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BHS Chemistry Fall 2013 • Except for the nucleus of H simplest type of hydrogen atom, all atomic nuclei are made of two kinds of particles, protons and neutrons. • Proton has a positive charge equal in magnitude to the negative charge of an electron. Atoms are electrically neutral because they contain equal numbers of protons and electrons. • A neutron is neutral. Particle Electron Charge Mass # -1 0 Proton +1 1 Location Electron cloud Nucleus Neutron 0 1 Nucleus Protons and neutrons are densely packed into the nucleus. Electrons orbit around the nucleus in a loose electron cloud. (click) Note the positive charge of the nucleus and the negative charge of the electron cloud. Also, note that overall, the charges cancel! The periodic table contains information about each element. Atomic Number The number of protons 1 H Hydrogen 1.00797 Symbol Name Atomic Mass The number of protons + neutrons Atomic number (Z) of an element is the number of protons in the nucleus of each atom of that element. Element Carbon Phosphorus Gold # of protons 6 15 Atomic # (Z) 79 79 6 15 •Elements are placed in order of increasing atomic number •Atomic number identifies an element •Ex: all silver atoms contain 47 protons Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p+ + n0 Symbol p+ n0 e- Mass # Oxygen O 8 8 8 16 Sulfur S 16 16 16 32 Tin Sn 50 69 50 119 Element *So how can you calculate the number of neutrons in an element? Simple! Take the mass number and subtract the atomic number! Tin: 119-50 = 69 *If there is no charge indicated, the number of electrons is the same as the number of protons! mass # = protons + neutrons • always a whole number • NOT on the Periodic Table! © Addison-Wesley Publishing Company, Inc. Isotopes are atoms of the same element having different masses due to varying numbers of neutrons. (all have same number of protons and electrons but different number of neutrons) Look at Fig 3-9 in book (p 76) Isotope Protons Electrons Neutrons Hydrogen–1 (protium) 99.985% 1 1 0 Hydrogen-2 (deuterium) 0.015% 1 1 1 Hydrogen-3 (tritium) radioactive 1 1 2 Nucleus © Addison-Wesley Publishing Company, Inc. 235 92 U Isotopes are atoms of the same element having different mass number due to varying numbers of neutrons Isotopes are usually identified by specifying their mass number. Two methods for specifying isotopes: The mass number is written with a hyphen after the name of the element ex: hydrogen-3 is tritium Show the composition of a nucleus as the isotopes nuclear symbol ex: uranium-235 is written (Superscript is mass number, subscripts is atomic number) 235 92 U Mass number – atomic number = number of neutrons Mass number (p+ + no) 235 92 U Atomic number (number of p+) Element symbol Chlorine-37 atomic #: mass #: # of protons: # of electrons: # of neutrons: 17 37 17 17 20 37 17 Cl Average Atomic mass is the average of all the naturally isotopes of that element. Isotope Carbon-12 Carbon = 12.011 Symbol Composition of % in nature the nucleus 12C 6 protons 98.89% 6 neutrons Carbon-13 13C 6 protons 7 neutrons 1.11% Carbon-14 14C 6 protons 8 neutrons <0.01% For most chemical calculations it is more convenient to use relative atomic masses Standard used by scientists to govern units of atomic mass is the carbon-12 nuclide. Assigned a mass of 12 atomic mass units (12 amu) One atomic mass unit, or 1 amu, is exactly 1/12 the mass of a carbon-12 atom. 12C atom = 1.992 × 10-23 g • atomic mass unit (amu) • 1 amu = 1/12 the mass of a 12C atom • 1 p = 1.007276 amu 1 n = 1.008665 amu 1 e- = 0.0005486 amu © Addison-Wesley Publishing Company, Inc.