
The atom can be broken down into even
smaller parts.
◦ The Proton is a positively charged particle that
resides in the nucleus of an atom.
◦ The Neutron is a neutrally charged particle that
adds mass to the nucleus.
◦ Electrons are negatively charged particles that orbit
the nucleus in the electron cloud.
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
Elements are groups of atoms that are all the
same.
Elements are defined by how many protons
are in their nucleus.
◦ The element with 6 protons is known as Carbon.
◦ The number of protons is also called the Atomic
Number
of an element. The atomic number
99
Es
Einsteinium
of Einsteinium is 99.
This means there are
99 protons in an
atom of Einsteinium.
Note: There are the same
number of electrons as
protons in every atom.

Another important part of Chemistry is
Chemical Symbols.
◦ Chemical Symbols are abbreviations used to identify
the different elements.
◦ Some give clues to what the element is
 E.g. C-Carbon, He-Helium, F-Fluorine
◦ While others are derived from other languages
 E.g. Na-Sodium, W-Tungston, Pb-Lead

Chemical symbols always have the first letter
capitalized and the second letter is lower
cased.

The Atomic Mass Number of an element is
the atomic mass rounded to a whole number
◦ It represents the number of protons plus the
number of neutrons

The Atomic Mass is the weighted average of
all the isotopes of an element
◦ It is measured in atomic mass units (amu)
◦ Note: An isotope is an element with the same
number of protons, but a different number of
neutrons.
6
C
12.01
Atomic Number
Chemical
Symbol
Atomic Mass
*So, the atomic
mass number for
Carbon would be
12



Isotopes- Calculating the number of neutrons
Isotopes have different numbers of neutrons in
elements that have the same number of protons.
To find the number of neutrons subtract the
number of protons from the mass number.
◦ Mass number–number of protons = number of neutrons
◦ E.g. How many neutrons in Carbon-12 and Carbon-14?
12-6= 6 neutrons
14-6= 8 neutrons
Both are carbon, but they do not have the same number of
neutrons.

Calculating the atomic mass
◦ Atomic mass can be calculated for an atom by
multiplying the mass of each isotope by its percent
abundance, then adding the results.
◦ E.g. The element neon has three isotopic forms:
Neon-20 (90.92%), Neon-21 (0.257%), and Neon22 (8.82%). What is the atomic mass of neon?
(.9092 x 20) + (0.00257 x 21) + (0.0882 x
20.2 amu
Amu= atomic
22)
mass=units
Note: Make sure to change your percentage to a
decimal.

During chemical changes, atoms sometimes
lose or gain electrons to form charged
particles called ions.
◦ Atoms that gain electrons have a negative charge.
 These are called cations
 Depicted with a + sign and a number.
 E.g. Li+, Ca+2, Al+3
◦ Atoms that lose electrons have a positive charge.
 These are called anions
 Depicted with a – sign and a number
 E.g. Cl-, O-2, N-3