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Topic IX Counting atoms Bell Ringer Complete the following reaction: H2SO4+ Ba(OH)2 BaSO4(s) + 2H2O (l) Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting atoms Counting Atoms Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting atoms Symbol you will see to represent an element A Z X A – mass number Z – atomic number X- symbol of the element Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting Atoms Atomic Number- Review • Atoms of the same element all have the same number of protons. • The atomic number (Z) of an element is the number of protons of each atom of that element. • The mass number is the total number of protons and neutrons that make up the nucleus of an isotope. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting atoms Atomic Number and mass number Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting atoms Isotopes • Atoms of the same element that have different masses. • The isotopes of a particular element all have the same number of protons and electrons but different numbers of neutrons. • Most of the elements consist of mixtures of isotopes. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting Atoms Relative Atomic Masses • The standard used to compare units of atomic mass is the carbon-12 atom, which has been arbitrarily assigned a mass of exactly 12 atomic mass units, or 12 amu. • One atomic mass unit, or 1 amu, is exactly 1/12 the mass of a carbon-12 atom. • The atomic mass of any atom is determined by comparing it with the mass of the carbon-12. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting Atoms Relating Mass to Numbers of Atoms The Mole- “The Chemist’s Dozen” • SI unit to describe quantity. mole (abbreviated mol) • It is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. Avogadro’s Number • Avogadro’s number = 6.02 1023 – Number of particles in exactly one mole of a pure substance. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting Atoms Relating Mass to Numbers of Atoms Molar Mass • The mass of one mole of a pure substance is called the molar mass of that substance. • Molar mass is usually written in units of g/mol. • The molar mass of an element is numerically equal to the atomic mass of the element. Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Topic IX Counting Atoms Relating Mass to Numbers of Atoms Gram/Mole Conversions • Chemists use molar mass as a conversion factor in chemical calculations. • For example, the molar mass of helium is 4.00 g He or 1 mol He 1 mol He 4.00 g He Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved. Mole conversions Molar mass in grams = 1 MOLE = 6.02 X1023 particles Samples to try: moles and grams, moles and atoms, atoms and grams Chapter menu Resources Copyright © by Holt, Rinehart and Winston. All rights reserved.
