The document discusses atomic structure and the key parts of an atom: protons, neutrons, and electrons. It explains that protons are found in the nucleus and give an atom its atomic number. The number of protons never changes for a given element. It also defines atomic number as the number of protons and mass number as the total number of protons and neutrons in an atom's nucleus.

1. Atomic
Structure

2. Mendeleev arranged the elements in order
of increasing atomic mass (1869).

3. Modern Table: organized by atomic #

4. All atoms are composed of three
parts…
Protons + charge Found in Nucleus Heavy
Neutrons No charge Found in Nucleus Heavy
Electrons - Charge Found outside Weigh almost
Nucleus nothing
Which part of the
atom weighs the
Nucleus most?
Nucleus!!!

5. Atoms are always electrically neutral…
• So the number of __________ & __________
PROTONS ELECTRONS
is always equal!

6. Atomic Number
• The number of protons in an atom
• Gives the atom it’s identity because the
number of protons will never change!
Look at your periodic table:
What is atomic number of Magnesium (Mg)?
What is the atomic number of Oxygen (O)?

7. Which element has 7 protons?
Which element has 35 protons?
Which element has 10 protons?

8. Mass Number
• The number of PROTONS + NUETRONS in the
nucleus.
• Both of these parts are “heavy” so they
contribute to the mass of the element.
• Why don’t we count the electrons?
Mass Number= Protons + Neutrons

9. What is the mass number of these
atoms??

10. Atomic Number = # of protons
Mass Number = protons + neutrons
Name Symbol Atomic Mass # of # of # of
Number Number Protons Neutrons Electrons
Beryllium Be 4 9 4 5 4
Oxygen O 8 16 8 8 8
Potassium K 19 39 19 20 19