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Atomic Structure. History of the Atom & Atomic Structure. History of the Atom. 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 - PowerPoint PPT Presentation
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History of the Atom & Atomic Structure
Atomic Structure
History of the Atom
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!
History of the Atom
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.
History of the Atom
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. H2O exists, while H2.35O 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! Still thought atom indivisible!
History of the Atom
J.J. Thomson (1856-1940) Major Contribution: The Electron Cathode Ray Tube Experiment (1897)▪ Nobel prize (1906)
History of the Atom
J.J. Thomson
History of the Atom
Thomson’s Atomic Model
Also known as the Plum Pudding Model Was his theory correct?▪ No! Missing other parts of atom!
History of the Atom
Ernest Rutherford (1871-1937) Two Major Contributions:▪ The nucleus▪ The atom is mostly empty space
Gold Foil Experiment (1910)▪ Nobel prize in Chemistry (1908)
History of the Atom
Ernest Rutherford’s Gold Foil Experiment
History of the Atom
Rutherford’s Atomic Model
Was his theory correct?▪ Mostly! Missing neutrons and location of
electrons!
History of the Atom
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
History of the Atom
Atomic Line Spectra
History of the Atom
Bohr’s Atomic ModelElectrons
Nucleus
History of the Atom
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.
History of the Atom
Current Model of the Atom:Probability cloud where electrons found
Nucleus
Atomic Structure
Parts of the Atom Proton▪ Positive▪ Nucleus
Neutron▪ Neutral▪ Nucleus
Electron▪ Negative▪ Outside Nucleus
Atoms and the Periodic Table
Atoms are identified by their number of protons This is referred to as their atomic
number All atoms of the same element
have the same number of protons
Atoms & Charges
In atoms that have a neutral charge, the numbers of electrons equals the number of protons
Na Atom Na+ Ion
11 Protons 11 Protons
11 Electrons
10 Electrons
Cl Atom Cl- Ion
17 Protons 17 Protons
17 Electrons
18 Electrons
Mass of an Atom
The mass of an atom is the number of protons plus the number of neutrons This is also referred to as mass number
The mass of protons and neutrons are equal
Electrons and Mass of Atom 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.
Think of electrons like flies buzzing around an elephant (the nucleus)
Mass Number and Periodic Table
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
Isotopes
Isotopes are elements that have the same number of protons, but contains a different number of neutrons Example: carbon-12 and carbon-14▪ The number indicates the mass number▪ Both contain the same number of protons (6),
so carbon-14 must have two extra neutrons
Isotopes & Periodic Table
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!