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

8O

Oxygen15.999

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 Hydrogen-3

1 proton 1 proton 1 proton1 electron 1 electron 1 electron0 neutrons 1 neutron 2 neutrons

Summary•Atomic # = protons =

electrons (neutral atoms)•Mass # = protons + neutrons

= rounded atomic mass•Neutrons = mass # - atomic

#

Practice makes perfect…

ElementSymbol

Atomic Number

MassNumber

# of Protons

# of Neutrons

# of electrons

S 16

7 14

17 17 18

40 20

16 32 16 16

N

Cl

Ca

7 7 7

35 17

20

20 20