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Atomic Structure
• Atoms are the smallest ‘pieces’ of an element that still
retain all the properties of that element.
• All matter is made up of atoms
of elements.
• Atoms are mostly empty space,
– occupied by a tiny, core called the nucleus,
– and much tinier electrons orbiting around the nucleus in
an area called the electron cloud.
Atomic Structure
• The particles that make up an atom are so small, that
science has a measurement just for things this small – amu
or atomic mass units.
 Protons:
• Inside the
nucleus,
• positive
charge
• 1 amu
 Neutrons:
• Inside the
nucleus,
• neutral (no)
charge
• 1 amu
 Electrons:
• Outside the
nucleus in an
area called the
electron cloud
• negative charge
• So tiny, it would
take about 2000
electrons to equal
the mass of one
proton!..........
1/2000 amu
Atomic Structure
• Atoms of the same element will always have
the same number of protons!
– The atomic number tells you how many protons the element has.
– In it’s ‘resting’ state, an atom has the same number of electrons as
protons!
Atomic Structure
• The mass number tells you the mass of the atom –
electrons are extremely tiny compared to protons
and neutrons and can be lost or gained, so only
protons and neutrons are counted in the mass
number.
– Mass number = # protons + # neutrons
Isotopes
• Atoms of the same element ALWAYS have the same
number of protons………but, do not necessarily have the
same number of neutrons.
– Isotopes are atoms of the same element with different
numbers of neutrons….they are variations of the
element.
• Hydrogen has 3 isotopes – All have the same number of
protons (1), but different numbers of neutrons.
– Protium has a mass number of 1 ….. Most common form of hydrogen
– Deuterium has a mass number of 2
– Tritium has a mass number of 3
Periodic Table
• Classification systems are methods of grouping
things together in a way that makes sense, so that
like are with like – makes it easier to find things if
you know where to look.
• The periodic table was developed by Dmitri
Mendeleev in 1869, as a way of classifying elements
according to their properties.
Periodic Table
• The periodic table is arranged by increasing atomic number.
– During Mendeleev’s time, this arrangement left several blanks,
however, the table exhibited a regularly repeating pattern, which
could be used to predict the properties of elements that had not
been discovered yet.
– He was proven right when scandium, gallium and germanium
were discovered, and had properties very close to what he had
predicted.
• The periodic law states that elements arranged as they are
in the periodic table have similarities that reoccur in a
repeating, regular pattern.
Atomic Number
Elemental Data
26
Element Symbol – 1 or 2
letter abbreviation for the
element.
The first letter is ALWAYS
capitalized.
If there is a second letter, it is
ALWAYS lower case.
Fe
Iron
Element
Name
55.847
Average Atomic Mass – the average of the masses of all the isotopes
of that element. For iron, there are Fe-56, Fe-57, Fe-54 and Fe-58, and
over 90% are the Fe-56.
Periodic Table
• The horizontal rows are called periods –
– As you move left to right, you increase the number of protons
and the number of electrons by one.
• The vertical rows are called groups, of families –
– Elements in a group or family have
similar characteristics.
• These are the groupings that Mendeleev started out with – elements
with similar characteristics.
Metals
• The periodic table is generally divided into unequal ‘halves’
– Most are to the left of the dividing line are metals.
• Tend to lose electrons during bonding
– Elements to the right of the zigzag line, except for
Hydrogen, are nonmetals.
• Tend to gain electrons during bonding
Reactivity
• Metals tend to react with nonmetals…..
• Look at the group number – The magic number for
reactions is 8
– Atoms in Group 1 are highly reactive with Group 7
– Atoms in Group 2 are reactive with Group ____
• The farther to the left or right, the more reactive.
– Atoms in Group 1 are more reactive than Group 2
– Atoms in Group 7 are more reactive than Group 6
• What about atoms in Group 8…..
Noble Gases
– Group 18 are the noble gases and are very stable cuz they do not
react at all
• They have 8 electrons in their outermost energy level, which for atoms, is
PERFECT! They do not need or want to bond with other atoms.
• When you add energy to an atom, that energy can excite the electrons,
causing them to jump out to a farther energy level. This is not a stable
arrangement, and another electron will likely ‘fall’ down into the vacated
spot in the lower orbital. When it drops down, it loses energy that it no
longer needs. This emitted energy is often seen as a flash of light = neon
lights!