The Atomic Number

The Atomic Number

• Reminder: Elements are different types of atoms (like different LEGO blocks)

• Elements are defined by the number of protons in the nucleus– This is the atomic number– Locate it on your diagram, & label!

• The same element may have different #’s of electrons & neutrons ALWAYS have the same # of protons!

Atomic Mass

• # of protons + # of neutrons = atomic mass

• Located on bottom of element diagram

• So, we can calculate the # of neutrons by:Atomic Mass – Atomic Number = # of Neutrons

Isotopes

Isotopes

• Not all elements are the same!– Number of neutrons can vary!– What has to stay the same?

• Isotopes: atoms of the same element with different numbers of neutrons

What does this mean?

• These don’t affect the chemical or physical properties

• BUT there are some isotopes that are unstable atoms fall apart releasing energy (radioactive decay)

• Have different atomic masses– Why A.M on periodic table are not whole #’s! they

are averages!

How designate which one you are talking about?

• Nuclide notation: mass of isotope in upper left hand side of symbol; place atomic number on bottom left hand side– Example:

• Hyphen Notation: can refer to it by element name & mass – Example: Hydrogen-2

Average Atomic Masses

• Definition: weighted average of the atomic masses of naturally occurring isotopes of an element

• See periodic table to see averages• Math:(mass of A)(%abundance of A) + (mass of B)

(%abundance of B) +(mass of C)(%abundance of C)/100

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