 This is the Greek philosopher Democritus who began the search for a description of matter more than 2400 years ago.  He asked: Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided? 400 BC

Democritus asserted that space contained an infinite number of particles Named atomos, or "indivisible” Atoms are eternal and invisible; absolutely small, so small that their size cannot be diminished; totally full and incompressible. Atoms are homogeneous, differing only in shape, arrangement, position, and number

 To Democritus, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.  Atoms were infinite in number, always moving and capable of joining together.

 He deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.  Atoms of the same element are exactly alike.  Atoms of different elements are different.  Compounds are formed by the joining of atoms of two or more elements in specific ratios.

 Used Cathode Ray tube to determine the presence of – (electrons) and + (protons) particles.

 Plum Pudding Model -- the structure of an atom is something like pudding. He assumed that the basic body of an atom is a spherical object containing electrons & protons randomly confined in homogeneous jellylike material. Positive charges cancel the negative charges.

 The Japanese Nagaoka imagined the atom to be like the planet Saturn. He thought it had a positively charged center, with electrons in orbit around it’s middle. There was a problem with this picture of the atom. A charged particle, such as an electron, moving in a circle, radiates electromagnetic waves. As it radiates, it loses energy. Unless it can get energy from some other source, it will slow down and follow a spiral path into the center of the circle. A Saturnian atom would have collapsed.

 Atoms have a central positive nucleus surrounded by negative orbiting electrons.  This idea was the result of his famous Gold Foil Experiment(see next slide). This experiment involved the firing of radioactive alpha particles through gold foil.  This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space.  Most of particles passed straight through the foil but approximately 1 in 8000 were deflected.

 The Bohr Model is probably familiar to us as the "planetary model" of the atom is used to symbolize atomic energy.  Electrons orbit the nucleus much like planets orbiting the Sun.  However, the orbits are not confined to a plane as is approximately true in the Solar System).

 James Chadwick discovered a third type of particle, which he named the Neutron.Neutron  Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons protonsnucleus  Neutrons always reside in the nucleus of atoms and they are about the same mass and size as protons. Neutronsnucleusatomsprotons  Neutrons do not have any electrical charge; they are electrically neutral Neutronsneutra

 Based on the work of many scientists  Based on the mathematical approach of Quantum Mechanics  Electrons are assigned regions of space ( Orbitals ) not pathways ( Orbits )  Electrons are moving around the nucleus rapidly in no predictable path producing a cloud of e-’s over time. Think of a rapidly moving fan blade.

Electron Cloud

 Number of protons in an atom is ALWAYS equal to the Atomic Number

 Atoms – Protons and electrons are equal  Ions  Loss of electron makes positive ions  Gain of electrons makes negative ions

 Loss of Electrons  Atom Ion  Na  Na e- (1e- removed)    0 net +1 NET CHARGE  Gain of Electrons  Cl 2 + 2e -  2Cl - (2e- added to Cl 2 )    0 net -1 NET CHARGE

 Atomic Mass (rounded to integer)  - Atomic Number   Number of Neutrons in the nucleus  Atomic Mass – Atomic Number = # Neutrons

 Protons (P+) – always equal to Atomic #  Electrons (e-)  Atoms – Same as the # of Protons  Ion – Net charge after e-’s have been lost or gained in an attempt to become stable (HAPPY NUMBERS!)  Loss of e-’s = Positive charge  Gain of e-’s = Negative charge  Neutrons (N±) = Atomic Mass – Atomic #

 Energy Levels  Maximum  K Shell = 2 e-’s  L Shell = 8 e-’s  M Shell = 8 e-’s  N Shell = 18 e-’s  When you get to Chemistry, you will find that this gets more involved than shown here.

 How many protons(+) are present?  How many electrons(-) are present?  How many neutrons(±) are present?  In what energy levels are the e-’s found?

 How many protons(+) are present?  Atomic number = 8 = protons  How many electrons(-) are present?  In atoms, p+ = e- therefore 8 e-’s  How many neutrons(±) are present?  At. Mass – At. # = 16 – 8 = 8 neutrons  In what energy levels are the e-’s found?  8e-’s total = 2 in K shell, 6 in L shell

 How many protons(+) are present?  How many electrons(-) are present?  What is the charge of the ion?  How many neutrons(±) are present?  In what energy levels are the e-’s found?

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