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 Isotopes have the same number of electrons and protons, but a different number of neutrons Most elements consist of a mixture of isotopes

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

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

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,

Isotopes of Hydrogen

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,

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 Complete worksheet here on atomic notation

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)

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

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

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

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

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

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

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)

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

Gram/Mole Conversions The molar mass can be used as a conversion factor see the board and the book for sample problems What is the mass in grams of 3.50 mol of copper? 222g Cu A chemist produced 11.9 g of Al. How many mol of Al were produced? 0.441 mol Al

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