Unit 3 Atomic Structure Atomic Number Mass Number
Unit 3: Atomic Structure Atomic Number, Mass Number, and Isotopes
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
Atomic Number • The number of protons in the element. • Always a whole number • Example: Oxygen’s atomic number is 8, so there are 8 protons in Oxygen’s nucleus
Atomic Number • For neutral elements (no charge) the # of p+ = # of e • The number of protons will never change for an element • It is like our social security number, it identifies them.
Atomic Mass • Read right off the Periodic Table. • The number will have a decimal. • It is the average (weighted) of all the elements isotopes.
Atomic Number vs. Atomic Mass 8 O Oxygen 15. 999 Atomic number Atomic mass
Mass Number • Number of protons & neutrons combined • Always a whole number
Finding the Number of Neutrons • Neutrons = mass # - atomic # • Example: Oxygen’s mass number is 16 and its atomic number is 8. The number of neutrons will be 16 -8 = 8.
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
Examples: Neutral hydrogen isotopes Hydrogen-1 Hydrogen-2 1 proton 1 electron 0 neutrons 1 proton 1 electron 1 neutron Hydrogen-3 1 proton 1 electron 2 neutrons
Summary • Atomic # = protons = electrons (neutral atoms) • Mass # = protons + neutrons = rounded atomic mass • Neutrons = mass # - atomic #
Practice makes perfect… Element Symbol Atomic Number Mass Number # of Protons # of Neutrons # of electrons S 16 32 16 16 16 N 7 14 7 7 7 Cl 17 35 17 18 17 Ca 20 40 20 20 20
- Slides: 12