Defining the Atom

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Slide 16

Mass Number

Most of the mass of an atom is concentrated in its nucleus.

The mass number of an atom is the total number of protons and neutrons in an atom.

Ex: Helium (He) has 2 protons and 2 neutrons: its mass number is 4.

Ex: Carbon (C) has 6 protons and 6 neutrons: its mass number is12.

Slide 17

Mass Number

If you know the atomic number and mass number of an atom, you can determine its composition.

The number of neutrons in an atom is the difference between the mass number and atomic number.

# of neutrons = Mass # - Atomic #

Slide 18

Mass Number

The composition of an atom can be written in Shorthand notation using atomic number and mass number.

Ex: Look at Figure 4.8

The atomic number is the subscript and the mass number is the superscript.

How many electrons does a gold atom have?

How many neutrons does a gold atom have?