Chapter 4 structure of the atom
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Chapter 4 Structure of the Atom. Section 4.1. Democritus (460-370 BC) Matter is composed of empty space through which atoms move Atoms are solid, homogeneous, indestructible and indivisible Different kinds of atoms have different sizes and shapes

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Chapter 4 Structure of the Atom

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Chapter 4 structure of the atom

Chapter 4 Structure of the Atom


Section 4 1

Section 4.1

  • Democritus (460-370 BC)

    • Matter is composed of empty space through which atoms move

    • Atoms are solid, homogeneous, indestructible and indivisible

    • Different kinds of atoms have different sizes and shapes

    • The differing properties of matter are due to the size, shape, and movement of atoms

    • Apparent changes in matter result from changes in the groupings of atoms themselves


Chapter 4 structure of the atom

  • Aristotle (384-322 BC)

    • One of the most influential philosophers

    • Wrote extensively on politics, ethics, nature, physics, and astronomy

    • Most of his writings have been lost through the ages

    • Criticized Democritus, saying that he did not believe that the nothingness of empty space could exist


Chapter 4 structure of the atom

  • John Dalton (1766-1844)

    • All matter is composed of extremely small particles called atoms

    • All atoms of a given element are identical. Atoms of a specific element are different from those of any other element

    • Atoms cannot be created, divided into smaller particles, or destroyed

    • Different atoms combine in simple whole number ratios to form compounds

    • In a chemical reaction, atoms are separated, combined, or rearranged


Chapter 4 structure of the atom

  • The smallest particle of an element that retains the properties of the element is called an atom.

  • Atoms are extremely small (a copper atom has a diameter of 0.000000000128 m)

  • Atoms can be seen using a scanning tunneling microscope.

  • See them on page 96.


Section 4 2

Section 4.2

  • J.J. Thomson (1856-1940) discovered the electron through experiments using a cathode ray tube.


Chapter 4 structure of the atom

  • Robert Millikan (1868-1953) determined that the charge of an electron was negative and was able to calculate the mass of an electron (9.11 x 10-28 g)


Chapter 4 structure of the atom

  • Earnest Rutherford (1871-1937) is famous for discovering the nucleus of the atom. He used an experiment in which he passed alpha particles through a thin film of gold foil. He expected the alpha particles to pass through, but found instead that they were deflected at large angles. He determined that there was a dense positive core in the atom, which he called a nucleus.


Chapter 4 structure of the atom

Subatomic Particles

The nucleus is the tiny positive core of the atom which contains most of the mass of the atom.

The proton (p+) is the positively (1+) charged particle found in the nucleus of the atom. It has a relative mass of one.

The neutron (no) is the particle with no charge (0) found in the nucleus of the atom. It has a relative mass of one.

The electron (e-) is the negatively (1-) charged particle found in the electron cloud outside of the nucleus. It has very little relative mass by comparison.


Chapter 4 structure of the atom

ACTUAL MASS (g)

RELATIVE MASS

ELECTRICAL CHARGE

SYMBOL

PARTICLE

Electron

Proton

Neutron

Obj. 5…Subatomic Particles

PROPERTIES OF SUBATOMIC PARTICLES

e-

-1

9.11 x 10-28

1/1840

p+

+1

1.67 x 10-24

1

1

n0

0

1.67 x 10-24


Chapter 4 structure of the atom

  • Since the number of protons is equal to the number of electrons, the atom is electrically neutral


Section 4 3

Section 4.3

  • Henry Moseley (1887-1915) discovered that the atoms of each element contain a unique positive charge in their nucleus. The number of protons in an atom is referred to as the element’s atomic number.


Chapter 4 structure of the atom

  • Atomic #:

  • # of p+ in an atom

  • identifies element

(change atomic # = change of element).

  • # p+ = # e- in neutral atom

** (+) charge = less e- than p+

** (-) charge = more e- than p+

  • Atomic mass:

(a.k.a. mass #)

  • mass of the nucleus

  • p+ + n0

  • units are a.m.u. (atomic mass unit)


Apeman great shortcut

APEMAN – great shortcut

  • To make it easier, you might want to use APEMAN….

  • Atomic number = Protons or Electrons

  • Mass number – Atomic number = Neutrons

  • (mass number must be rounded)


Chapter 4 structure of the atom

  • Since an atom is electrically neutral, the number of protons equals the number of electrons.

  • Atomic # = # of protons = # of electrons


Chapter 4 structure of the atom

  • Most elements are a mixture a isotopes. The relative abundance of each isotope is constant.

  • For example, in bananas,93.25% of the K atoms have 20 no, 6.7302% have 22 no, and 0.0117% have 21 no. In any other source of K, the relative percentage of the isotopes will be the same.


Chapter 4 structure of the atom

  • In order to identify the various isotopes of an element, chemists add a number after the elements name. The number added is called the mass number and it represents the sum of the number of protons and neutrons in the nucleus.

    Potassium-39 Potassium-40 Potassium-41

  • P+ 19 19 19

  • No 20 21 22

  • E- 19 19 19

  • Number of neutrons = mass number – atomic number


Chapter 4 structure of the atom

  • Because the mass of an atom is so small, chemists have developed a method of measuring the mass of an atom relative to the mass of a specifically chosen atomic standard. The standard is the carbon-12 atom. Scientists assigned the C atom a mass of 12. One atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom. 1 amu is approximately equal to the mass of a single p+ or no.


Chapter 4 structure of the atom

  • Atomic mass of an element is the weighted average mass of the isotopes of that element.

    Example: The atomic mass of Chlorine (Cl) is 35.453 amu. Chlorine exists naturally as 75% chlorine-35 and 25% chlorine–37.

    .75770 x 34.969 amu = 26.496 amu

    .24230 x 36.966 amu = 8.957 amu

    Weighted average atomic mass of Cl =

    (26.496 + 8.957) = 35.453 amu


Chapter 4 structure of the atom

  • The mass # of an element (periodic table) is the weighted

avg. of allisotopes that exist in nature.

63.55 g/mole

  • Isotopes are naturally occurring.

- abundance of isotope is just as important as mass!

  • Ex...

Natural copper (Cu) consists of 2 isotopes ...

Copper - 63 (mass = 62 .930 g/mole)

69%

Copper - 65 (mass = 64 .930 g/mole)

31%

  • To calculate avg. mass...

mass x abundance for each isotope

Step 1 :

add the two values from step 1 together

Step 2 :

62 .93 x .69 =

43.42

43.42

+

20.13

64 .93 x .31 =

20.13


The teacher lied

The teacher lied…

  • There are reactions that involve an atom of one element changing into an atom of another element. These reactions, which involve an atom’s nucleus, are called nuclear reactions.


Chapter 4 structure of the atom

  • Radioactivity is the emission of radiation.

  • Radiation is the radioactive material that is emitted during radioactivity.

  • Radioactive atoms emit radiation because their nucleus is unstable. Unstable nuclei lose energy by emitting radiation in a spontaneous process called radioactive decay.


Types of radiation

Types of Radiation

  • Alpha radiation – made up of 2 p+ and 2 no . Is equivalent to He-4 nucleus. During alpha decay, a nucleus loses 4 amu and 2 protons, to become a new element.


Chapter 4 structure of the atom

  • Beta radiation – consists of fast moving electrons called beta particles. Each beta particle is an electron with a 1- charge. Beta decay changes a n0 into a p+ and an e-.

  • Gamma radiation – Gamma rays are high energy radiation that possess no mass. Because they possess no electrical charge, they are not deflected by electric or magnetic fields. Gamma rays usually accompany alpha and beta radiation and they account for most of the energy lost during the radioactive decay process. Because gamma rays are massless, they cannot result in the formation of a new atom.


Chapter 4 structure of the atom

+

+

230

90

208

83

222

86

208

82

226

88

Bi

Rn

Th

Pb

Ra

+

230

90

Th

  • Practice:

0

e

beta

-1

0

gamma

0

4

He

alpha

2


Chapter 4 structure of the atom

MASS CHANGE

ATOMIC # CHANGE

NEUTRON CHANGE

SYMBOL

TYPE

0

0

ALPHA ()

by 1

by 4

by 1

by 2

by 2

BETA ()

He

4

2

+2

GAMMA ()

e

0

-1

no change

no change

no change

no change


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