Atoms and
Elements
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
Symbol: one
or two letter
abbreviation
Examples:
C
Carbon
O
Oxygen
Mg
Magnesium
Br
Bromine
Au
Lb
K
Na
Latin
Gold-aurum
Lead-plumbum
Potassium-kalium
Sodium-natrium
Elements – Atomic Number
DEFINES the
element
• 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
Nitrogen
Helium
Atomic Number =
2 (protons)
Atomic Number =
7 (protons)
Mass Number =
4 amu (p + n)
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
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
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
• Averaged together to
represent all carbon
Same # protons, different # neutrons
Elements - SUMMARY
Structure of an Atom
• Outside the nucleus
electron
– Electron
• Atomic mass
• Charge
nucleus
(containing protons
and neutrons)
0
(-)
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
1
(+)
– Neutrons
• Atomic mass
• Charge
1
none
Symbols
• Either 1 or 2 letters symbols
– First letter Capital, second lower case
– Some based on Latin name
Example
Latin
C
Carbon
Au Gold-aurum
O
Oxygen
Lb Lead-plumbum
Mg
Magnesium
K
Potassium-kalium
Br
Bromine
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.