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PreAP Chemistry Chapter 3

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PreAP Chemistry Chapter 3. Democritus was the early Greek philosopher who is credited with the concept of the atom (atomos) –which means invisible. Democritus was the early Greek philosopher who is credited with the concept of the atom (atomos) –which means invisible

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Democritus was the early Greek philosopher who is credited with the concept of the atom (atomos) –which means invisible

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Democritus was the early Greek philosopher who is credited with the concept of the atom (atomos) –which means invisible

Law of Definite Proportions states that the elements in a specific compounds always contain the same proportions by mass.

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Law of Multiple Proportions states that if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

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Dalton is an English school teacher who proposed the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

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Dalton is an English school teacher who proposed the law of conservation of mass, the law of definite proportions, and the law of multiple proportions.

He is also known as the Father of the Atomic Theory

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Dalton’s atomic theory

1.All matter is composed of very small particles called atoms

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Dalton’s atomic theory

  • All matter is composed of very small particles called atoms
  • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in these properties.
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Atoms cannot be subdivided, created, or destroyed

  • 4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
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Atoms cannot be subdivided, created, or destroyed

  • 4.Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  • 5.In chemical reactions, atoms are combined, separated, or rearranged.
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Two aspects of Dalton’s atomic theory proven to be incorrect:

  • We now know atoms are divisible.
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Two aspects of Dalton’s atomic theory proven to be incorrect:

  • We now know atoms are divisible.
  • b. Atoms of the same element can have different masses.
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Two regions of an atom

1. Nucleus -- small region near the center of the atom.

Composed of two particles—the proton which is positively charged and the neutron which is neutral (zero) charged.

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2. Electron Cloud --composed of negatively charged electrons. It is very large in comparison to the nucleus.

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2. Electron Cloud --composed of negatively charged electrons. It is very large in comparison to the nucleus.

Subatomic particles—protons, neutrons, electrons

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2. Electron Cloud --composed of negatively charged electrons. It is very large in comparison to the nucleus.

Subatomic particles—protons, neutrons, electrons

Cathode Ray tubes — the instrument used in the discovery of the electron

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J. J. Thomson is the man credited with the discovery of the electrons in the late 1800’s

Millikan discovered the mass of the electrons

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Knowledge of electrons led to two inferences about atomic structure:

1.Because atoms are electrically neutral, they must contain positive charge to balance the negative electrons.

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Knowledge of electrons led to two inferences about atomic structure:

1.Because atoms are electrically neutral, they must contain positive charge to balance the negative electrons.

2.Because electrons have so little mass, atoms must contain other particles to account for most of their mass

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Nucleus of the atom—discovered by Lord Ernest Rutherford

Gold foil experiment—actually done by Hans Geiger and Ernest Marsden

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Observations:

  • Majority of the alpha (α)particles penetrated foil undeflected.
  • b.About 1 in 20,000 were slightly deflected
  • c.About 1 in 20,000 were deflected back to emitter
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Conclusions: 1. Mass of the atom and the positive charge are concentrated in small regions called nucleus

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Conclusions: 1. Mass of the atom and the positive charge are concentrated in small regions called nucleus

2. Most of the atom is empty

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Conclusions: 1. Mass of the atom and the positive charge are concentrated in small regions called nucleus

  • Most of the atom is empty
  • 3.Magnitude of charge on the nucleus is different for different atoms
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4. Number of electrons outside the nucleus = number of units of nuclear charge (to account for the fact that the atom is electrically neutral)

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4. Number of electrons outside the nucleus = number of units of nuclear charge (to account for the fact that the atom is electrically neutral)

Atoms are electrically neutral because they contain equal numbers of protons and electrons

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1. What kinds of particles are being emitted by the radioactive source? What is their charge?

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What kinds of particles are being emitted by the radioactive source? What is their charge?

  • Alpha Particles, positive
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What happens when the charged particles strike the surface of the gold foil?

  • Most go straight through
  • Some were slightly deflected
  • Some were greatly deflected
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What was the purpose of the fluorescent screen?

  • See where the alpha particles hit
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What did Rutherford conclude from this gold foil experiment regarding the amount of empty space in the atom? Why was he able to conclude this?

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What did Rutherford conclude from this gold foil experiment regarding the amount of empty space in the atom? Why was he able to conclude this?

  • Most of the atom is empty space because most of the particles went straight through.
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How does the deflection path of a charged particle that strikes the center of a gold atom differ from that path of a particle that passes only near the center?

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How does the deflection path of a charged particle that strikes the center of a gold atom differ from that path of a particle that passes only near the center?

  • If it hits the center it is deflected back to the emitter. If it passes near the center it is slightly deflected.
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Based on this experiment, where is most of the positive charge and mass of an atom found?

  • Nucleus – in the center
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What would have been observed if the positive charge of an atom had been thinly spread out throughout the atom?

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What would have been observed if the positive charge of an atom had been thinly spread out throughout the atom?

  • All the alpha particles would have behaved the same – gone straight through.
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Nuclear Forces the short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together

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Nuclear Forces the short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together

Mass of one proton = mass of neutron = mass of 1837 electrons

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Atomic number—the number of protons in the nucleus of the atom.

--number of protonsidentifies the

element and is equal to the number of electrons (of a neutral atom)

--symbol is Z

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Isotopes are atoms of the same element that have different masses because they have different numbers of neutrons but they still have similar chemical properties

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Isotopes are atoms of the same element that have different masses because they have different numbers of neutrons but they still have similar chemical properties

Mass number – approximate atomic mass of an atom (nucleus)

-- the number of protons + neutrons

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Nuclide is the general term for any isotope of any element

Atomic Mass Unit (amu) is exactly 1/12 the mass of a carbon-12 atom

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Average atomic mass is the weight average of the atomic masses of the naturally occurring isotopes of an element.

Ave. Atomic mass = %abundace(mass of isotope 1) + %abundance(mass of isotope 2) +…..

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Moles

Mole (mol) is the amount of a substances that contains as many particles as there are atoms in exactly 12 g of carbon-12

(Mole comes from the German word for “pile”)

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Avogadro’s number – 6.022 ×1023 – is the number of particles in exactly 1 mole of a pure substance. Just as one dozen = 12 somethings, one mole = 6.022 ×1023 somethings.

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The mass of one mole of a pure substance (element or compound) is called the molar mass of that substance. The unit is

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