1. Atoms, Ions and Isotopes
A closer look at the importance of protons, electrons and isotopes

2. Table of Contents If you wish to focus on a single topic,
skip to the appropriate slide
Review: What is an Atom? Slide 3
Atomic Number Slide 4
Mass Number Slide 8
How to Write a Complete Atomic Symbol Slide 16
Ions Slide 17

3. Review: What is an atom?
An atom contains protons and neutrons, located inside the nucleus, and electrons, located outside the nucleus.
We have identified the charge on each particle as being positive (protons), negative (electrons) or neutral (neutrons).
In a neutral atom, the number of protons and the number of electrons must be equal.

4. Atomic Number
(all about protons)

5. Atomic Number
The identity of an atom is defined by the number of PROTONS in the atom.
This number is referred to as the Atomic Number.

6. Atomic Number Starting at the beginning:
An atom with one proton has an atomic number of one.
This atom is identified as Hydrogen.
An atom with two protons has an atomic number of two.
This atom is identified as Helium.
An atom with three protons has an atomic number of three.
This atom is identified as Lithium.
We could keep going. What would be the atomic number of an atom with four protons?

7. Atomic Number
Remember: An atom with one proton (an atomic number of one) will ALWAYS be a Hydrogen atom.
A Hydrogen atom will ALWAYS have one proton (an atomic number of one).
The identity of an atom is defined by the number of protons!

8. Mass Number
(Working with neutrons, too)

9. Mass Number Let’s consider a hydrogen atom.
As we stated previously, Hydrogen has an atomic number of one, meaning it has one proton. But what about the rest of the atom? How many electrons does it have? How many neutrons?
STOP and THINK: If we consider a neutral hydrogen atom, how many electrons must it have?

10. Mass Number
If we remember that protons have a charge of +1 and electrons have a charge of -1, we can determine the number of each in a neutral atom.
Hydrogen has one proton, for a total charge of +1. Therefore, to be neutral, it must have one electron.
But what about the number of neutrons?
At this point – we don’t know! There’s nothing in the atomic number, or the fact that it’s a neutral atom, to tell us how many neutrons there are. This is why we need a mass number.

11. Mass Number
The mass number for an atom is defined by the number of PROTONS and NEUTRONS in the atom.

12. Mass Number Let’s consider three options for our Hydrogen atom.
Option 1: It has zero neutrons
Option 2: It has one neutron
Option 3: It has two neutrons
In each case, we still have a Hydrogen atom – the atomic number hasn’t changed.
The mass number, however, would be different.
Option 1: 1 proton + 0 neutrons gives a mass number of 1
Option 2: 1 proton + 1 neutron gives a mass number of 2
Option 3: 1 proton + 2 neutrons gives a mass number of 3

13. They have the same atomic number, but a different mass number.
Atoms with the same number of protons but differing numbers of neutrons are called ISOTOPES.
They have the same atomic number, but a different mass number.

14. Mass Number
In the case of Hydrogen, all three of these isotopes are naturally occurring. However, they don’t occur with equal frequency – 1H is the most common!
The same applies to many other types of atoms. Carbon, for example, has three naturally occurring isotopes (12C, 13C, 14C). How many protons, neutrons, and electrons are present in a neutral atom of each isotope?

15. Mass Number 12C has: 6 protons 6 neutrons 6 electrons 13C has:

16. Writing Full Atomic Symbols
We can write a full atomic symbol to give information regarding the atomic number and mass number of an atom. 1 2 3 H H H 1 1 1
Mass Number
Displayed at the upper left
Atomic Symbol
As determined from the periodic
table. The first letter in the symbol
is always a capital. The second
letter (when present) should be
lowercase.
Atomic Number
Displayed at the lower left,
often omitted, since this can be determined from the symbol

17. Ions
(And finally, electrons)

18. Ions
To this point, we’ve been considering neutral atoms.
However, atoms are not always neutral!
When an atom has an unequal number of protons and electrons, the result is an atom with an electrical charge, known as an ION.

19. Ions
When an atom has more protons than electrons, it has a positive charge. This type of ion is called a CATION.

20. Ions
When an atom has more electrons than protons, it has a positive charge. This type of ion is called an ANION.

21. Ions Let’s go back to Hydrogen. STOP and THINK:
What is the atomic number for Hydrogen?
How many protons are in a Hydrogen atom?
How many electrons are in a neutral Hydrogen atom?

22. Ions
A neutral Hydrogen atom has an atomic number of 1, contains one proton and one electron.
(If you came up with a different answer,
you may wish to go back and review a bit!)
The next step is to consider a non-neutral atom.
STOP and THINK: What would happen if Hydrogen were to lose its electron?
1. How many protons does it have now? Electrons?
2. What would be the charge on the Hydrogen
atom now?

23. Ions
1. Well, we didn’t say anything about losing a proton, so the Hydrogen atom still has one proton. We started with one electron, and lost one electron, so now the atom has zero electrons.
With different numbers of protons and electrons, this atom is now called an ION.
2. The charge on each proton is +1, and there is one proton. Each electron has a charge of -1, but there aren’t any electrons. If we add those up, we get a total charge of +1 on this ion

24. Ions
To include a charge when writing the full atomic symbol for an ion, place the value (1,2,3, etc) and sign (+ or -) of the charge in the upper right hand corner. 1 1+ H 1
The value and the sign of the charge are shown in the upper right corner. If the charge has a value of 1, it is possible to display only the sign (H+ rather than H1+)