CHEM 101 Introduction to Fundamental Chemistry. Spring Quarter 2008 SCCC. Lecture 17.

Transcription

1 CHEM 101 Introduction to Fundamental Chemistry Spring Quarter 2008 SCCC Lecture 17

2 Forces Between Particles Noble Gas Configurations Ionic Bonding Ionic Compounds Naming Binary Ionic Compounds The Smallest Unit of Ionic Compounds Covalent Bonding People have been doing admirably well with what is tricky material. The section on covalent bonding will take some practice before you feel comfortable with the material.

3 We write a simple ion by first writing the element symbol followed by the charge of the ion as a superscript. e.g. Li +, Mg 2+, Al 3+, Fe 2+, Cu 2+, Cl -, S 2-, O 2-,etc Cations are named by giving the name of the parent metal atom and adding the word ion. e.g. Li + lithium ion, Mg 2+ magnesium ion, Al 3+ aluminum ion, etc

4 Some metal atoms from the d block and groups IIIA-VA can form more than one ion. e.g. Element Iron Chromium Cobalt Copper Common Ionic Forms Fe 2+, Fe 3+ Cr 2+, Cr 3+, Cr 6+ Co 2+, Co 3+ Cu +, Cu 2+ When we write the names of these ions we need to specify the charge by putting it in Roman numerals after the element name. Cr 2+ Chromium(II) ion, Co 3+ Cobalt(III) ion, etc

5 Anions are named by adding the suffix ide to the stem name of the parent non-metal atom and adding the word ion. The stem name is typically the first syllable of the atom name (see table 4.2). e.g. Br - bromide ion, Cl - chloride ion, O 2- oxide ion, S 2- sulfide ion and P 3- phosphide ion.

6 Binary ionic compounds are named by giving the name of the cation first followed by the name of the anion and dropping all occurrences of the word ion. e.g. FeO Iron(II) oxide, NaCl Sodium chloride What is the name of the binary ionic compound formed between magnesium and chlorine? What is its formula? Magnesium chloride MgCl 2

7 In our early lectures we defined a molecule as the limit of physical subdivision. Molecules exist as particles containing the number of atoms specified by their formula. e.g. a water molecule is a particle containing 2 hydrogen atoms and one oxygen atom and has the formula H 2 O.

8 The molecular weight (mass of 1 mole) of a water molecule is equal to the sum of the atomic weight of oxygen plus two times the atomic weight of hydrogen. M = x = gmol -1

9 Ionic compounds do not exist as discrete molecules. Instead they exist as crystals where ions of opposite charges occupy positions known as lattice sites. Ions combine in the ratio that results in zero charge to form ionic compounds. NaCl

10 A quantity called the formula weight may be obtained by adding the atomic weights of the ions in the formula of an ionic compound. e.g. The formula weight of MgCl 2 is FW = x35.45 = gmol -1 Formula weights are used in a similar way to molecular weights.

11 Non-metals may also complete their octets by sharing electrons. This may occur between atoms of the same type: e.g. H 2, O 2, N 2, Cl 2, F 2, I 2, etc Or between different types of atoms: e.g. CO 2, H 2 O, CH 4, etc

12 Consider two hydrogen atoms separated by a large distance. Each has 1 electron in a 1s atomic orbital Now lets bring the two atoms together so there orbitals overlap.

13 The atomic orbitals overlap to form a new molecular orbital. This is a stable configuration as each H atom can have a full 1s susbshell (like He) where the electrons spend most of their time shared between the atoms. In this arrangement each nucleus feels an inwards attraction to the two electrons. This is called covalent bonding.

14 We can draw Lewis diagrams showing the arrangement of valence electrons in covalent compounds. In these diagrams we represent each pair of electrons between atoms as a line. So for the H 2 molecule discussed previously the Lewis diagram would be: H H All other electrons are represented by dots as described previously.

15 I use a five step method to draw the Lewis diagram for a covalent molecule formed between non-metals. 1. Add up the valence electrons and write this number down. e.g. For CH 4 C has 4 valence electrons Each H has 1 valence electron 8 valence electrons total.

16 2. Write down the central atom this is the first atom in the formula. e.g. for CH 4 C

17 3. Connect each of the outer atoms to the central atom with single lines (each one electron pair). Subtract the number of electrons used from the total number of electrons. e.g. for CH 4 H H C H H 8-(4x2)=0

18 4. If there are any electrons left over add electrons as lone pairs to the outer atoms until their octets are complete. Subtract these electrons from the total. 5. If there are still electrons left over add these as lone pairs to the central atom. If the octet of the central atom is not complete then we need to add electrons from the outer atoms to form double and triple bonds until its octet is complete.

19 HOME WORK READ Prepare for Lab (pre-lab due 9am tomorrow) Continue revision for midterm.