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SNC 1D1 – Explaining the Periodic Table
Atomic Number
• What makes atoms unique is the number of protons they
contain. The number of protons is called the atomic
number.
•No two different elements can have the same number of
protons.
• a hydrogen atom has one proton and thus has an atomic
number of 1.
• two different elements can never have the same
properties.
• the atomic number is usually found at the top of the box
for each element in the periodic table.
Mass Number and Atomic Mass
• The mass number consists of the nuclear contents of an
atom, its protons and neutrons.
• A single atom of lithium contains 3 protons and 4 neutrons
and thus has a mass number of 7.
• In almost all elements there exists a small fraction of atoms of
the element that have different numbers of neutrons
• Atoms with the same number of protons but different
numbers of neutrons are called isotopes.
• Mass number is used to describe the nuclear content of one
isotope (usually the most abundant) of an element.
• Atomic mass is the weighted average of all of the isotopes of
an element.
• Atomic masses on the periodic table are not whole numbers
because they contain the mass numbers all of the isotopes of
an element and can thus be fractions.
• In many cases you can determine the most abundant isotope
of an element by rounding the atomic mass to the nearest
whole number.
• The atomic mass of carbon is 12.01 u so the most abundant
isotope of carbon will have a mass number of 12 u.
• The atomic mass is measured in atomic mass units (u).
Determining the Number of Neutrons.
Find the number of neutrons in the most common isotope of
aluminum:
Atomic mass of aluminum:
26.98u (27 rounded)
Atomic number:
13
number of neutrons = atomic mass – atomic number
= 27(rounded) - 13
= 14 neutrons
The most common isotope of aluminum contains 14 neutrons.
Bohr-Rutherford Diagram of an Atom
• A Bohr-Rutherford diagram shows the numbers and locations
of protons, neutrons and electrons in an atom. We can
determine these numbers from the atomic number and the
atomic mass (mass number) on the P.T.
• The number of protons = atomic number
• The number of neutrons = difference between mass number
(rounded atomic mass) and the atomic number
• The number of electrons equals the number of protons in a
neutral atom
P = protons
E = electrons
N = neutrons
Bohr Rutherford Diagram for Sodium:
Atomic mass for sodium:
Atomic number:
Neutrons:
P (protons) = 11
E (electrons) = 11
N (neutrons) = 12
22.99 u (23 u rounded)
11
23 – 11 = 12
Do Bohr-Rutherford Diagrams for the Following Elements:
nitrogen
calcium
neon
P
E
N
P
E
N
P
E
N
• All elements in period 1 of the periodic table are placing
electrons in their first orbit. (maximum 2 electrons)
• All elements in period 2 of the periodic table are placing
electrons in their second orbit. (maximum 8 electrons)
• All elements in period 3 of the periodic table are placing
electrons in their third orbit. (maximum 8 electrons)
• Elements 19 (potassium) and 20 (calcium) place electrons in
the fourth orbit.
• You are not responsible for drawing Bohr-Rutherford diagrams
for elements past element 20 (calcium).
Patterns in the Periodic Table
•
As you go down each family the number of electron orbits
increases, a new orbit is added with each row.
•
Within each family all atoms have the same number of
electrons in their outermost orbit. For example all group 1
elements have one electron in their outer orbit and are very
reactive.
•
In general, reactivity increases as you move down a column
in the P.T. The outermost electrons are further from the
nucleus and are not as easily attracted to the positive
nuclear forces. They are easier to take away and more
reactive.
Homework:
Pg. 240 #1 - 10