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Atomic Structure

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Atomic Structure
Subatomic Particles: particles that are smaller than atoms, they are the atomic building blocks
1. Protons: positively charged particles, with a mass of 1amu
2. Neutrons: neutral particles with NO charge, with a mass of 1amu
3. Electrons: negatively charged particles, with negligible mass, 0amu
Atomic Structure: there are two main regions of the atom
1. Nucleus: the center of the atom that contains all of the atom's mass, protons and neutrons are
here.
*sometimes protons and neutrons are called nucleons
2. Energy Levels: the region of space outside of the nucleus, electrons are here.
Note: Electrons located in the outermost level (far from the nucleus) are known as valence
electrons.
Describing Atoms:
Mass Number (Atomic Mass): the total number of protons and neutrons in an atom's nucleus
Atomic Number: the number of protons in an atom, this identifies the type of element!
• because the atom is neutral overall, there must be an equal number of protons and electrons
Dalton's Atomic Theory:
1. All matter is composed of atoms, that cannot be broken down, created, or destroyed.
2. All atoms of the same element are identical.
3. Different elements have different types of atoms.
4. Chemical reactions occur when atoms are rearranged.
5. Compounds are composed of atoms of different elements combined in fixed proportions. (whole
number ratios)
J.J. Thomson's
Cathode Ray Tube Experiment
Cathode Ray Tube: a glass tube that encloses a vacuum and can be energized by electricity
Observations:
• A beam of very small particles travels from one end of the tube to the other when electrified.
• The beam is bent by a magnetic field, it bends towards the positive pole and away from the
negative pole.
Conclusion:
• The atom is composed of smaller particles known as Electrons!
Rutherford's Gold Foil Experiment:
Alpha Particles:
• positively charged radiation emitted by Uranium
Observations:
1. Most of the alpha particles passed straight through the foil.
2. Some alpha particles were deflected at great angles.
Conclusions:
1. The atom is mostly empty space.
2. The atom contains a small, dense, positively charged nucleus.
Bohr Model of the Atom
principle energy levels: regions where electrons are located around the nucleus; levels become
larger as the distance from the nucleus increases
Ground State: all electrons are as close to the nucleus as possible
(low energy, stable)
Excited State: electrons occupy energy levels that are further from the nucleus
(high energy, unstable)
energy level
1
2
3
4
5
6
maximum # of electrons
2
8
18
32
50
72
Wave-Mechanical Model
Electrons do NOT move around the nucleus in circular orbits. Rather, they travel in waves, which
results in "cloudy" regions of space around the nucleus where electrons can be found.
Lewis Dot Structure (Electron Dot Diagram): a diagram that shows the symbol of the element
surrounded by dots that represent the valence electrons of an atom of that element
orbital: a region of high probability of finding an electron, 2 electrons can occupy each orbital
Isotopes: atoms of the same element, and therefore contain equal numbers of protons, with different
numbers of neutrons, atoms of the same element with different masses
•
same # of protons (atomic number)
•
different # of neutrons
Percent Abundance: a measurement of the amount of naturally occurring isotopes of an element.
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