Elements and Isotopes. 2-1 The Nature of Matter

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1 Elements and Isotopes An element's mass number (atomic mass) is the total number of protons and neutrons. Isotopes are atoms of the same element with a different number of neutrons. Most atomic masses reported on the periodic table are decimals because they are the weighted average of all the isotopes of that atom factoring in their abundance. Carbon is especially important for life as we know it and the study of biology because life is carbonbased, making up 19.4% the mass of living matter. Carbon forms organic compounds, the building blocks of life due to carbon s ability to form 4 covalent bonds with itself and other atoms. 5

2 Elements and Isotopes Isotopes are identified by their atomic mass. The isotopes of carbon are 12 (most common), 13 (least common) & 14. Isotopes have the same number of electrons. Isotopes have the same chemical properties. Some isotopes are radioactive due to their instability? Radioactive isotopes are scientifically useful in radiometric dating, as chemical tracers, for cancer treatment, food preservation, power generation, and inducing mutations. 6

3 Chemical Compounds A compound consists of atoms of 2 or more different elements chemically bonded in specific proportions and possessing properties different than the elements that make it up. Molecular oxygen (O 2 ) is not a compound, because it is made up of only one kind of atom. The composition of compounds is symbolized using a chemical formula. Write the chemical formulas for the following: Carbon Dioxide, Water, Hydrochloric Acid, Sulfuric Acid, Sodium Chloride, Hydrogen Peroxide, Glucose. 7

4 Ionic Bonds Sodium atom (Na) Sodium ion (Na+) Chlorine atom (Cl) Chloride ion (Cl - ) A neutral sodium atom has 11 electrons. A neutral chlorine atom has 17 electrons? The rings and distances known as energy levels or energy shells. The inner-most energy level appears to be able to hold a maximum of 2 electrons. The second and third energy levels appear to hold a maximum of 8 electrons. The outer most electrons that participate in a chemical bonding are called valence electrons and are found in the valence shell. When the neutral sodium atom loses an electron, its overall charge will be +1. When the neutral chlorine atom gains an electron, its overall charge will be -1. An atom with an overall charge is called an ion. The oppositely charged sodium and chlorine ions will be attracted to each other. This is an ionic bond. This is the weakest type of chemical bond. This is an ionic compound. The name of this compound is sodium chloride. The chemical formula is NaCl. There is no such thing as a molecule of an ionic compound. 8

5 Covalent Bonds Before binding with hydrogen, there are 6 valence electrons in a neutral oxygen atom. Two more electrons would fill oxygen's valence energy level. Before binding with oxygen there is 1 valence electron in each neutral hydrogen atom. One more electron would fill hydrogen's valence energy level. As shown in the figure, oxygen can fill its valence energy level by sharing 1 electron each with 2 hydrogen atoms. The hydrogen atoms can fill their valence energy level by sharing 1 electron with oxygen. Each atom is sharing 2 electrons with another atom. This is called a single covalent bond. Covalent bonds are stronger than ionic bonds. The chemical formula for this compound is H 2 O. Compounds formed by covalent bonds are molecular compounds. When it involves sharing four or six electrons, they are referred to as double and triple covalent bonds, respectively. 9

6 Van der Waals Forces Van der Waals Forces are attractions between MOLECULES due to unequal distribution of charges. Van der Waals Forces are weaker than even ionic bonds. This Tokay Gecko is able to cling to smooth surfaces, like glass, and walk upside down on the ceiling due to the Van der Waals Forces between the MOLECULES at the tip of microscopic bristles on its toes and the MOLECULES of the surfaces it is climbing on. Although weak, the additive forces of millions of bristles is surprisingly strong! 10