The Structure of Matter: Compounds & Molecules Chemical bond - the attractive force that holds atoms or ions together Chemical bond - the attractive force that holds atoms or ions together Chemical structure – the arrangement of bonded atoms or ions within a substance Chemical structure – the arrangement of bonded atoms or ions within a substance

Compounds & Molecules 2 Bond length – average distance between nuclei of two bonded atoms Bond length – average distance between nuclei of two bonded atoms Bond angle – angle made by 2 bonds to the same atom Bond angle – angle made by 2 bonds to the same atom

Compounds & Molecules 3 What’s the difference between compounds and mixtures? What’s the difference between compounds and mixtures? Figure out the relative #s of atoms or ions in a compound from its chemical formula. Figure out the relative #s of atoms or ions in a compound from its chemical formula.

Compounds Vs. Mixtures 1 Compounds have very different properties from the elements that make them. Compounds have very different properties from the elements that make them. Here, elements lose solo properties when they bond together. Here, elements lose solo properties when they bond together.

Compounds Vs. Mixtures 2 Compounds = same substances or different ones that are chemically bonded to each other like O 2 or the H’s & O’s in two molecules of water, 2H 2 O. Compounds = same substances or different ones that are chemically bonded to each other like O 2 or the H’s & O’s in two molecules of water, 2H 2 O. Reactions that form these involve breaking of bonds & rearrangement of atoms into new bonds. Reactions that form these involve breaking of bonds & rearrangement of atoms into new bonds.

Compounds Vs. Mixtures 3 Mixtures = different substances that are just placed together not bonded and still keep their individual properties. Mixtures = different substances that are just placed together not bonded and still keep their individual properties. Compounds have chemical formulas which show the types & numbers of atoms or ions that make up their simplest units. Compounds have chemical formulas which show the types & numbers of atoms or ions that make up their simplest units.

Compounds Vs. Mixtures 4 Compounds always have the same elements in the same combinations. Compounds always have the same elements in the same combinations. Consider C 6 H 12 O 6 for glucose, a product of photosynthesis. Consider C 6 H 12 O 6 for glucose, a product of photosynthesis.

Compounds Vs. Mixtures 5 Anywhere that one finds a molecule of glucose in the universe, this compound will have 6 carbon, 12 hydrogen & 6 oxygen atoms all chemically bonded together. Anywhere that one finds a molecule of glucose in the universe, this compound will have 6 carbon, 12 hydrogen & 6 oxygen atoms all chemically bonded together.

Chemical Structure: Bonding Within Compounds Chemical formulas don’t show how atoms are connected Chemical formulas don’t show how atoms are connected In other words, they do not reveal bond angles & bond lengths between atoms. In other words, they do not reveal bond angles & bond lengths between atoms.

Bonding Within Compounds 2 However, models do show bond lengths and angles. However, models do show bond lengths and angles. Structural formulas can show bond angles too but not bond lengths. Structural formulas can show bond angles too but not bond lengths.

Models of Compounds 3 Ball-and-stick models give an idea of bond lengths and bond angles. Ball-and-stick models give an idea of bond lengths and bond angles. Space filling models show the space that bonded atoms occupy well. Space filling models show the space that bonded atoms occupy well.

Models of Compounds 4 However, space filling models make it difficult to “see” the bond lengths and angles between the atoms in a compound. However, space filling models make it difficult to “see” the bond lengths and angles between the atoms in a compound.

Structure Affects Properties 1 Network structure includes the same bond angles throughout the compound and multiple bonds on repeating centers. Network structure includes the same bond angles throughout the compound and multiple bonds on repeating centers. This results in strong solids with unusually strong bonds that take more energy to break. This results in strong solids with unusually strong bonds that take more energy to break.

Structure Affects Properties 2 For example in quartz, 4 oxygen atoms bond each silicon atom at 109.5˚ angles. For example in quartz, 4 oxygen atoms bond each silicon atom at 109.5˚ angles. High melting points (mp) & boiling points (bp) result from network structures b/c it takes more heat energy to break stronger bonds. High melting points (mp) & boiling points (bp) result from network structures b/c it takes more heat energy to break stronger bonds.

Structure Affects Properties 3 Networks can be made of bonded positive and negative ions in cube shaped crystals. Networks can be made of bonded positive and negative ions in cube shaped crystals. Molecular or covalent compounds have weaker bonds & as a result lower melting & boiling points. Molecular or covalent compounds have weaker bonds & as a result lower melting & boiling points.

Structure Affects Properties 4 Ionic compounds – bonds between ions where these charged atoms have given away or taken electrons. Ionic compounds – bonds between ions where these charged atoms have given away or taken electrons. Covalent compounds – bonds between atoms where these atoms share electrons. Covalent compounds – bonds between atoms where these atoms share electrons.

Structure Affects Properties 5 How do weaker bonds cause lower mp and bp? How do weaker bonds cause lower mp and bp? Different structures in molecular compounds result in different mp and bp even among grouped compounds. Different structures in molecular compounds result in different mp and bp even among grouped compounds.

Structure Affects Properties 6 Consider Table 4-2 on page 113 and Figure 4-8 on page 114. Consider Table 4-2 on page 113 and Figure 4-8 on page 114. Structure affects even the strength of attractions beween molecules. Structure affects even the strength of attractions beween molecules. For example, intermolecular attraction between molecules of H 2 0 is stronger than attraction between those of H 2 S, dihydrogen sulfide. For example, intermolecular attraction between molecules of H 2 0 is stronger than attraction between those of H 2 S, dihydrogen sulfide.

Structure Affects Properties 7 What do the decreasing mp and bp for sugar (C 12 H 22 O 11 ), H 2 0 & H 2 S in this order tell us about their relative attractions for each other? What do the decreasing mp and bp for sugar (C 12 H 22 O 11 ), H 2 0 & H 2 S in this order tell us about their relative attractions for each other? Rank them in order of least to greatest intermolecular attraction. Rank them in order of least to greatest intermolecular attraction.

Structure Affects Properties 8 Notice that sugar & water have very different stuctures w/ expectedly different properties. Notice that sugar & water have very different stuctures w/ expectedly different properties. However, water & dihydrogen sul- fide have more similar structures than properties. Why is this? However, water & dihydrogen sul- fide have more similar structures than properties. Why is this?

Structure Affects Properties 9 Hydrogen bonding – extra strong intermolecular attractions between hydrogen and oxygen atoms in nearby compounds. Hydrogen bonding – extra strong intermolecular attractions between hydrogen and oxygen atoms in nearby compounds.