom. • Electrons in the outermost shell (valence shell) are called valence electrons. • Think of the shells as being 3-D like spheres, not 2-D like circles. • It has 2 + 8 + 8 = 18 electrons, and therefore, 18 protons. • It has three electron shells, so it is in period 3. • It has eight electrons in the outer (valence) shell. See page 174 (c) McGraw Hill Ryerson 2007

2. 4.1 Atomic Theory and Bonding (Part 2) • Electrons appear in shells in a very predictable manner. • There is a maximum of two electrons in the first shell, eight in the 2nd shell, and eight in the 3rd shell. • The period number = the number of shells in the atom. • Except for the transition elements, the last digit of the group number = the number of electrons in the valence shell. • The noble gas elements have full electron shells and are very stable. See page 175 (c) McGraw Hill Ryerson 2007

3. 4.1 Atomic Theory and Bonding (Part 2) • When two atoms get close together, their valence electrons interact. • If the valence electrons can combine to form a low-energy bond, a compound is formed. • Each atom in the compound attempts to have the stable number of valence electrons as the nearest noble gas ( called a stable octet). • Metals may lose electrons and non-metals may gain electrons (ionic bond), or atoms may share electrons (covalent bond). 8 See pages 176 - 177 (c) McGraw Hill Ryerson 2007

4. 4.1 Atomic Theory and Bonding (Part 2) • Ionic bonds form when electrons are transferred from positive ions to negative ions. (c) McGraw Hill Ryerson 2007

5. 4.1 Atomic Theory and Bonding (Part 2) • Ionic bonds are formed between positive ions and negative ions. • Generally, this is a metal (+) and a non-metal (-) ion. • For example, lithium and oxygen form an ionic bond in the compound Li2O. + Electrons are transferred from the positive ions to negative ions Li+ O2- Li+ lithium oxide, Li2O oxygen lithium Draw a Bohr Diagram of NaCl…. See pages 176 - 177 hydrogen electrons are shared fluorine (c) McGraw Hill Ryerson 2007

6. 4.1 Atomic Theory and Bonding (Part 2) • Covalent bonds form when electrons are shared between two non-metals. • Electrons stay with their atom but overlap with other shells. Draw A Bohr Diagram of CO2 + Hydrogen fluoride See pages 176 - 177 hydrogen electrons are shared fluorine (c) McGraw Hill Ryerson 2007

7. 4.1 Atomic Theory and Bonding (Part 2) • Lewis diagrams illustrate chemical bonding by showing only an atom’s valence electrons and the chemical symbol. See page 178 (c) McGraw Hill Ryerson 2007

8. 4.1 Atomic Theory and Bonding (Part 2) • Lewis diagrams can be used to represent ions and ionic bonds. • For positive ions, one electron dot is removed from the valence shell for each positive charge. • For negative ions, one electron dot is added to each valence shell for each negative charge. • Square brackets are placed around each ion to indicate transfer of electrons. Draw A Lewis Diagram for Calcium Fluoride – 2+ – •• • • • • •• •• • • • • •• •• • • • • •• •• •• • • •• •• • • • • •• •• • • • • •• •• • • • • •• •• • • • • •• Be Cl Cl Be Cl Cl Be Cl Each beryllium has two electrons to transfer away, and each chlorine can receive one more electron. Since Be2+ can donate two electrons and each Cl– can accept only one, two Cl– ions are necessary. beryllium chloride See page 179 (c) McGraw Hill Ryerson 2007

9. 4.1 Atomic Theory and Bonding (Part 2) • Lewis diagrams can also represent covalent bonds. • Like Bohr diagrams, valence electrons are drawn to show sharing of electrons. • The shared pairs of electrons are usually drawn as a straight line. Bondingpair Lone pair Draw a Lewis diagram for carbon monoxide.. See page 179 (c) McGraw Hill Ryerson 2007

10. 4.1 Atomic Theory and Bonding (Part 2) • Diatomic molecules, like O2, are also easy to draw as Lewis diagrams. • Diatomic molecules are composed of only 2 atoms the same or different. Two of the same are called homonuclear. •• • • • • •• •• • • • • •• •• • • • • •• •• • • • • •• •• •• • • •• •• • • •• •• O O O O O O Several non-metals join to form diatomic molecules. Valence electrons are shared, here in two pairs. This is drawn as a double bond. THE END OF SECTION 4.1 See page 180 (c) McGraw Hill Ryerson 2007

11. PLO • C1: Differentiate between atom, ions, and molecules using knowledge of their structure and components • Define and give examples of ionic bonding ( e.g., metal and non-metal) and covalent bonding ( e.g., two non-metals, diatomic elements) (section 4.1) • With reference to elements 1 to 20 on the periodic table , draw and interpret Bohr models, including protons, neutrons, and electrons of, • -atoms ( neutral) • - ions (charged) • - molecules • - covalent bonding (e.g., O2, ­CH4) • - ionic compounds ( e.g., CaCl2) (section 4.1) (c) McGraw Hill Ryerson 2007

12. PLO continued • Identify valence electrons using the periodic table ( excluding lanthanides and actinides) (section 4.1) • distinguish between paired and unpaired electron's for a single atom • draw and interpret, Lewis diagrams showing single bonds for simple ionic compounds and covalent molecules ( e.g., NaCl, MgO, BaBr2, H2O, CH4, NH3) (section 4.1) • distinguish between lone pairs and bonding pairs of electrons in molecules (c) McGraw Hill Ryerson 2007