Chapter 2

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University of Toronto Scarborough
Biological Sciences
Mark Fitzpatrick

BIOA01 J LEC3 CHAPTER 2 N All matter is composed of atoms N mass of one electron - 9 1028 N Protons + Neutrons = Atomic Mass N Protons + Electrons = Charge N Elements J pure substance Type Charge Mass (da) Info Provided containing only on kind of atom N Elements are arranged in the Proton +1 1 Atomic number periodic table N Six main elements J Hydrogen, Neutron 0 1 Isotope Carbon, Nitrogen, Oxygen, and Phosphorus Electron -1 negligible Reactivity N The number of protons identifies an element N Number of protons = atomic number N All elements except hydrogen have one or more neutrons N Mass number is the number of protons + number of neutrons N Mass number = mass of atom if Daltons N Isotopes J forms of an element with different numbers of neutrons, thus different mass numbers N Atomic weight: average of mass numbers of isotopes in their normally occurring proportions N Radioisotopes are unstable; they give off energy in the form of alpha, beta, and gamma radiation from the nucleus. This radioactive decay transforms the atom, including changes in the number of protons Energy from radioactive decay can interact with surrounding material Radioisotopes can be incorporated into molecules and Z^2_}oo N Reactivity of atoms = electrons o The number of electrons determines how atoms will interact o Chemical reactions involve changes in the distribution of electrons between atoms N Locations of electrons in an atom are described by orbitals. o Orbital J region where electron is found at least 90 percent of the time. o Orbitals occur in series called electron shells or energy levels. o Orbitals are filled in a specific sequence. N Reactive atoms have unpaired electrons in their outermost (valence) shell. N Atoms can share electrons, loose or gain electrons, resulting in atoms bonded together to form molecules.
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