1. Atoms – a closer look at elements
As mentioned before, atoms are composed of protons, neutrons, and electrons

2. Review from the earlier notes
Protons are positive, have a mass roughly the same as a neutron, found in the nucleus, define an element
Neutrons have no charge, have a mass roughly the same as a proton, found in the nucleus, are responsible for isotopes
Electrons are negative, very small mass, smallest subatomic particle, found outside of the nucleus, are responsible for ions (atoms with a charge, add number to electron total if negative, subtract if positive).

3. So, what does it mean to define an element?
An element is defined as the number of protons that it has in its nucleus.
Therefore, the number of protons equals the element’s Atomic Number.
The atomic number is the same for all atoms of the same element.

4. More about the Atomic number
Equals the number of electrons for NEUTRAL atom
Determines an elements placement on the periodic table
Represented by letter Z
So Z = protons = electrons (if neutral, if not see above note)

5. Let’s look at a Periodic Table
How many protons does titanium (Ti) have?
Antimony has 51 protons, what is its atomic number? What abbreviation is given to Antimony?
Magnesium (Mg) is a neutral atom, how many electrons does it have?

6. Ag Mass number (A) Sum of the protons and neutrons of an atom
So, Mass number = protons + neutrons
To find neutrons given protons and mass number:
Neutrons = mass number - protons
108 Ag
This is called a nuclear symbol 47
Top is mass number
Bottom atomic number
Note: Both numbers on left

7. Practice Problem
What is the mass number of an element with 10 protons and 10 neutrons? What element are we dealing with? How would you represent this symbolically?
20 amu Ne 20 10

8. More Practice
How many protons, neutrons and electrons does the following neutral atoms have? Fe
26 protons, 30 neutrons, 26 electrons Xe
54 protons, 77 neutrons, 54 electrons 56 26
131 54

9. Two More – now with a charge
16 protons
16 neutrons
18 electrons
20 protons
21 neutrons
19 electrons

10. Group Work – Make and complete this chart on the white boards
Element/Ion
Atomic Number
Mass Number
Protons
Neutrons
Electrons

11. Look at the masses on the Periodic Table
So, if the atomic mass is equal to the sum of protons and neutrons, why isn’t the mass a whole number on all Periodic Tables of the Elements?
Even though an atom has to have a certain number of protons, the number of neutrons can vary slightly. These different versions of the same atom are called…
Isotopes

12. Isotopes Same number of protons but different number of neutrons
Examples are Carbon -13 verse Carbon -12

13. Average Atomic Masses Relative atomic mass: average masses of isotopes
average atomic mass is also known as Atomic weight (AW).
Atomic weights are listed on the periodic table.
*Don’t confuse these with mass numbers*

14. Naturally occurring Carbon is 98. 892 % 12C + 1. 108 % 13C
Naturally occurring Carbon is % 12C % 13C. Find the average atomic mass
Step 1: Multiply the mass number of each isotope by its percentage
Step 2: Add the numbers together
Step 3: Divide by 100
Step 4: Check your answer. Mass should be between highest and lowest mass

15. An Isotope Example - Chlorine
Let’s look at Chlorine, which has an atomic mass of 35.5
Atomic # = ___, which means that there are ___ protons. 17 17
There are two naturally occurring isotopes, Cl-35 and Cl-37
This means that Cl-35 has ___ neutrons while Cl-37 has ___ neutrons. 18 20
Cl-35 occurs naturally 75.8% of the time
Cl-37 occurs naturally 24.2% of the time
(35 x x 24.2)/100 =
35.5

16. Try this one
Calculate the average mass of the isotopes of Uranium with:
50.0% at 239 amu
29.4% at 235 amu
20.6% at 238 amu
amu

17. Homework
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