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Chemistry-140 Lecture 4. Chapter Highlights atomic theory structure of the atom; electrons, protons, neutrons atomic mass, mass number and isotopes atomic number periodic table allotropes. Chapter 2: Atoms & Elements. Chemistry-140 Lecture 4. Philosophy.

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Chapter 2: Atoms & Elements


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Chemistry-140 Lecture 4

  • Chapter Highlights
  • atomic theory
  • structure of the atom; electrons, protons, neutrons
  • atomic mass, mass number and isotopes
  • atomic number
  • periodic table
  • allotropes

Chapter 2: Atoms & Elements

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Chemistry-140 Lecture 4

Philosophy

Chemistry is a pyramidal science, that is, all higher learning draws on the material already absorbed.

A large base of knowledge provides a stable platform for understanding higher concepts.

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Chemistry-140 Lecture 4

  • Each element is composed of small particles called atoms.
  • All atoms of a given element are identical
  • Atoms are neither created nor destroyed in chemical reactions.
  • Compounds are formed when atoms of more than one element combine

Dalton’s Atomic Theory of Matter

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Chemistry-140 Lecture 4

  • Law of constant composition: all samples of a given substance must have the same composition (#4).
  • Law of conservation of matter: matter cannot be created nor destroyed, only rearranged (#3).
  • Law of multiple proportions: if two samples have different compositions, they must be samples of two different substances.

Law’s From Dalton’s Theories

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Chemistry-140 Lecture 4

  • Combinations of elements are in
  • ratios of small whole numbers.
  • In forming water, 8.0 g of oxygen combines with 1.0 g of hydrogen.
  • In forming hydrogen peroxide 16.0 g of oxygen combines with 1.0 g of hydrogen.
  • Using atomic theory, we conclude that hydrogen peroxide contains twice as many atoms of oxygen per hydrogen.
  • H2O2 versus H2O

Consequences of Dalton’s Theories

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Chemistry-140 Lecture 4

  • Cathode rays were discovered in the 1800s and were found to be beams of electrons streaming from the negatively charged cathode toward the positively charged anode.
  • curvature is related to the mass and velocity of the particles

The Discovery of Atomic Structure

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Chemistry-140 Lecture 4

The Discovery of Atomic Structure

  • J. J. Thompson: A balancing of the magnetic and electrical fields allows measurement of the charge-to-mass ratio of the electron
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Chemistry-140 Lecture 4

The Discovery of Atomic Structure

  • Robert Millikan: a balancing of gravity with attraction to the positively charged plate allowed measurement of the charge on the electron (and the mass of the electron)
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Chemistry-140 Lecture 4

  • Radioactivity is the spontaneous emission of radiation from a substance. eg. X-rays, g-rays, a-particles, and b-particles
  • X-rays and g-rays are high-energy light
  • a-radiation is a stream of helium nuclei, He2+ known asa- particles.
  • b-radiation is a stream of high speed electrons that originate in the nucleus (b-particles)

Radioactivty and Atomic Theory

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Chemistry-140 Lecture 4

The Nuclear Atom

  • Most of the mass and all of the positive charge was concentrated in the nucleus while the negative charge and most of the volume was accounted for by the electrons outside the nucleus.
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Chemistry-140 Lecture 4

  • Subsequent experiments led to the discovery of both positive (protons) and neutral (neutrons) particles in the nucleus.
  • Protons were discovered in 1919 by Rutherford.
  • Neutrons were discovered by the British scientist James Chadwick in 1932

The Nuclear Atom; Continued

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Chemistry-140 Lecture 4

  • In a chemical context,
  • there are three important subatomic particles
  • The proton is found in the nucleus
  • charge = + 1.602 x 10-19 C (+1 electronic charge)
  • mass = 1.67262 x 10-24 g (1.0073 amu)
  • The neutron is found in the nucleus
    • charge = 0
    • mass = 1.67495 x 10-24 g (1.0087 amu)
  • The electron is found outside the nucleus
  • charge = -1.602 x 10-19 C(-1 electronic charge)
  • mass = 9.10939 x 10-28 g (5.486x10-4 amu)

Modern View of Atomic Structure

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Chemistry-140 Lecture 4

  • The heaviest atom has a mass of only 4 x 10-22 g
  • and a diameter of only 5 x 10-10 m.
  • Useful units:
  • 1 amu (atomic mass unit) = 1.66054 x 10-27 kg
  • 1 pm (picometer) = 1 x 10-12 m
  • 1 Å (Angstrom) = 1 x 10-10 m (not SI unit but very common)

Putting a Scale on Atoms

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Chemistry-140 Lecture 4

A Simple Calculation With Atoms

Question:

The diameter of a penny is 19 mm. The diameter of a copper atom is 2.6 Å. How many copper atoms could be arranged side by side in a straight line across the diameter of a penny?

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Chemistry-140 Lecture 4

Answer:

The unknown is the number of Cu atoms. We can take the relationship that 1 Cu atom = 2.6 Å as a conversion factor relating number of atoms and distance.

Thus: # Cu atoms = (19mm)(3.8 x 106 Cu atom/mm)

= 7.3 x 107 Cu atoms

A Simple Calculation With Atoms

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Chemistry-140 Lecture 4

  • Atomic number is the number of protons in the nucleus.
  • All atoms of the same element have the same atomic number.
  • Mass number is the sum of the number of protons & neutrons The number of neutrons in the nucleus is given by the mass number minus the atomic number.
  • Isotopes are atoms of the same element that have different numbers of neutrons. Therefore isotopes have the same atomic number but different mass numbers.
  • An atom of a specific isotope is called a nuclide.

Isotopes, Atomic Numbers and Mass Numbers

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Chemistry-140 Lecture 4

  • In reference to a specific isotope, we use the symbolism;
  • where mass number = (protons + neutrons)
  • atomic number = (protons)
  • usually do not include atomic number in symbol as the symbol implies a particular atomic number
  • example: 14C is "carbon fourteen"
  • also some isotopes have special names:
  • 1H = hydrogen, 2H = deuterium, 3H = tritium

Isotopes, Atomic Numbers and Mass Numbers

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Chemistry-140 Lecture 4

Measuring Atomic Masses

  • A mass spectrometer is used to ionize samples and separate the ions produced according to their mass
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Chemistry-140 Lecture 4

Measuring Atomic Masses

  • A sample of the element antimony (Sb) is separated into ions according to isotopic mass and natural abundance
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Chemistry-140 Lecture 4

Example 2.3:

Silver has two isotopes, one with 60 neutrons and the other with 62 neutrons. What are the mass numbers and symbols of these isotopes?

A Question of Isotopes

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Chemistry-140 Lecture 4

Answer:

Silver has an atomic number of 47, so it has 47 protons in the nucleus. The two isotopes therefore have mass numbers of

Isotope 1: A = 47 protons + 60 neutrons = 107

Isotope 2: A = 47 protons + 62 neutrons = 109

The first isotope has a symbol

the second is

A Question of Isotopes

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Chemistry-140 Lecture 4

  • Atomic mass of an element is the average of all the isotopes weighted by their natural abundance

Atomic Mass

Example 2.4:

Bromine has two naturally occurring isotopes, one with a mass of 78.918336 amu and a percent abundance of 50.69%, the other with mass 80.916289 and a percent abundance of 49.31%. Calculate the atomic mass of bromine.

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Chemistry-140 Lecture 4

Answer:

The atomic mass of any element is the average of the masses of all the isotopes in a representative sample. To calculate the atomic mass, you multiply the mass of each isotopes by its percent abundance divided by 100.

Average atomic mass of bromine = atomic mass of bromine

[(0.5069)(78.918336) + (0.4931)(80.916289)] amu

= 79.90 amu

Atomic Mass

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Chemistry-140 Lecture 4

Textbook Questions From Chapter #2

Atoms: 24, 25

Isotopes: 30

Atomic Mass: 32, 34

Good Problems: 56, 62 Fun(?): 46

chemistry 140 lecture 5
Chemistry-140 Lecture 5
  • Elements are listed in ascending order of atomic number
  • Periods are the horizontal rows in the periodic table.
  • Groups (Families) are the vertical columns in the periodic table.

The Periodic Table

chemistry 140 lecture 51
Chemistry-140 Lecture 5
  • Metals: elements to the left of the periodic table (excluding hydrogen). Generally lustrous, ductile, malleable and conduct heat and electricity.
  • Nonmetals: elements to the right of the periodic table. Lack the properties of metals, are brittle as solids, do not conduct well, and are often dull in appearance. Many are gases.
  • Semimetals (metalloids): elements at the boundary between metals and nonmetals. Properties are intermediate between those of the metals and nonmetals. Boundary is often shown as a "stepped" line on the periodic table, from B to At

The Periodic Table

chemistry 140 lecture 52
Chemistry-140 Lecture 5
  • Some Groups have special names:
  • Examples
  • Group 1 alkali metals
  • Group 2 alkaline earth metals
  • Group 17 halogens
  • Group 18 noble gases

The Periodic Table

chemistry 140 lecture 53
Chemistry-140 Lecture 5
  • Some elements exists in more than one form:
  • Example:
  • C: graphite, diamond, fullerene(s)
  • O: dioxygen, ozone
  • P: red, white, black
  • Different forms of the same element are known as allotropes

Allotropes