Chapter 2 - Chapter 2 Atoms and Molecules The Chemical...

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Chapter 2: Atoms and Molecules: The Chemical Basis of Life ELEMENTS AND ATOMS - elements- substances that can’t be broken down into simpler substances - Chemical symbols : usually first letter of English or latin name (ex: Sodium = Na b/c its Natrium in latin) - Oxygen, carbon, hydrogen and nitrogen - responsible for more than 96% of mass of most organisms - others are there in smaller quantities : Ca, P, K, Mg - trace elements- required in minute amounts -- only required in small amounts (ex: Iodine,Cu) - atom - smallest portion of element that retains chemical properties (can’t be seen under microscope but some scanning tunneling microscopy can take pictures of big atoms) - matter- anything that has mass and takes up space An Atom is uniquely identified by its number of protons - atomic number- # of protons, fixed amount - bohr model - shows arrangement of electrons, concentric circles.--inaccurate *space outside nucleus is huge compared to nucleus, electrons don’t travel in fixed concentric circles Protons plus neutrons determine atomic mass - subatomic particles measured in amu’s or Dalton’s - proton= 1 amu, neutron= 1 amu, electron = 1/1800 amu - atomic mass - protons + neutrons, mass of electrons ignored because its soo small Isotopes of an element differ in number of neutrons - isotopes - atoms of same element with different number of neutrons and different masses - mass of element is expressed by average of masses of isotopes (weighted by relative abundance) - have same chemical characteristics b/c they have same number of electrons - some isotopes are unstable and break down to stable isotope (usually become new elements) - radioisotopes - emit radiation when they decay - autoradiography - used to detect radioactive decay -- dark silver grains in photographic film - isotopes are interchangeable in molecules -- because they have the same properties - radioisotopes are useful for dating fossils because they can substitute for their isotopes * can also determine sequence of genetic info in DNA and trace biochemical pathways - radioisotopes are used in medicine to trace blood flow in arteries, test thyroid gland function, etc. - radiation interferes with cell division so it’s used to treat cancer b/c it can stop cancer cells from spreading Electrons move in orbitals corresponding to energy levels - orbitals - regions of 3D space, maximum 2 electrons - energy of electron depends on orbital it occupies - electrons in orbitals w/ similar energy are at the same principal energy level and make up an electron shell - the farther away from the nucleus, the more energy an electron has - valence electrons - most energetic electrons, occupy valence shell (outer shell) à key role in chemical rxns - electron can gain energy and move away from nucleus, or give up energy and move closer to nucleus - during photosynthesis: light energy absorbed causes electrons to get excited and move up an energy level CHEMICAL REACTIONS -chemical behavior of atom determined mostly by valence electron arrangement -
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