Protons, Neutrons, Electrons/Isotopes/Aver age Atomic Mass
Jan 21, 2016
Protons, Neutrons, Electrons/Isotopes/Average
Atomic Mass
If an atom was the size of a football stadium …
• the nucleus (protons + neutrons) would be a marble on the 50 yard line
• the electrons would be smaller than gnats out in the stands
An Atom is Like…Gnat-sized electron
Marble -sized nucleus
Important things to remember about atoms!
Most of an atom is empty space
Electrons are so small, we can pretend that their mass is zero
The mass of an atom is from its nucleus (protons & neutrons)
Atom Facts!
Tells you the identity of the element
(Each element has a different atomic #)
Atomic Number
Atomic Mass
Given the following atomic numbers, determine the identity of the elements:
Atomic # Element
16
9
53
Practice!
Sulfur
Fluorine
Iodine
Also tells you the number of protons
(Each element has a different # of protons)
Protons
Atomic Mass
Determine the number of protons for each element:
Element # Protons
chlorine
arsenic
oxygen
Practice!
17
33
8
Isotopes = Different versions of the same element
Isotopes of Carbon
Isotopes
Important things to remember about the isotopes of an element!
Same Element
Same # Protons
Different # Neutrons
Different Masses
Isotope Facts!
Relative Percent Abundance = The overall percent of each isotope of an element
Isotopes of Carbon
% Abundance
1
1H or Hydrogen-1 1 proton 0 neutrons
Mass number
Hydrogen-3OR
21H or Hydrogen-2
1 proton 1 neutron
31H or Hydrogen-3
1 proton 2 neutrons
Isotope Symbols
The mass number = # protons + # neutrons (So, if you know the # protons, you can figure out the #
neutrons, and vice versa!)
Example: 31H or Hydrogen-3
# Protons = 1
Mass Number = 3
Mass number = # protons + # neutrons
3 = 1 + x
x = # Neutrons = 2
Mass Number
Determine the # of protons and neutrons for each of the following isotopes:
168O
2110Ne
Flourine-19
Boron-11
Practice!
/8 protons, 8 neutrons
10 protons, 11 neutrons
9 protons, 10 neutrons
5 protons, 6 neutrons
The Atomic Mass is the average
weighted mass of the isotopes of an
element
Atomic Mass
Atomic Mass
To calculate the average weighted mass (atomic mass of an element:
Step 1Multiply each isotope’s mass x its %
abundance. (Make sure the % abundance is in decimal form!)
Step 2Add all of the answers from step 1 above.
Ave. Weighted Mass
Calculate the average weighted mass (the atomic mass) of nitrogen.
Isotope Mass Relative % Abund.
Nitrogen-14 14.003 amu 99.63
Nitrogen-15 15.000 amu 0.37
Step 1
(14.003)(0.9963) = 13.951 amu
(15.000)(0.0037) = 0.055 amu
Step 2
13.951 + 0.055 = 14.006 amu
Mass % Abund. (as a decimal)
Ave. Weighted Mass
Calculate the average weighted mass (the atomic mass) of Silicon.
Isotope Mass Relative % Abund.
Silicon-28 27.977 amu 92.23
Silicon-29 28.976 amu 4.67
Silicon-30 29.974 amu 3.10
Step 1:
(27.977)(0.9223) = 25.803 amu
(28.976)(0.0467) = 1.353 amu
(29.974)(0.0310) = 0.929 amu
Practice!
Step 2:
25.803+1.353+0.929 =
28.085 amu
Tells you the number of electrons too!
Remember! The atomic # also tells you the number of protons
(so…the # protons = # electrons in an atom)
Electrons
When an atom loses or gains one or more electrons, it becomes an ion.
There are two kinds of ions:• Cations: Ions formed when atoms lose
one or more electrons – have a positive charge
Ex. Ca2+, Na1+, Al3+
• Anions: Ions formed when atoms gain one or more electrons – have a negative charge
Ex. F1-, P3-, O2-
Ions!
Determine the number of electrons for each element:
Element # Electrons
S
S2-
Al
Al3+
Practice!
16
18
13
10
Finished!