1. Atoms, Ions and Isotopes

2. Subatomic Particles Particle Symbol Charge Relative Mass (amu)
Electron e
Proton p
Neutron n

3. Atomic Number the number of protons in an atom
All atoms of the same element have the same number of protons
Atomic Number

4. If an atom is neutral, the net charge is zero
Number of Electrons
If an atom is neutral, the net charge is zero
Number of protons = Number of electrons

5. Mass Number Mass Number = protons + neutrons
number of neutrons = mass number – atomic number
*Mass number is NOT on the periodic table

6. Atomic Symbols atomic number 11 Show the mass number and atomic number
Give the symbol of the element
mass number
23 Na or Sodium-23 (Na-23)
atomic number

7. More Atomic Symbols 16 31 65 8 15 30 O P Zn 8 p+ 15 p+ 30 p+
O P Zn
8 p p p+
8 n (16-8) 16 n (31-15) 35 n (65-30)
8 e e e-

8. Isotopes
Atoms with the same number of protons, but different numbers of neutrons.
Same atomic number
Different mass numbers
Isotopes of chlorine
35Cl 37Cl
chlorine chlorine – 37
Cl – Cl - 37

9. Ions An atom that has gained or lost electrons
An atom that loses electrons has a positive charge (+)
Positive ions are called cations
ex. Na+, Mg+2
An atoms that gains electrons has a negative charge
Negative ions are called anions
Ex. Cl-, O-2
How many protons, neutrons, and electrons are there in the Calcium ion (Ca+2)?
How many protons, neutrons, and electrons are there in the Fluoride ion (F -)?

10. Learning Check AT 5 16 14 14 An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is
C. The element is
1) Si 2) Ca 3) Se
D. Another isotope of this element is
1) 34X 2) 34X 3) 36X

11. Masses of Atoms
A scale designed for atoms gives their small atomic masses in atomic mass units (amu)
An atom of 12C was assigned an exact mass of amu
Relative masses of all other atoms was determined by comparing each to the mass of 12C

12. Calculating Average Atomic Mass
You need to know…
Percent(%) abundance of isotopes
Mass of each isotope
Weighted average =
(mass of isotope 1)(% abundance/100) +(mass of isotope 2)(% abundance/100) + …

13. Atomic Mass of Magnesium
Isotopes Mass of Isotope Abundance
24Mg = 24.0 amu %
25Mg = amu %
26Mg = amu %
Average Atomic Mass = (.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)
Atomic mass (average mass) Mg = 24.3 amu Mg
24.3

14. Learning Check AT7
Gallium is a metallic element found in small lasers used in compact disc players. In a sample of gallium, there is 60.2% of gallium-69 (68.9 amu) atoms and 39.8% of gallium-71 (70.9 amu) atoms. What is the average atomic mass of gallium?

15. Solution AT7 Atomic mass Ga 41.5 amu + 28.2 amu = 69.7 amu
68.9 amu x (60.2/100) = amu for 69Ga
Ga-71
70.9 amu x (39.8/100) = amu for 71Ga
Atomic mass Ga
41.5 amu amu = amu