Chapter 2: The AtomAtom: The smallest unit of an element that
retains the chemical properties of the element
Atoms consist of three primary particles.
Electrons, Protons & Neutrons
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• Nucleus - small, dense, positively charged region in the center of the atom. Contains:
- protons - positively charged particles, with mass of 1 amu.
- neutrons - uncharged particles, with mass of 1 amu.
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region of negative charge populated by:
- electrons - negatively charged particles, very little mass
Table 2.1 Selected Properties of the Subatomic Particles
Name Charge Mass(amu) Mass (grams)
Electrons (e) -1 5.4 x 10-4 9.1095 x 10-28
Protons (p) +1 1.00 1.6725 X 10-24
Neutrons (n) 0 1.00 1.6750 x 10-24
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CAZ X
Mass
Number
mass number (A) - sum of the number of protons and neutrons
Atomic Number
Charge of particle
Symbol of the atom
atomic number (Z) - the number of protons in the atom
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Calculate the number of protons, neutrons and electrons in each of the following:
B115
Fe5526
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Mat
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re• Isotopes - atoms of the same element
having different masses
– contain same number of protons– contain different numbers of neutrons
Isotopes of Hydrogen
H H H 31
21
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Hydrogen(Hydrogen - 1)
Deuterium(Hydrogen - 2)
Tritium(Hydrogen - 3)
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• Some isotopes are radioactive
• Find chlorine on the periodic table.
• What is the atomic number?› 17
• What is the mass given?› 35.45
• This is not the mass number of an isotope.
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• It is called the atomic mass - the weighed average of the masses of the isotopes that make up chlorine.
• Chlorine consists of chlorine-35 and chlorine-37 in a 3:1 ratio.
• The weighed average is an average corrected by the relative amounts of each isotope present in nature.
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• Ions - electrically charged particles that result from a gain or loss of one or more electrons by the parent atom.
• Cation - positively charged– result from the loss of electrons– 23Na 23Na+ + 1e-
• Anion - negatively charged– results from the gain of electrons– 19F + 1 e- 19F-
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Ions are Charged Atoms
How many protons neutrons and electrons are in the following ion?
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K3919
-23216S
Development of the Atomic Theory
• Dalton’s Atomic Theory - the first experimentally based theory of atomic structure of the atom.– John Dalton– early 1800’s
• Much of Dalton’s Theory is still regarded as correct today. See starred items.
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The
ory Postulates of Dalton’s Atomic Theory
1. All matter consists of tiny particles called atoms.*
2. An atom cannot be created, divided, destroyed, or converted to any other type of atom.
3. Atoms of a particular element have identical properties.
4. Atoms of different elements have different properties.*
5. Atoms of different elements combine in simple whole-number ratios to produce compounds (stable aggregates of atoms.)*
6. Chemical change involves joining, separating, or rearranging atoms.*
* These postulates are still regarded as true.
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The
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The
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Subatomic Particles: Electrons, Protons and Neutrons
• Electrons were the first subatomic particles to be discovered using the cathode ray tube.
Indicated that the particles were negatively charged.
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The
ory • Protons were the next particle to be
discovered– Protons have the same size charge but
opposite in sign.
– Proton is1837 times as heavy as electron.
• Neutrons – Postulated to exist in 1920’s but not
demonstrated to exist until 1932
– Almost the same mass as the proton.
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The Nucleus12
• The initial ideas of the atom did not have a “nucleus.”
• “Plum Pudding Model”
• Earnest Rutherford’s “Gold Foil Experiment” lead to the understanding of the nucleus.
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The
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“It was almost as incredible as if you fired a 15-inch shell at a piece of tissue and it came back and hit you.”
-Earnest Rutherford.
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Light and Atomic Structure 13
• Spectroscopy - absorption or emission of light by atoms (energy changes)
– Used to understand the electronic structure.
• To understand the electronic structure, we must first understand light (or electromagnetic radiation.)– travels in waves from a source
– speed of 3.0 x 108 m/s
The Bohr Atom
Electrons exist in fixed energy levels surrounding the nucleus.
Quantization of energy
• Initial understanding of the atom by Niels Bohr
Promotion of electron occurs as it absorbs energy
Excited State
Energy is released as the electron travels back to lower levels.
Relaxation
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• Orbit - what Bohr called the fixed energy levels.
• Ground state - the lowest possible energy state.