1. The Structure of Atoms and Ions
Mr. Shields Regents Chemistry U01 L03

2. Atomic Number
Mosley’s and Rutherford’s work explained the key role of Protons in defining an element.
The number of protons an element has
determines it’s ATOMIC NUMBER
Every element has a unique atomic #
2. Therefore every element has a different number of protons
This makes it different from every other element

3. Atomic Number When we write the atomic number it is written to the
Left of the element symbol and at the bottom.
Atomic no. 3

4. Mass Number
An Atoms Mass Number is the sum of both Protons & Neutrons in the nucleus.
Protons and neutrons have about the same mass
For example Carbon’s Atomic No. = 6.
So if Carbon also had 8 Neutrons it’s Mass Number would be?
6p + 8n = 14

5. Mass Number When we write the Mass Number it is written to the
Left of the element symbol and at the top.
Atomic Mass
Number = 7

6. Atomic Charge We know atoms must be electrically neutral. Why? +/-
If atoms are neutral but have Positive Protons
In the nucleus then there must be one electron to balance the positive charge of each Proton
Therefore the ATOMIC NUMBER not only indicates
the number of PROTONS but ALSO the number of
ELECTRONS

7. Notation 12C 6 How many neutrons in this atom?
12 = no. of protons + neutrons.
6 = no. of protons.
12C 6
How many neutrons in this atom?
12 – 6 = 6 neutrons

8. Notation
You might also see atomic mass designated as C-12 or O-17. The number following the symbol is the mass number - not the atomic number.
With this designation you would have to look up the atomic number since it’s not designated.

9. C-14 15N 7 Try these … Mass number = ? Atomic number = ?
# of neutrons = ?
# of electrons = ?
C-14
Mass number = ?
Atomic number = ?
# of neutrons = ?
# of electrons = ?
15N 7
How many electrons in U-238? 92
How many neutrons in U-238?
146

10. Ions Atoms can gain or lose Electrons -
This ability to gain or lose electrons determines how an element will react to form bonds
After an element that gains or loses one OR MORE electrons it is called an ION - - +

11. Positive Ions A Positive Ion is called an Cation
Gaining or losing electrons does not affect the number of protons an element has
When an atom loses an electron it becomes POSITIVE (+)
A Positive Ion is called an Cation
Remember this as “C+ “
Note: The number of Protons now exceeds the number of Electrons

12. Positive Ions
Charge is indicated in the Upper Right of the element symbol
Include the Sign of the charge plus the magnitude
Ex. +2, +3
Exception: +1 is written only as a + with no number
Example:
Na  Na e- (or Na - 1e-  Na+ )
Cu  Cu e-

13. Negative Ions When an atom GAINS an electron it becomes NEGATIVE
This Ion is called a Anion
Remember this as “ A- “
The number of ELECTRONS now exceeds the number of protons
As before charge is written in the upper right
Charge is now negative so the number is written with a “-” sign
Cl + 1e-  Cl-
S e-  S-2

14. Ions QUESTION (1): Write the chemical equation
for the loss of 1 electron from the Rubidium atom?
Rb  Rb+ + 1 e-
QUESTION (2): Nitrogen can gain three electrons. Write the chemical equation for the gain of these three electrons.
N e-  N-3

15. Putting it all together
How many protons, neutrons, & electrons in each of the following:
23Na+1
34S-2
19F-1 11 16 9
16 p, 18 n, 18 e
9 p, 10 n, 10 e
11 p, 12 n, 10 e