Physical Science 8th grade (1).pdf

As a good example aspirin acetylsalicylic acid is

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of which it is made. As a good example, aspirin (acetylsalicylic acid) is made from carbon, hydrogen, and oxygen according to the chemical formula H 8 C 9 O 4 (Figure 8.4). This compound has the property of relieving swelling and reducing pain in humans. Properties depend on the exact chemical formula By themselves, the elements (H, C, O) do not have the property of reducing pain. Other molecules formed from the same elements have very different properties than aspirin. For example, polyethylene plastic wrap and formaldehyde (a toxic preservative) are also made from carbon, oxygen, and hydrogen. The beneficial properties of aspirin come from the specific combination of exactly 8 hydrogen, 9 carbon, and 4 oxygen atoms. If the ratio of elements was changed, for example removing even one hydrogen, the resulting molecule would not have the properties of aspirin. Properties also depend on molecular structure The structure of a molecule is also important to the properties of a compound. The same 21 atoms in aspirin can be combined in other structures with the same chemical formula! The resulting molecules are something completely different (diagram above) and do not have the beneficial properties of aspirin. Both chemical formula and structure determine the properties of a compound . Figure 8.4: An aspirin molecule. Like many modern medicines, the active ingredient in aspirin was first discovered in nature. In fact, aspirin’s pain-relieving properties were known and used in its natural form long before scientists learned of it. Research the discovery of aspirin to find out the intriguing story of this widely-used medicine.
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162 U NIT 3: A TOMS AND THE P ERIODIC T ABLE Figure 8.5: Sodium chloride is an ionic compound in which each positive sodium ion is attracted to all of its negative chloride neighbors and vice versa. ion - an atom that has an electric charge different from zero. Ions are created when atoms gain or lose electrons. ionic bond - a bond that transfers an electron from one atom to another resulting in attraction between oppositely charged ions . Ionic compounds An ion is a charged atom Not all compounds are made of molecules. For example, sodium chloride (NaCl) is a compound of sodium (Na) and chlorine (Cl) in a ratio of one sodium atom per chlorine atom. The difference is that in sodium chloride, the electron is essentially transferred from the sodium atom to the chlorine atom. When atoms gain or lose an electron they become ions . An ion is a charged atom. By losing an electron, the sodium atom becomes a sodium ion with a charge of +1. By gaining an electron, the chlorine atom becomes a chloride ion with a charge of -1 (when chlorine becomes an ion, the name changes to chlor ide ). Ionic bonds Sodium and chlorine form an ionic bond because the positive sodium ion is attracted to the negative chloride ion. Ionic bonds are bonds in which electrons are transferred from one atom to another.
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