Alone they were not enough to stop an alpha particle

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Alone they were not enough to stop an alpha particle
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What he got
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How he explained it Atom is mostly empty Small dense, positive piece at the center Alpha particles are deflected if they get close enough to positive center
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Niels Bohr (1885-1862) Electrons have orbits about the nucleus (planetary theory) Electrons could only exist at given energy levels An energy level is where an electron is likely to be moving Energy levels were like steps on a ladder An electron can only be at any given step at any given time
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Modern Atomic Theory Bohr Model—shows electrons in orbit around protons and neutrons Quantum-mechanical model— doesn’t show exact location of electrons, just probable place
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Structure of the Atom There are two regions The nucleus Protons and neutrons Positive charge Almost all of the mass Electron cloud Most of the volume of an atom Region where electron can be found
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Subatomic particles
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Counting the pieces Atomic number = number of protons Same as the number of electrons in a neutral atom Mass number = the number of protons + neutrons
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Atomic Mass Unit AMU Mass of a proton = 1.67 x 10 -27 g A pretty inconvenient number New unit referenced to mass of an isotope of carbon: carbon -12 Carbon-12 has 6 protons and 6 neutrons Has a mass of 12.00000 amu – an atomic mass unit Therefore 1 proton and 1 neutron has a mass of 1 amu.
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So why not whole numbers for atomic masses in periodic table? Reported numbers are average atomic mass units, reflecting the abundance of isotopes for any given number. In nature most elements occur as a mixture of two or more isotopes
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Isotopes Atoms of the same element can have different numbers of neutrons Different mass numbers Called isotopes
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