1. The Structure of Atoms
Section 4.2

2. Something to keep in mind …
You want to be able to … describe the general structure of the atom, and explain how the properties of the first 20 elements in the Periodic Table are related to their atomic structures

3. Defining the Atom
Atom – the smallest particle of an element that retains the properties of that element.
When we say “small,” we mean “SMALL!”
A typical copper penny contains
29,000, ,000,000,000,000 atoms
Current world population is about
7, 382,000,000 people

4. Just how small are atoms?

5. Parts of the Atom There are two main parts in each atom
Nucleus (+ charge)
Protons (+1 charge)
Neutrons (no charge)
Electron Cloud (- charge)
Electrons (-1 charge)
Lots and lots of empty space

6. How can we tell atoms of different elements apart?
There are differences in numbers
of subatomic particles.

7. First – we need to become familiar with the Periodic Table
Quick Practice! Use your Periodic Table to complete the “Elements and Their Symbols” worksheet.

8. Atomic Number
The atomic number of an element is the number of protons in the nucleus of the atom of that element.
Where can I find this?
Look on the Periodic Table

9. Every atom of Cobalt contains 27 protons
Atomic Number
Locate any element on the Periodic Table
The atomic number = the number of protons in every atom of that element
Every atom of Cobalt contains 27 protons

10. Electrons How do you know the number of electrons in an atom?
Since atoms are neutral, the number of electrons in an atom must equal the number of protons.
It takes one electron to cancel out the charge from one proton.

11. This is the Atomic Mass of Cobalt
Atomic mass can be used to determine the number of neutrons in an “average” atom
This number is found on the Periodic Table
This is the Atomic Mass of Cobalt

12. Periodic Table Practice Sheet
Quick Practice!
Periodic Table Practice Sheet
Use the periodic table to obtain information about the number of protons and electrons for a given element.

13. Number of Protons + Number of Neutrons = Mass Number (AMU)
The total number of protons and neutrons in the nucleus is the mass number of the atom.
Mass number is NOT found on the Periodic Table
Number of Protons + Number of Neutrons = Mass Number (AMU)
The units for mass number are “AMU” which stands for atomic mass unit.

14. Calculating Mass Number Example 1
Determine the mass number of an atom that has 8 protons and 8 neutrons? And, what is the identity of this element?
How do you find the mass number?
8 protons + 8 neutrons = 16 AMU
How do you identify the element?
8 protons means that the atomic number is 8, which means this is oxygen.

15. How do you determine the number of neutrons?
Mass Number Example #2
Determine the number of neutrons in an atom of Boron that has a mass number of 11.
How do you determine the number of neutrons?
Mass number – Atomic Number = Number of Neutrons
11 (Mass number) – 5 (Atomic Number) = 6 Neutrons

16. Example #3…
How many protons, neutrons and electrons are there in one atom of Helium-4?
Number of Protons = atomic number
2 protons
Number of Electrons = Number of Protons
2 electrons
Number of Neutrons =
Mass number – Number of Protons
4 – 2 = 2 Neutrons

17. Isotopes
Isotopes are atoms of the SAME element with different numbers of neutrons.
Therefore, isotopes for the same element will have the SAME ATOMIC NUMBER but have DIFFERENT MASS NUMBERS

18. Expressing Isotopes
There are two ways to write the symbol for an isotope.
#1 is Nuclear Symbol
Isotopes are written as the chemical symbol with two additional numbers to the left side of it.
115B
Mass number is a superscript
Atomic number is a subscript.

19. Expressing Isotopes Lithium–7 Li-7
#2 = Hyphen notation, which is a shorthand for the nuclear symbol.
Lithium–7
Li-7
The name of the element or symbol is written first
There is a dash and then the mass number is written

20. Putting it all together
What is the atomic number?
What is the atomic mass?
39.95 amu
How many protons, neutrons and electrons are in an atom of Rubidium if the mass number is 80?
37 protons, 37 electrons
43 neutrons
80 (mass number)- 37 (atomic number) = 43 neutrons
37 protons and 37 electrons

21. Confusing Terms! Atomic Number Atomic Mass Mass Number
This tells you the number of protons in an atom of an element
This also tells you the number of electrons in an atom of an element, because elements are neutral.
This IS found on the Periodic Table
Atomic Mass
This tells you the mass (in grams) of one mole of atoms of an element
Can be used to determine the number of neutrons in an “average” atom
Mass Number
The total number of protons and neutrons in the nucleus is the mass number of the atom.
This is NOT found on the Periodic Table

22. What’s Next?
We will use what we learned here to determine the number of particles in atoms and isotopes of elements.