Chapter 2 cont’ Atoms and Elements Recall: Atomic Number Number of protons Z Mass Number Protons + Neutrons Whole number A Abundance = relative amount (in %) found in a sample

Neon %221210Ne-22 or 0.27%211110Ne-21 or 90.48%2010 Ne-20 or Percent Natural Abundance A, Mass Number Number of Neutrons Number of ProtonsSymbol

Atomic Mass we previously learned that not all atoms of an element have the same mass ◦ isotopes we generally use the average mass of all an element’s atoms found in a sample in calculations we call the average mass the atomic mass 3

Mass Spectrometry masses and abundances of isotopes are measured with a mass spectrometer atoms or molecules are ionized, then accelerated down a tube their path is bent by a magnetic field, separating them by mass ◦ similar to Thomson’s Cathode Ray Experiment a mass spectrum is a graph that gives the relative mass and relative abundance of each particle relative mass of the particle is plotted in the x-axis relative abundance of the particle is plotted in the y-axis 4

Example Magnesium has three naturally occurring isotopes with masses of amu, amu, and amu and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium Lithium has two naturally occurring isotopes: lithium-6 and lithium-7. If the average atomic mass of lithium is amu, which isotope is the most abundant? How do you know?

Collection Terms A collection term states a specific number of items. 1 dozen donuts = 12 donuts 1 ream of paper = 500 sheets 1 case = 24 cans 2 In chemistry, how do chemists know the number of atoms in an element or compound?

A Mole of Atoms A mole is a unit of measurement used in chemistry to express amounts of a chemical substance, the same number of particles as there are carbon atoms in 12.0 g of carbon. a collection term “dozen” 3 Avogadro’s Number = x mole = N A = x of anything

Relationship Between Moles and Mass The mass of one mole of atoms is called the molar mass The molar mass of an element, in grams, is numerically equal to the element’s atomic mass, in amu 8 E.g 1 H atom = 1.01 amu 1 mol H = x atoms of H = ???? g

Examples Give the molar mass for each A. 1 mole of Li atoms =________ g B.1 mole of Co atoms =________g C.1 mole of S atoms= ________g 9

Examples Assuming all pennies are pure copper and each has a mass of 2.5 g ◦ Without doing calculation, determine the number atoms of copper present in 1 mole ◦ How many pennies does it take to make a mole?

Examples In a 3.0 moles of O 2 molecules ◦ How many oxygen molecules are there ? ◦ How many oxygen atoms are there? Calculate the moles of carbon in g of pencil lead Calculate the mass (in grams) of moles of titanium

Converting between mass, moles and atoms g C mol C g C mol C atoms