Atoms
Atom- the smallest part of an element
• Atoms of one element are different from another element.
• contains protons, electrons, and neutrons
Atomic Mass Unit – standard unit of mass for an atom & equivalent to the mass
of a proton.
• 1 amu = 1.66 x 10-24g (very tiny!)
• abbreviated with amu or u.
• Protons and neutrons are approximately 1 amu each.
• electrons are approximately 1/2000 the mass of a proton- so small it is
considered negligible.
Bohr Model of Helium
Helium has 2 protons, 2 neutrons, and 2 electrons.
Where do they go?
n
+
neutron
Proton
nucleus
electron
electron
orbit
+n
n+
Atomic Properties
Charge
Mass
(amu)
Location
Proton
+1
1
Nucleus
Electron
-1
0
Cloud
Neutron
0
1
Nucleus
Subatomic
Particle
Elements
• Element- the simplest form of matter with a unique set of properties and cannot be broken
down into smaller components by chemical means.
• Atomic Number- the number of protons in the nucleus of an atom.
• Every element has a unique number of protons
• Elements are arranged on The Periodic Table by their atomic numbers
• Atomic Charge- The balance between the number of protons (positive) and electrons
(negative)
• Atoms are neutral. # of protons = # of electrons
• Mass number- the number of protons and neutrons added together in an atom.
• Protons and neutrons are both approximately 1 amu.
• Electrons are too small, not added.
• Hydrogen (H) has 1 proton and 1 electron. What is its atomic mass?
1 amu + 0 amu
= 1 amu
• Beryllium has 4 protons, 4 electrons, and 5 neutrons.
4 amu + 0 amu
+ 5 amu
= 9 amu
Nuclear Symbols
• Nuclear Symbols are used to represent atoms and their isotopes
• Isotopes – atoms with the same atomic number but different atomic masses due to
different number of neutrons
• Example: uranium-238
mass number →
• A nuclear symbol tell us:
• The element (ex. uranium)
• top number is the mass number
atomic number →
• # protons + # neutrons
• bottom number is the atomic number
• # protons
•
You can determine the number of neutrons by subtraction
238 (mass #) - 92 (atomic #) = 146 (# of neutrons)
• What are the nuclear symbols for uranium-235, iodine-181, and Xe-135?
Isotopes
Isotopes are atoms of the same element but with different atomic masses
– number of protons are the same (therefore same element)
– number of neutrons are different
Example: The element carbon has an atomic number of 6.
– it has 6 protons.
– Some carbon atoms have 6 neutrons and some have 8 neutrons.
Carbon-12
Carbon-14
6 protons
+ 6 neutrons
12 atomic mass units
6 protons
+ 8 neutrons
14 atomic mass units