4. Atomic & Mass Number
• Atomic number:
• Mass number:
• Atomic Number =
number of protons in nucleus
• Mass number =
protons + neutrons
5. Number of electrons
• For a neutral atom:
number of protons = number of electrons
• Atomic number:
(# of protons)
• Number of electrons:
• Mass number:
(p+
+ n0
)
6. Isotope Notation
• Atomic Number = number of protons in nucleus
• Mass number = protons + neutrons
• Number of neutrons = Mass number – atomic number
• Number of neutrons:
12. Radioactive
• Low Mass
– Elements 1-20
– Stable – n/p ratio = 1
– Unstable – Above or
Below
• Middle Mass
– Elements 21-83
– Stable – n/p ratio =
• 1 to 1.5
Unstable – Above or
Below
•NOTE: All atoms of elements above 83 are
radioactive!!!
13. Radioactive
• Radioactive
– Elements 84 and above
(all isotopes)
– n/p ratio is too large
• Conclusions:
– Number n0
= number p+
Stable
– Atoms above or below this ratio Unstable
– More mass Unstable (radioactive)
14. Radioactive
• Lithium - 8
• Number protons:
• Number neutrons:
• n/p =
• Lead - 214
• Number protons:
• Number neutrons:
• n/p =