bio study guide - 1 ? .Whatcausedthe...

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1. Presently about 22% of the Earth's atmosphere is oxygen but at the time of  the first appearance of life there was virtually no oxygen.  What caused the  increase in oxygen concentration? Why is it important for organisms that the  oxygen concentration is at its present level?  -The evolution of cyanobacteria, photosynthetic prokaryotes led to the  build of oxygen in the atmosphere because oxygen is a product of  photosynthesis.  Most organisms utilize aerobic respiration which  requires oxygen.  2. Define the terms atomic number , mass number , atomic mass .  What elements  are represented by the chemical symbols C, H, O, N, S, P, Na, K, Cl, Ca, Fe,  Mg Cu, B?  1. - Atomic number - the number of protons in an element.  Mass number -  total number of protons and neutrons in an atom. Atomic mass-  approximately the mass number of an atom expressed in daltons.   Carbon, hydrogen, oxygen, nitrogen, sulfur, phosphorus, sodium,  potassium, chlorine, calcium, magnesium, copper, boron. 3. What are isotopes and radioisotopes? What are the three naturally occurring  isotopes of carbon? Which isotope is most common and which isotope is  radioactive?  1. - Isotopes are atoms with the same atomic number but with different mass  numbers.  Radioisotope is an isotope in which the nucleus is unstable  and is decaying with the release of energy and particles. Carbon  isotopes:  12 C,  13 C,  14 C.   12 C is the most common and  14 C is a radioisotope.
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4. Define covalent, polar covalent and ionic chemical bonds.  Give and example  of a molecule with each type of bond.  How can you determine the number of  covalent bonds a particular type of atom will form? Explain using the  examples O 2 and CO 2 .     1. - A covalent bond is formed between two atoms that share electrons  equally.  A polar covalent bond is formed between two atoms that  share electrons unequally so that one atom is relatively more negative  than the other atom. An ionic bond is formed between two atoms  when one atom completely pulls an electron from the second atom and  the two atoms are held together because one is now negatively charged  (containing an extra electron) and the other is positively charged  (because it has lost an electron).   Covalent bond - gaseous hydrogen,  H 2  or oxygen, O 2 . Polar covalent bond - H 2 O.  Ionic - NaCl. In general,  the number of covalent bonds an atom can form is equal to the number  of unpaired electrons in has in its outer shell. Oxygen has an atomic  number of 8 and therefore it has 6 electrons in its outer shell.  It has 
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