Unit 3: Atomic Structure

1. Unit 3: Atomic Structure Atomic Number, Mass Number, and Isotopes

2. After today, you will be able to… • Find the atomic mass, atomic number, and symbols of elements off of the Periodic Table • Calculate the number of neutrons in a particular atom • Explain what an isotope is

3. Atomic Number • The number of protons in the element. • Always a whole number • Example: Oxygen’s atomic number is8, so there are 8protons in Oxygen’s nucleus

4. Atomic Number • For neutral elements (no charge) the # of p+ = # of e- • The number of protons willnever change for an element • It is like our social security number, it identifies them.

5. Atomic Mass • Read right off the Periodic Table. • The number will have a decimal. • It is the average (weighted)of all the elements isotopes.

6. 8 O Oxygen 15.999 Atomic Number vs. Atomic Mass Atomic number Atomic mass

7. Mass Number • Number of protons & neutrons combined • Always a whole number

8. Finding the Number of Neutrons • Neutrons = mass # - atomic # • Example: Oxygen’s mass number is 16 and its atomic number is8. The number of neutrons will be 16-8 = 8.

9. Isotopes • Atoms with the same number of protons, but different number of neutrons. • They are chemically alike • Have different mass numbers • Written like this: • Element–Mass Number

10. Examples: Neutral hydrogen isotopes Hydrogen-1 Hydrogen-2 Hydrogen-3 1 proton 1 proton 1 proton 1 electron 1 electron 1 electron 0 neutrons 1 neutron 2 neutrons

11. Summary • Atomic # = protons = electrons (neutral atoms) • Mass # = protons + neutrons = rounded atomic mass • Neutrons = mass # - atomic #

12. Practice makes perfect…  16 32 16 16 N 7 7 7 Cl 35 17 Ca 20 20 20