Download 4-1 Atomic Structure

4-1: Atomic Structure
Tuesday, August 18, 2015
5:49 PM
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Nucleus - an atom's central region which is made up of protons and neutrons
Proton - a subatomic particle that has a positive charge and that is found in the nucleus of an atom
Neutron - a subatomic particle that has no charge and that is found in the nucleus of an atom
Electron - a subatomic particle that has a negative charge
Orbital - a region in an atom where there is a high probability of finding electrons
Valence electron - an electron that is found in the outermost shell of an atom and that determines the atom's
chemical properties
I. What are Atoms?
a. Democritus - Greek philosopher in the 4th century BCE
i. Suggested that the universe was made of invisible particles called atoms
*the word atom is derived from the Greek word for "unable to be divided"
ii. Believed movements of atoms caused changes in matter
iii. Unable to provide any evidence needed to convince people - no scientific basis
b. John Dalton - English schoolteacher proposed another atomic theory in 1808
i. Theory had a scientific basis
ii. Considered the foundation for modern atomic theory
iii. Dalton's theory stated
1) that atoms could not be divided
2) that all atoms of a given element were exactly alike
3) that atoms of different elements could join to form compounds
c. Atoms are the building blocks of molecules
i. An atom is the smallest part of an element that still has the element's properties
ii. Atoms can join to form molecules
II. What's in an Atom?
Less than 100 years after Dalton published his atomic theory, scientists determined that atoms consisted of
still smaller particles and could be broken down even further.
a. Atoms are made of protons, neutrons, and electrons
i. Protons have a positive charge (+)and are located in the nucleus
ii. Neutrons have no charge (0) and are also located in the nucleus
iii. Electrons have a negative charge (-) and exist outside the nucleus
Ch4 Atoms and the Periodic Table Page 1
b. Unreacted atoms have no overall charge
i. Atoms do not have a charge, because they have an equal number of protons (+) and electrons (-).
III. Models of the Atom
Like most scientific models and theories, the model of the atom has been revised many times to explain new
discoveries.
a. Niels Bohr - Danish scientist
i. In 1913 suggested that electrons in an atom move in set paths around the nucleus
much like planets in orbit around the sun
Niels Bohr
Bohr model of the atom
b. Electrons act more like waves
Ch4 Atoms and the Periodic Table Page 2
b. Electrons act more like waves
i. By 1925, Bohr's model of the atom no longer explained electron behavior
ii. A new, modern model proposed that electrons behave more like waves on a vibrating string
c. An electron's exact location cannot be determined
i. It is impossible to determine the exact location of an electron in an atom and the electron's speed and
direction.
ii. We use the term electron cloud to help us understand this
iii. Consider this diagram:
iv. io
The darker the shading, the better the chance of finding an electron at that location.
d. Electrons exist in energy levels
1) Electrons with different amounts of energy exist in different energy levels
2) The number of filled energy levels an atom has depends on the number of electrons
3) Energy level 1 can hold 2 electrons
4) Energy level 2 can hold 8 electrons
5) Energy level 3 can hold 18 electrons
6) Energy level 4 can hold 32 electrons
e. Electrons are found in orbitals within energy levels
i. The regions in an atom where electrons are likely to be found are called orbitals.
ii. The four kinds of orbitals are s, p, d, and f.
1) s orbital - the simplest orbital; spherical in shape
2) p orbital - dumbbell shaped and can exist in 3 different orientations (x-axis, y-axis, & z-axis)
3) d orbital - 5 possible configurations
4) f orbital - possible configurations
** d and f orbital are much more complicated than s and p. We will not work with these two
very much.
Ch4 Atoms and the Periodic Table Page 3
very much.
f. Every atom has between one and eight valence electrons
i. An electron in the outermost energy level of an atom is called a valence electron.
ii. Valence electrons determine an atom's chemical properties and its ability to form bonds.
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