Chemistry – Ch 3-3
St. Augustine Preparatory Academy
August 24, 2015
Atoms
• All atoms of a particular element have the
same number of protons (positive charge
particles).
• If you take away or add a proton, the atom
becomes a different element
Atoms
• Since atoms are neutrally charged the number
of electrons that an atom has must be the
exact same as the number of protons.
• We can also calculate the number of neutrons
that an atom has if we know it’s atomic mass
(sometimes call molar mass) and the number
of protons.
• The atomic mass has the units g/mol (grams
per mole)
Using the Periodic Table
The above example is Carbon
- We find the number of protons by looking at the
atomic number: Carbon has 6 protons
- Since atoms are neutral, carbon has 6 electrons
- To find the number of neutrons, we round the atomic
mass and subtract the atomic number: Carbon has 6
neutrons
Summary
• # of Protons: equals the atomic number
• # of electrons: equals the number of protons
(the atomic number)
• # of neutrons: equals the number of protons
subtracted from the rounded atomic mass
Examples to Try
• For the following examples, record the atomic
number, atomic mass, element symbol,
number of protons, electrons, and neutrons
(roughly 7 - 10 minutes)
1. Calcium 2. Helium 3. Tungsten 4. Boron
5. Mercury 6. Gold 7. Manganese
Example: Iron
Atomic Number: 26 Atomic Mass: 55.845g/mol
Symbol: Fe Protons: 26 Electrons: 26
Neutrons: 56 – 26 = 30 neutrons
Isotopes
• Recall, all atoms of the same element, must
have the same number of protons.
• However, atoms of the same element can
have a different number of neutrons
• For example: carbon can have anywhere
between 8 and 22 neutrons.
– However, many of these isotopes are created in a
lab and are not stable (The least stable isotope is
carbon-8, with a half-life of 2.0 x 10−21 s)
Writing isotopes properly
• Two methods to do this:
1. Hyphen notation: Mass number is written
after the name of the element
- Examples: Carbon-12, Carbon-13
2. Nuclear symbol: Composition of the nucleus
and the elements symbol
Examples:
12
- mass #: 12 atomic #: 6
C
6
- mass #: 13 atomic #: 6
13
6C
Practice
• Write the following in hyphen notation or by
nuclear symbol.
1) Carbon-14
2) Helium-4
3) Oxygen-17
4) 4 He
2
5)
2
1
H