Element Notations Notes
Atomic Number (Z)
The atomic number tells us the number of protons that are in an atom’s nucleus.
If the number of protons in an atom’s nucleus changes, the identity of the atom changes.
We identify the elements by the number of protons.
Isotopes
Scientists found that atoms of an element can exist with the same number of protons but
with a different number of neutrons.
When an atom has a number of neutrons unequal to its number of protons, it is called an
isotope.
Different isotopes of the same element have essentially the same chemical properties but can
have different physical properties.
Mass Number (A)
This number helps us to identify which isotope of an element we are talking about.
The mass number is a whole number found by adding the number of protons (or atomic
number, “Z”) to the number of neutrons found in an atom’s nucleus (“N”). A = Z + N
Isotopes are named by writing the element name, followed by a hyphen and the element’s
mass number. e.g. Carbon-12, Carbon-13, Carbon-14
Isotopic Notation
Another way to indicate different isotopes is by using the element’s symbol and writing a
superscript in front that tells the isotope’s mass number (# protons + # neutrons) and a
subscript tells the isotope’s atomic number (# protons).
Example: The carbon-14 isotope would be written:
14
6
C
Atomic Mass
The atomic mass is another name for the mass number (which is the mass of an atom’s
protons and neutrons).
The mass of one proton or neutron is 1.008 amu (atomic mass unit).
The atomic mass seen on the periodic table is an average of all an element’s isotopes.
Example calculation of atomic mass using Nitrogen (element #7):
Take the sum of Nitrogen’s protons + neutrons: 7 + 7 =14
Then multiply that by 1.008 (the mass of one P or N): 14 x 1.008 amu = 14.112 amu
Valence Electrons
Valence electrons are the electrons in the outermost energy level. They are responsible for an
atom’s chemical behavior.
Valence electrons are not necessarily the last electrons to fill an orbital. However, they are
the electrons with the highest “n” value, the highest principle quantum number.
Valence electrons will NEVER be in the d or f orbitals.
(This is because they fill a higher “n” value s or p orbital before filling a lower “n” value d or
f orbital.)
Example: Argon
Conventional electron configuration: 1s22s22p63s23p6
To determine the number of valence electrons, first find the s and/or p orbitals with the
highest n value then total the electrons in those orbitals.
Argon has 2 electrons in the 3s orbital and 6 electrons in the 3p orbital so
6 + 2= 8 valence electrons.
Example 2: Nickel [Ar]4s23d8 thus Nickel’s has 2 valence electrons (from the 4s sublevel).
Electron Dot Notation
For electron-dot notation, each valence electron is represented as a dot beside the element
symbol. If there are multiple valence electrons, they are put in pairs on the four sides of the
symbol. This works because any element will never have more than 8 valence electrons.
Earlier, we found that Nickel (Ni) had 2 valence electrons. This is written as
Ni: