Atomic
Structure
Modern Atomic Theory
• Atom – smallest particle of an element that
retains the properties of the element
• Subatomic Particles
– Protons
– Neutrons
– Electrons
Protons
• Positive Charge (+1)
• Located in the nucleus
• Mass = 1 amu (atomic mass unit)
– 1 amu = 1.66 x 10-24 grams
• Symbol: p+ or H+
Neutrons
•
•
•
•
Neutral Charge
Located inside the nucleus
Mass = approximately 1 amu
Symbol: n
Electrons
• Negative Charge (-1)
• Located outside the nucleus
• Mass = 1/1836 of a proton (basically mass
less)
• Symbol: e- or e-1
Nucleons
• Particles located inside the nucleus
– Protons and Neutrons
• All mass is located in the center of the
atom (inside the nucleus), nucleus is very
dense
Atomic Number
• The number of protons in an atom
• Bold number on the periodic table
• For a neutral atom:
Number of protons = number of electrons
Why???
Atomic Mass/Mass Number
• The mass of an atom (expressed in amu)
• Equal to the number of protons + the
number of neutrons in an atom
Why aren’t electrons included in the mass?
Nuclear Symbol
•
The nuclear symbol consists of three
parts
– the symbol of the element
– the atomic number of the element
– the mass number of the specific isotope
Examples
Indicate how many protons, neutrons
and electrons are in each of the following
atoms.
1. Aluminum-27
2. Xe-131
3. 56Fe
Gram Atomic Mass
• The mass of 1 mole of the element
– 1 mole = 6.02 x 1023 particles
• Expressed in grams instead of amu
Mass Examples
1. 1 atom of K =
2. 1 mole of K =
3. 1 CO2 molecule =
4. 1 mole of CO2 =
Isotopes
• Same element, same atomic number, number of
protons are the same
• Different masses, because the number of
neutrons varies
Example: indicate the number of protons,
neutrons, and electrons in carbon-12 and
carbon-14
Weighted Average Atomic Mass
• Mass found on the periodic table is the
weighted average of the naturally
occurring isotopes
• Accounts for the percent abundance and
mass of each of the elements isotopes
How to Calculate
• Each exact atomic mass is multiplied by its
percent abundance (expressed as a decimal).
Then, add the results together and round off to
an appropriate number of significant figures.
Example: Nitrogen
Mass
Exact Mass Percent
Number (amu)
Abundance
14
14.003074
99.63
15
15.000108
0.37
Examples
• Calculate the weighted average atomic mass for
Magnesium.
Mass
Number
Exact Mass
(amu)
Percent
Abundance
24
23.985043
78.99
25
24.985837
10.00
26
25.982593
11.01
Ions
• Occur when a neutral atom gains or loses
a electrons
• Caused by the transfer of electrons
• Results in a charged atom
• Protons are never lost or gained
Cation
• Positive Ion
• Results from the loss of an electron
• Elements on the left side of the table
(metals) tend to lose electrons to form
cations
Anion
• Negative ion
• Results from the gain of an electron
• Elements on the right side of the table
(nonmetals) tend to gain electrons to form
anions
Ion Examples
Indicate the number of protons, neutrons
and electrons in each:
1.
2.
3.
4.
5.
6.
35Cl
atom:
35Cl- ion:
39K atom:
39K+ ion:
40Ca atom:
40Ca2+ ion: