Atomic Theory and Structure Overview Organizing Matter Relative Masses Atomic History and Theory The Modern Periodic Table Structure of an Atom Ions Atomic Mass Isotopes

Organizing Matter As scientists were discovering different elements, it became apparent that they needed a way to organize them At first they were organized by their atomic mass Dimitri Mendeleev – the “founder” of the periodic table, did this.

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Relative Masses Carbon was found to be 12X as heavy as Hydrogen ____________ Hydrogen has such a small mass it becomes difficult to compare masses relative to it Instead, we compare the relative atomic mass of all elements to that of Carbon-12 Therefore, we define the unified atomic mass unit (u) as of the mass of a carbon-12 atom _____________ So… H has a mass of 1 u, C is 12 u and O is 16 u

Mendeleev’s Periodic Law Arranging elements in order of increasing atomic mass worked relatively well, but some discrepancies resulted: For example: K and Ar. The mass of Potassium is less than that of Ar. Today, elements on the periodic table are arranged by increasing Atomic Number (number of protons)

Dalton’s Atomic Theory – The Ball Model: 1800s Referred to the atom as a small, hard, indestructible sphere that cannot be subdivided An atom is …………… An element is a substance made up of only 1 type of atom ____________

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Ernest Rutherford’s Gold Foil Experiment - 1911 “+” charged particles (alpha particles) were fired at a thin sheet of gold foil ____________

What he thought would happen given the “Plum Pudding” Model... Some alpha particles would be slightly diverted due to their ……………….. The above diagram shows what we would expect the result of Rutherford’s experiment to be if the “plum pudding” model of the atom is correct.

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Rutherford’s Findings Most of the particles passed right through A few particles were deflected _____________ Egads! It’s like shooting bullets at tissue paper and watching them bounce back

The Rutherford Atomic Model Based on his experimental evidence he concluded that the atom: Is mostly empty space Contains a small, dense, positively charged nucleus at the centre ____________

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The Bohr-Rutherford Model ____________ Electrons move in orbits (energy levels) around the nucleus Electrons more or less stay in their orbit unless an increase in energy causes them to move

Modern Periodic Law Elements arranged in order of increasing atomic number and show a periodic repetition of properties

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Check Your Understanding Determine the number of protons, neutrons, and electrons for the following elements. *Round each atomic mass to the nearest whole number* _____________ 13 Al 26.98 27 - 13 protons = ____________ 79 Au 196.97 ______________ 197 - 79 = ______________ ________________ 99 Es 253.08 253 - 99 = _____________ _______________ 40 Zr 91.22 91 – 40 = ____________

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Atomic Mass The number of protons in the nucleus determines the identity of the element and is referred to as the ___________ ___________

Calculating Atomic Mass An element’s atomic mass (A), having an atomic number (Z) and a specific number of neutrons (N) ____________ Most of the atomic masses on the periodic table are not integers (1, 2, 3, etc.). __________ Isotopes are elements that have the same number of protons ____________ Therefore, they have a different atomic mass

Isotopes Periodic table states Average Atomic Masses, which take into account _______________of isotopes of each element, as well as ___________ Symbol Isotope Protons Neutrons Electrons Abundance 35Cl Chlorine-35 17 18 75.77% 37Cl Chlorine-37 20 24.23%

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Calculating Average Atomic Mass When calculating average atomic mass, the fraction of each isotope must be determined Typically, we are given abundance in a percentage. So we need to convert to a decimal before using it to calculate atomic mass *AM represents atomic mass*

Check Your Understanding Calculate …………………. Isotope Relative Atomic Mass Abundance (%) C-12 12 98.89 C-13 13.003 1.11 Given Information: Equation: Solve Therefore……………..

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Radioisotopes Isotopes that are radioactive Radioactive isotopes release radiation as they decay and change into other isotope of a given element These isotopes are unstable emit and radiation as their nucleus changes _____________

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What are Isotopes used for? ______________ Deuterium is the isotope of hydrogen with a nucleus of one proton and one neutron. It occurs naturally in water to a small concentration The main use at present is as heavy water, which is deuterium oxide (equivalent to H2O), _________________