Structure of an Atom
Charges
• Electron – negative
• Proton – positive
• Neutron - neutral
Parts
• Nucleus – neutrons & protons
• Orbitals - electrons

Elements
• Pure chemical substances made up of one type of atom
• Building blocks of matter
• Over 115 elements known - and more being discovered

Elements – Symbol / Name
Examples:
C
O
Carbon
Oxygen
Mg
Br
Magnesium
Bromine
Symbol: one or two letter abbreviation
Latin
Au Gold-aurum
Lb Lead-plumbum
K Potassium-kalium
Na Sodium-natrium

Elements – Atomic Number
DEFINES the element
Atoms are neutral… so have same # electrons (-) as protons (+)
• ALL Carbon atoms have six protons, and
… ALL atoms with six protons are Carbon
• BUT … they can have different numbers of neutrons
These are called isotopes of carbon

Elements – Atomic Mass
Calculated from Mass
Number
• MASS NUMBER = mass of all particles in one atom (protons, neutrons and electrons)
• Measured in “atomic mass units” (amu)

Elements – Mass Number
• MASS NUMBER = mass of all particles in one atom (protons, neutrons and electrons)
• Mass of 1 proton = 1 amu
Mass of 1 neutron = 1 amu
Mass of 1 electron = really, really tiny!
• SO… mass number = # protons + # neutrons
Carbon
• 6 protons x 1 amu/proton = 6 amu
• 6 neutrons x 1 amu/neutron = 6 amu
• 6 electrons x really tiny mass = 0 amu
TOTAL = 6 amu + 6 amu + 0 amu
= 12 amu

Elements – Mass Number
Helium Nitrogen
Atomic Number = 2 (protons)
Mass Number = 4 amu (p + n)
Atomic Number = 7 (protons)
Mass Number = 14 amu (p + n)

Elements – Atomic Mass
• BUT… Why is the atomic mass for Lithium
(atomic number 3; with 3 protons) = 6.94?!?!
• …. Isotopes NOTE: Mass number is used to name isotope
Lithium-6
- 3 protons, 3 neutrons
mass number = 6
- 7.5% of all lithium
Lithium-7
- 3 protons, 4 neutrons
mass number = 7
- 92.5 % of all lithium

Elements – Atomic Mass
• BUT… Why is the atomic mass for Lithium
(atomic number 3; with 3 protons) = 6.94?!?!
• Calculation: Weighted Average (info. only):
– 7.5 % Lithium-6 at 6 amu
– 92.5 % Lithium-7 at 7 amu
• (0.075 x 6 amu) + (0.925 x 7 amu) =
(0.45 amu) + (6.48 amu) = 6.93 amu

Elements – Atomic Mass
• The atomic mass of an element is calculated by doing a weighted average of the mass
numbers of all naturally existing isotopes.
• If you round the atomic mass of an element to the nearest whole number, you get the
mass number of the most common isotope.
7.0

Elements – Symbol / Name
Same # protons, different # neutrons
Atomic Mass
• “Average” for all carbon
• Calculated from mass numbers of isotopes
• On the periodic table
All have same # protons
Mass Number
• Calculated for each individual isotope
• Used to name the isotope
• Averaged together to represent all carbon

Elements - SUMMARY

Structure of an Atom
• Outside the nucleus
– Electron
• Atomic mass
• Charge
0
(-) electron nucleus
(containing protons and neutrons)

Structure of an Atom
• Electrons are arranged in orbits
– Orbit 1 holds
2 electrons
– Orbit 2 holds
8 electrons
– Orbit 3 holds
8 electrons…

Structure of an Atom
• Nucleus – the center of an atom contains…
– Protons
• Atomic mass
• Charge
– Neutrons
• Atomic mass
• Charge
1
(+)
1 none

Symbols
• Either 1 or 2 letters symbols
– First letter Capital, second lower case
– Some based on Latin name
Example
C Carbon
O
Mg
Br
Oxygen
Magnesium
Bromine
Latin
Au Gold-aurum
Lb Lead-plumbum
K Potassium-kalium
Na Sodium-natrium

The Periodic Table
• Elements are arranged by size and chemical properties.
• Rows
– Period
• Size
• Columns
– Group
• Properties

Properties
• Properties are related to atomics structure.
• The number of electrons in the outer most orbit (valence electrons) determine how an atom will behave in chemical reactions.