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Atoms, Ions and Isotopes
Subatomic Particles
Particle
Symbol
Charge
Relative
Mass (amu)
Electron
e-
1-
0
Proton
p+
1+
1
Neutron
n
0
1
2
Atomic Number
• the number of protons in an atom
• All atoms of the same element have the
same number of protons
Atomic Number
3
Number of Electrons
• If an atom is neutral, the net charge is zero
– Number of protons = Number of electrons
4
Mass Number
Mass Number = protons + neutrons
• number of neutrons = mass number – atomic number
*Mass number is NOT on the periodic table
5
Atomic Symbols
• Show the mass number and atomic number
• Give the symbol of the element
mass number
23 Na or Sodium-23 (Na-23)
atomic number
11
6
More Atomic Symbols
16
O
8
8 p+
8 n (16-8)
8 e-
31
65
P
Zn
15
30
15 p+
16 n (31-15)
15 e7
30 p+
35 n (65-30)
30 e-
Isotopes
• Atoms with the same number of protons, but
different numbers of neutrons.
– Same atomic number
– Different mass numbers
–
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
Cl – 35
chlorine – 37
Cl - 35
8
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 -)?
Learning Check AT 5
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
C. The element is
1) Si
2) Ca
3) 34
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
16
14
14
10
3) 36X
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 12.00
amu
• Relative masses of all other atoms was determined by
comparing each to the mass of 12C
11
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) + …
12
Atomic Mass of Magnesium
Isotopes
24Mg
=
25Mg
=
25.0 amu
10.13%
=
26.0 amu
11.17%
26Mg
Mass of Isotope Abundance
24.0 amu
78.70%
(.7870 x 24.0) + (.1013 x 25.0) + (.1117 x 26.0)
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
13
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?
14
Solution AT7
Ga-69
68.9 amu x 60.2 = 41.5 amu for
69Ga
100
Ga-71 (%/100)
70.9 amu x 39.8
=
28.2 amu for
=
69.7 amu
100
Atomic mass Ga
15
71Ga
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