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Atomic Structure Chemistry Ms. Piela

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Democritus (460 BC 360 BC) Ancient Greek philosopher No experiments performed! Major Contribution: The Atom He proposed that everything was made of these atoms and they were all indivisible Was his theory correct? No! There are subatomic particles!

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John Dalton (1766-1844) Major Contribution: Atomic Theory (1808) This began the modern era of chemistry Four Principles: Elements are composed of indivisible particles called atoms. All atoms of a given element are identical.

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John Dalton Compounds are composed of atoms of one or more elements, and will form only in whole-number ratios. This is called the Law of Multiple Proportions i.e. H 2 O exists, while H 2.35 O does not A chemical reaction involves the combination, separation, or rearrangement of atoms, not their creation or destruction This is called the Law of Conservation of Mass Was his theory correct? Mostly! Parts 1 & 2 have problems!

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John Dalton When we attempt to conceive the number of particles in an atmosphere [gas], it is somewhat like attempting to conceive the number of stars in the universe; we are confounded with the thought. But if we limit the subject, by taking a given volume of any gas, we seem persuaded that the number of particles must be finite - John Dalton on his approach to the theory of atoms, 1808

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J.J. Thomson (1856-1940) Major Contribution: The Electron Cathode Ray Tube Experiment (1897) Nobel prize (1906)

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J.J. Thomson

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Thomsons Atomic Model Also known as the Plum Pudding Model Was his theory correct? No! Missing parts of atom!

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Ernest Rutherford (1871-1937) Two Major Contributions: The nucleus The atom is mostly empty space Gold Foil Experiment (1910) Nobel prize in Chemistry (1908)

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Ernest Rutherfords Gold Foil Experiment

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Rutherfords Atomic Model Was his theory correct? Mostly! Missing neutrons and location of electrons!

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Niels Bohr (1885-1962) Major Contribution: Planetary Model of the Atom Nobel Prize in Physics (1922) for spectrum of hydrogen Atomic Line Spectra Bohr observed that when light was given off from an atom, there were only single lines visible Bohr proposed that each line represented an electron in a different orbit

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Atomic Line Spectra

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Bohrs Atomic Model Electrons Nucleus

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Current Theory of the Atom Many scientists contributed to developing quantum mechanics, which is the current model of the atom Known as the electron cloud model The cloud is an area of probability where the electron is found These electrons, moving at extremely high speeds, effectively occupy the entire area of the cloud, the same way that moving fan blades effectively occupy the entire area through which they pass.

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Current Model of the Atom: Probability cloud where electrons found Nucleus

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Parts of the Atom Proton Positive Nucleus Neutron Neutral Nucleus Electron Negative Orbitting Nucleus

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Atomic Number Atoms are identified by their number of protons This is referred to as their atomic number Think of atomic # like a social security card for each element

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In atoms that have a neutral charge, the numbers of electrons equals the number of protons When an atom gains a charge, it is called an ion Sodium Ion (Na + ) 11 Protons10 Electrons Sodium Atom (Na) 11 Protons11 Electrons

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Chlorine Ion (Cl - ) 17 Protons18 Electrons Chlorine Atom (Cl) 17 Protons17 Electron

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Mass Number The mass of an atom is the number of protons plus the number of neutrons This is referred to as mass number The mass of protons and neutrons are equal

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Why is the electron not part of the mass? It takes roughly 1800 electrons to equal the mass of 1 proton, so it is left out. If we wanted the mass of an elephant, would we weigh the flies buzzing around it? Doesnt count!

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The periodic table does not give the mass number, but always the atomic number For simplicity, we round the number on the periodic table to get the mass number Round this number to whole number

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Isotopes are elements that have the same number of protons, but contains a different number of neutrons Compare: carbon-12 vs. carbon-14 The number indicates the mass number Both contain the same number of protons (6), so carbon-14 must have two extra neutrons

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The masses given on the periodic table are an average of all the isotopes on the planet We refer to the masses on the periodic table as the average atomic mass of an element This explains why the atomic masses are not whole numbers it is an average!