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Chapter 4: Section 1
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The Atom is the smallest particle of an element
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The Atomic theory grew as a series of models that developed from experimental evidence. As more evidence was collected, the theory and models were revised.
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All elements are composed of atoms that cannot be divided
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All atoms of the same element are exactly alike and have the same mass. Atoms of different elements are different and have different masses
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An atom of one element cannot be changed into an atom of another element.
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Atoms cannot be created or destroyed in any chemical change, only rearranged ◦ Law of ____________ __ ____
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Every compound is composed of atoms of different elements, combined in a specific ratio
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Discovered that atoms had negatively charged particles and reasoned that the atom had to contain positively charged particles because the atom is electrically neutral
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Inferred that an atom’s positive charge must be clustered in a tiny region in its center called a nucleus
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Electrons moved around the nucleus He also named protons
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Theory came from Gold foil experiment. (see figure 4 & 5, pages 104 & 105) some of the alpha particles had run into massive concentrations of positive charge and, since like charges repel, had been hurled straight back by them
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Showed that electrons could only have specific amounts of energy, leading them to move in certain orbits around the nucleus
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Electrons do not orbit the nucleus like planets, but can be anywhere in a cloud like region around the nucleus
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An electron’s movement is related to its energy level. The specific amount of energy the electron has Electrons of different energy levels are found in different places.
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James Chadwick discovered the neutrons, which are found in the nucleus, they have no charge and almost as much mass as the protons
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The model consists of an atom made up of a nucleus which contains the protons and neutrons surrounded by a cloudlike region of moving electrons contained in energy levels
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Protons are shown by a plus sign(1+), AMU = 1
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Electrons are shown by a minus sign(1-), AMU = 1/ 1, 836
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◦ Neutrons are shown by a zero sign(0), ◦ AMU = 1
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Every atom of an element has the same number of protons
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The atomic number identifies the element
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Since an element is electrically neutral, the atomic number also equals the number of electrons.
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The number of neutrons in an atom can vary.
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◦ Atoms with the same number of protons but different numbers of neutrons are called isotopes. ◦ Figure 10, page 108
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An isotope is identified by its mass number
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The sum of protons and neutrons in the nucleus
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◦ Number of neutrons can be found by subtracting the atomic number FROM the Mass number. Mass number minus Atomic number = Neutron number
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