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3.3: Counting Atoms

3.3: Counting Atoms. How do I count and calculate the mass of atoms?. Counting Atoms. Atoms are far too small to be counted individually, so we use the concept of a weighted average. Suppose you need 1000 mints. Each mint weights 1 gram. How could you gather 1000 mints

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3.3: Counting Atoms

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  1. 3.3: Counting Atoms How do I count and calculate the mass of atoms?

  2. Counting Atoms • Atoms are far too small to be counted individually, so we use the concept of a weighted average. • Suppose you need 1000 mints. • Each mint weights 1 gram. • How could you gather 1000 mints without actually counting each individual mint?

  3. Atomic Number • Atoms of different elements have different numbers of protons. • Atoms of the same element all have the same number of protons!!! • Theatomic number (Z) of an element is the number of protons of each atom of that element. It also indicates the number of neutrons (in a normal atom). • Atomic number identifies an element! • For all neutral atoms: protons = electrons

  4. Isotopes • Isotopesare atoms of the same element that have different masses. • Meaning they have varying number of neutrons. • Most elements exist as a mixture of different isotopes. Three isotopes of hydrogen

  5. Mass Number • Themassnumberis the total number of protons and neutrons that make up the nucleus of an isotope.

  6. Designating Isotopes • Hydrogen isotopes are unusual in that they have names. • Isotopes are usually identified by specifying their mass number. • Two methods for specifying isotopes: • Hyphen notation hydrogen-3 ( 3 is the mass number (number of p+ and n0)) • Nuclearsymbol Uranium-238

  7. Finding the number of particles in Isotopes Example: How many protons, electrons, and neutrons are there in an atom of chlorine-37? • The mass number is 37 • It indicated the number of protons and neutrons in the nucleus

  8. Solution We know: • atomic number of chlorine = number of protons and = number of electrons (atomic number is found on the periodic table.) • mass number = protons + neutrons # neutrons = mass number - protons

  9. Solution • Number of protons = atomic number = 17 • Number of electrons = atomic number = 17 • Number of neutrons = mass number – atomic number = 37 – 17 = 20 Protons = 17 Electrons = 17 Neutrons = 20

  10. Learning Check – Left side • How many protons, electrons, and neutrons are there in an atom of bromine-80? • Write the nuclear symbol for carbon-13 • Write the hyphen notation for the isotope with 15 electrons and 15 neutrons.

  11. Learning Check- Left side Fill in the following table.

  12. Relative Atomic Masses • The mass of an individual atom, expressed in grams, is very, very small. • One atom of oxygen-16 = 2.656 x 10-23 g. = .00000000000000000000002656 g • So, the standard measurement chemists use atomic mass units or amu. • Oneamu= 1/12 the mass of a carbon-12 atom. Atomic Mass (amu)

  13. Average Atomic Masses of Elements • Why are atomic masses not whole numbers? • Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element.

  14. Calculating Average Atomic Mass • What is the average atomic mass, in amu, of copper? Naturally occurring copper consists of 69.15% copper-63, which as an atomic mass of 62.929 601 amu; and 30.85% copper-65, which has an atomic mass of 64.927 794. .6915 (62.929601) + .3085 (64.927794) = • When you average the masses and adjust for percentage, you get an average atom’s mass of63.55 amu.

  15. Summary • Atomic number = # p+ = # e- • Mass # - atomic # = # n0 • # of neutrons varies with the isotope

  16. How many atoms are in a given mass of a compound? • How can we relate the number of atoms in a sample to a samples mass? • We need an equality statement to use in the conversion of mass to # of atoms when we use dimensional analysis. • Our equality statements are: • The Mole • Avogadro’s Number • Molar Mass

  17. The Mole • Amole, mol, is the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12. • A counting unit. • 1 mol = 6.022 x 1023 atoms

  18. Avogadro’s Number • Avogadro’snumber – 6.022 141 79 x 1023 – is the number of particles in exactly one mole of a pure substance. • IT IS RIDICULOUSY LARGE!!!!! 602,214,179,000,000,000,000,000 • If every person living on Earth (6 billion) worked to count the atoms in one mole of an element, and if each person counted continuously at a rate of atom per second, it would take about 3 million years for all the atoms to be counted.

  19. Molar Mass • The mass of one mole of a pure substance is called the molarmassof that substance. • Units of molar mass = g/mol • Numerically equal to the atomic mass found on the periodic table. • What is the molar mass of Lithium?

  20. Comparison of 1-Mol Samples

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