2ObjectivesExplain the role of atomic number in determining the identity of an atom.Define an isotope and explain why atomic masses are not whole numbers.Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.
3Atomic NumberMoseley ( ) discovered that atoms of each element contain a unique number of positive charges in their nuclei.Therefore, it was concluded that the number of protons identifies an atom as an atom of a particular element.The number of protons in an atom is referred to as an element’s atomic number.
4Atomic NumberAtomic number determines an element’s position in the periodic table - from left to right and top to bottom, elements are arranged by increasing atomic number.CarbonElement nameATOMIC NUMBERChemical symbolAverageatomic mass6C12.011
5Atomic Number Atomic number = number of protons = number of electrons Recall that atoms are neutral.The number of protons, therefore, MUST be equal to the number of electrons.Atomic number = number of protons = number of electronsCarbon, for example, has an atomic number of 6 and 6 protons and 6 electrons.
7IsotopesRecall that Dalton was wrong about atoms being indivisible. He was also wrong when he stated that all atoms of an element are identical.All atoms of an element DO have the same number of protons and electrons. The number of neutrons, however, MAY DIFFER from atom to atom.Atoms that have the same number of protons but different numbers of neutrons are called isotopes.
83 Isotopes of Hydrogen Each has 1 proton. They differ in the number of neutrons they contain.Because of this, each atom has a different mass.Note that the isotopes of hydrogen have special names.
9Isotopes Elements are found in nature as a mixture of isotopes. The percent abundance of the isotopes (percent of each isotope in a sample) is constant no matter where the sample is found.Isotopes have the same chemical behavior because chemical behavior is determined by the number of ELECTRONS an element has.
10Isotopes and Mass Number To make it easy to identify the various isotopes of an element, scientists add a number after the element’s name.This number is the mass number - the sum of the number of protons and neutrons in the nucleus of an atom of the isotope.Mass # = # protons + # neutrons
11ExampleOxygen-17oxygen has an atomic number of 8, sooxygen-17 has 8 protons and electrons.Since mass # = # protons + # neutrons, then# neutrons = mass # - #protons. So, in oxygen-17, # neutrons = or 9.Oxygen-17 has 8 protons, 8 electrons, and 9 neutrons.
12Isotopes Chemists use another shortened notation to identify isotopes. Mass numberIsotopesAtomic numberChemists use another shortened notation to identify isotopes.The chemical symbol for the element is used. Written to the left of the symbol, a superscript indicates the mass number and a subscript indicates the atomic number.
14Atomic Mass UnitsRecall that the masses of protons, neutrons, and electrons are VERY small numbers. These numbers are difficult to work with.Scientists therefore developed a way to measure their mass relative to a standard atom (the carbon-12 atom).One atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.
15Atomic Mass Units The masses of the particles in terms of amu are: electron = amu (We round to 0 amu)proton = amu (We round to 1 amu)neutron = amu (We round to 1 amu)Note that the mass of a proton & neutron is slightly different; also, correct masses of atoms will not be whole numbers but decimals.
16Atomic MassThe atomic mass of an element is the weighted average mass of the isotopes of that element.Atomic mass, therefore, takes into account the percent abundance of each isotope as well as the mass of an atom of that isotope.The isotope in greater abundance will have a greater effect on determining the atomic mass.
17Analyzing Atomic MassAnalyzing an element’s atomic mass can SOMETIMES give you a clue as to what the most abundant isotope for that element is.The atomic mass of fluorine is amu. From this you can conclude that most fluorine occurs in the form of fluorine-19.
18Calculating Atomic Mass Multiply the % abundance of each isotope by its mass.34.969(75.77%) =amu36.966(24.23%) =Add the mass contributionsof all the isotopes together.amu is the atomic mass of chlorine.Chlorine Chlorine-37Atomic mass: amu Atomic mass: amu% abundance: 75.77% % abundance: 24.23%
19Practice Problems magnesium-24: atomic mass 23.985, % abundance 78.99% Use the following information to calculate the atomic mass of magnesium:magnesium-24: atomic mass , % abundance 78.99%magnesium-25: atomic mass , % abundance 10.00%magnesium-26: atomic mass , % abundance 11.01%Use the following information to calculate the atomic mass of X; then identify X.6X = amu; 7.5% abundance7X = amu; 92.5% abundance