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Counting Atoms

Counting Atoms. Section 3.3. Atomic Number. Atomic number: the number of protons of each atom Represented by “ Z ” The atomic number identifies the element. Isotopes. Isotopes: atoms of the same element that have different masses

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Counting Atoms

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  1. Counting Atoms Section 3.3

  2. Atomic Number • Atomic number: the number of protons of each atom • Represented by “Z” • The atomic number identifies the element

  3. Isotopes • Isotopes: atoms of the same element that have different masses • Isotopes have the same number of electrons and protons, but a different number of neutrons • Most elements consist of a mixture of isotopes

  4. Mass Number • Mass number: the total number of protons and neutrons that make up the nucleus of an isotope • Mass number = p+ + n0 • Isotopes are usually identified by specifying their mass number

  5. Designating Isotopes • There are two methods to specify isotopes • Method 1 is called hyphen notation: write the name of the element, then a hyphen, then the mass number • Method 2 is called the nuclear symbol: in front of the symbol, superscript the mass number and subscript the atomic number

  6. Hydrogen • Hydrogen is special in that it also has specific names for each isotope • Hydrogen with only 1 p+ and no n0 is called protium, hydrogen-1, • Hydrogen with 1 p+ and 1n0 is called deuterium, hydrogen-2, • Hydrogen with 1 p+ and 2 n0 is called tritium, hydrogen-3,

  7. Isotopes of Hydrogen

  8. Practice: name the isotope both ways • helium with 1 neutron • carbon with 7 neutrons • oxygen with 8 neutrons • uranium with 142 neutrons • helium-3, • carbon-13, • oxygen-16, • uranium-234,

  9. More Practice • How many protons, electrons, and neutrons are there in an atom of chlorine-37? • 17 electrons, 17 protons, 20 neutrons • How many protons, electrons, and neutrons make up an atom of bromine-80 • 35 protons, 35 electrons, 45 neutrons

  10. Relative Atomic Masses • If we express the mass of atom in grams, it would be extremely small • Relative mass is used instead • Carbon-12 is the standard atom and has been assigned a mass value of exactly 12 atomic mass units (amu)

  11. More about Relative Atomic Mass • All other atomic masses are determined by comparing it with the mass of the carbon-12 atom • See the chart on page 80 for some typical atomic masses • The masses could also be expressed by adding up the mass of the e-s, p+s, and n0s

  12. Average Atomic Mass • Average atomic mass: the weighted average of the atomic masses of the naturally occurring isotopes of an element • The average atomic mass depends on both the mass and the relative abundance of each of the element’s isotopes

  13. Calculating Average Atomic Mass • Copper consists of 2 naturally occurring isotopes • A sample of copper contains 69.15% of copper-63 (62.929601 amu) and 30.85% of copper-64 (64.927794 amu) • Change the percentages to decimals

  14. Continued • Solve like this: • (0.6915)(62.929601 amu ) + (0.3085)(64.927794 amu)= 63.55 amu • The average atomic mass of copper is 63.55 amu • Always round to 2 decimal places

  15. The Mole • The SI unit for the amount of a substance • Mole: the amount of a substance that contains as many particles as there are atoms in exactly 12 g of carbon-12 • It is a counting unit

  16. Avogadro’s Number • Avogadro’s number: the number of particles in exactly one mole of a pure substance; 6.022 x 1023 particles

  17. Molar Mass • Molar mass: the mass of one mole of a pure substance • Usually written in units of g/mol • The molar mass of an element is numerically equal to the atomic mass of the element in atomic mass units (found on the periodic table)

  18. More about Molar Mass • Molar mass is usually rounded to two decimal places • What is the molar mass of carbon? • 12.01 g/mol • What is the molar mass of chlorine? • 35.45 g/mol

  19. Gram/Mole Conversions • The molar mass can be used as a conversion factor • see the board and the book for sample problems

  20. Conversions with Avogadro’s Number • Since 6.022 x 1023 is the number of particles in a mole, that can be used as a conversion factor • See the board and your book for sample problems

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