Physical Science 513
Unit 3
3.2 – Atoms and Elements
Lesson Objectives
6.
Demonstrate an understanding of Chapter 18
vocabulary words
7.
Describe the difference between an ion and an
atom
8.
Explain how isotopes of a given element are
different from each other
9.
Use a chemical symbol to calculate the number
of protons, neutrons, and electrons
The Atom

Definitions
– Atomic number: The number of protons in an atom
– Mass number: The number of protons + neutrons in
an atom
– Atomic mass: The average mass of an atom in a natural
sample of the element
– Ion: An atom that has lost or gained one or more
electrons so that it is no longer electrically neutral
– Isotope: Atoms with the same number of
protons but different number of neutrons
Chemical Symbols
Example: Magnesium
24
12

Mg
Atomic Number (# of protons in nucleus)?
– 12
– Written in lower left hand corner of chemical symbol

Mass Number (# of protons and neutrons)?
– 24
– Written in upper left hand corner of chemical symbol
– Can also be written after element name
 Magnesium-24 or Mg-24
Isotopes

Will isotopes of magnesium have different
atomic numbers?
– No
– Why not?

Will isotopes of magnesium have different mass
numbers?
– Yes!
– Why?
– Example: Mg-24, Mg-25, and Mg-26
– How many neutrons are in each of these?
Isotopes
Isotopes are atoms of the same element that
contain a different number of neutrons
 Hydrogen has 3 naturally occurring isotopes:

– Protium: 1p, 0n
– Deuterium: 1p, 1n
– Tritium: 1p, 2n

Isotopes are identified by their mass number

Protium has a mass number of 1
– Can be written as hydrogen-1
1
– Can also be written as H
1
Ions

Will a magnesium ion have a different atomic
number?
– No
– Why not?

Will a magnesium ion have a different mass
number?
– No
– Why not?
Ions
Ion - an atom that has lost or gained one or
more electrons
 No longer electrically neutral

All that will change is the CHARGE
24
+2

Mg

12

How many protons, neutrons, and
electrons are represented above?
Atomic Mass
Periodic tables give atomic mass instead
of mass number
 Atomic mass is the sum of all of the
masses of the electrons, protons and
neutrons
 The average atomic mass of the various
isotopes of each element is shown on the
periodic table
 What is the atomic mass for Magnesium?

– 24.31