Unit 2 Structures and Properties. Chapter 5 Chemical Bonding

Transcription

1 Section 5.1 Elements and Compounds Unit 2 Structures and Properties Chapter 5 Chemical Bonding LEWIS DOT DIAGRAMS (electron dot) show only the valence electrons (Group Number) around the chemical symbol. UNPAIRED ELECTRONS single electrons in the outer shell - they participate in bonding while electron pairs usually don t {good to explain that Lewis dot are done many different ways but we will stick to this one} ection 5.2 Bond Formation Group 8 elements are found alone in nature and do not naturally bond to any other atoms because all of its electrons are paired. ie its outer energy level is full. OCTET RULE atoms will gain/lose or share electrons to gain a noble gas configuration - usually refers to 8 but H and He are 2 CHEMICAL BONDS are attractive electrostatic forces that hold atoms together. There are three types of bonds: 1. Covalent Bonding - a COVALENT BOND is the electrostatic attraction between the nuclei of 2 atoms and a pair of shared electrons. - Individual units in covalent bonding are called molecules - the atoms SHARE electrons to follow the octet rule. - electrons that are shared between atoms are called SHARED PAIRS. - one pair of shared electrons makes one bond and is called a SINGLE COVALENT BOND - two pairs of shared electrons make two bonds and is called a DOUBLE COVALENT BOND - three pairs of shared electrons make three bonds and is called a TRIPLE COVALENT BOND - some covalent compounds bond in a special way to form continuous 3- dimensional arrays that are extremely strong (eg diamond). These are called NETWORK SOLIDS. Different shapes of a network solid of the same element are called ALLOTROPES (eg diamond and graphite)

2 2. Ionic Bonding IONIC BOND an electrostatic attraction between the positive and negative ions caused by the transfer of electrons Lewis structures: The oppositely charged ions are Attracted to one another by electrostatic forces. This attraction is called an IONIC BOND In an ionic bond there is no distinct pair of elements that can be identified. Each ion is attracted to many oppositely charges ions in a way that forms a 3-D array called a CRYSTAL LATTICE. The smallest neutral unit of an ionic compound is called the FORMULA UNIT. 3. Metallic Bonding METALLIC BOND an electrostatic attraction between the positively charged metal ions and the pool of valence electrons that moves freely among them. Properties of metals: - conduct electricity/heat - malleable - ductile - lustrous

3 Back to Section 5.1 / Section 6.3 Only a few elements exist in pure form, as elements. Most are found as compounds. Table 5.1 (p.160) Property Ionic Compound Covalent Compound Metal State of matter Solid Solid, liquid, or gas Solid-except Hg MP/BP High Low to high High Conducts electricity Yes Not usually Yes Solubility in water High Low to high No Derivation Metal and non-metal Non-metals Metal Representative Particle Formula unit Molecule Bond Formation Transfer electrons Share electrons Cation in sea of e -

4 Periodic Trends in Atomic Size Shielding Effect occurs when inner electrons help shield out electrons from the nuclear charge. Note: the shielding effect is not an issue within a period because the amount of inner electrons remains the same. (ie there are no extra rings ) Atomic Size Trend As you go down the periodic table energy levels or rings are being added. These rings become further apart because the out electrons are being shielded from the strong nuclear charge by the inner electrons. Therefore the atomic size increases. The increase in nuclear charge (while staying in the same energy level) pulls the outermost electrons (rings) tighter to the nucleus thus decreasing the atomic size. Periodic Trends in Ionization Energy IONIZATION ENERGY the energy to remove a single electron from a gaseous atom (to form an ion) Ionization energy decrease Because the outer ring Gets further away from The nucleus. Therefore, The nucleus has less pull On the electrons so it doesn t take as much energy to remove an electron. Since the nucleus has a greater attraction for the electrons, it is harder for the electrons to leave. Therefore, it increases the amount of energy it needs to leave

5 Periodic Trends in Ionic Size POSITIVE IONS are always smaller than the atoms from which they form. Explanation: the loss of outer electrons results in an increased attraction of the nucleus pulling the rings closer together. NEGATIVE IONS are always larger than the atoms from which they form. Explanation: the increased number of outer electrons decreases the effect of the nuclear attraction pushing the rings further apart. Section 5.3 Bonds as a Continuum ELECTRONEGATIVITY the ability of an atom to attract the shared electrons toward itself (when involved in a bond). Use Figure 5.17 for values. {may want to show trend rather than use figure 5.18} Electronegativity trend - TYPE OF BOND nonpolar covalent moderately polar covalent very polar covalent > ionic Problems: (use electronegativity values on p 174)

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