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Dalton's Atomic Theory
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Definition
1.Each element is composed of tiny indestructible particles called atoms.
2.All atoms of a given element have the same mass and other properties that distinguish them from the atoms of other elements.
3. Atoms combine in simple, whole-number ratios to form compounds. |
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Plum Pudding Model of the atom (1904)
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| In the model suggested by J. J. Thomson, negatively charged electrons (yellow) were held in a sphere of positive charge (red). |
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| Rutherford: Nuclear theory of the atom |
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1.Most of the atom’s mass and all of its positive charge are contained in a small core called the nucleus.
2.Most of the volume of the atom is empty space through which the tiny, negatively charged electrons are dispersed.
3. The number of negatively charged electrons outside the nucleus is equal to the number of positively charged particles (protons) inside the nucleus, so that the atom is electrically neutral. |
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Positive charge in the nucleus of the atom
Mass 1.0073 amu- very similar to neutron mass
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Neutral charge- in nucleus of atom
Mass= 1.0087 amu
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Negative charge- empty space surrounding nucleus
Mass very minute- .00055amu |
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•Dmitri Mendeleev, a Russian chemistry professor, proposed from observation that when the elements are arranged in order of increasing relative mass, certain sets of properties recur periodically.
Periodic law set by observation |
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•occupy the left side of the periodic table and have similar properties:
–Metals are good conductors of heat and electricity.
–Metals can be pounded into flat sheets (malleability).
–Metals can be drawn into wires (ductility).
–Metals are often shiny (lustrous).
–Metals tend to lose electrons when they undergo chemical changes. |
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•occupy the upper right side of the periodic table.
•The dividing line between metals and nonmetals is the zigzag diagonal line running from boron to astatine.
–Nonmetals have more varied properties; some are solids at room temperature, while others are gases.
–As a whole, nonmetals tend to be poor conductors of heat and electricity.
–Nonmetals tend to gain electrons when they undergo chemical changes. |
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•lie along the zigzag diagonal line dividing metals and nonmetals.
–Metalloids, also called semimetals, display mixed properties.
–Metalloids are also called semiconductors because of their intermediate electrical conductivity, which can be changed and controlled.
–This property makes semiconductors useful in the manufacture of electronic devices that are central to computers, cell phones, and other modern gadgets.
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Sum of protons and neutrons
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| The number of neutrons in an isotope is a difference between the mass number and the atomic number |
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(Fraction of isotope 1 × Mass of isotope 1) +
(Fraction of isotope 2 × Mass of isotope 2) +
(Fraction of isotope 3 × Mass of isotope 3) + …
•Naturally occurring chlorine consists of
75.77% chlorine-35 (mass 34.97 amu) and 24.23% chlorine-37 (mass 36.97 amu).
Its atomic mass is:
(.7577 x 34.97 amu) + (.2423 x 36.97 amu)= 35.45 amu
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