Download atomic structure

ATOMIC
STRUCTURE
ESSENTIAL
QUESTION…..
How can you identify an element
and/or isotope based on its
subatomic particles?
WHAT IS AN
ATOM
COMPOSED OF?
• COMPOSED OF THREE SUBATOMIC
PARTICLES:
– PROTON (POSITIVELY CHARGED)
– NEUTRON (NO CHARGE)
BOTH IN THE NUCLEUS
– ELECTRON (NEGATIVELY CHARGED)
ALL FOUND OUTSIDE THE NUCLEUS
(ELECTRON CLOUD)
ABOUT THE PARTICLES…..
• PROTONS (p+)
– Proton is nearly 2000 times more massive than the electron, but
equal in charge and opposite in sign to the electron
– Number of protons in the nucleus is electrically balanced by an
equal number of electrons
• NEUTRONS (n0)
– Mass almost exactly equal to that of the proton
• ELECTRONS (e-)
– Mass of electron much less than the mass of a proton and neutron
– Equal number of protons and electrons = NEUTRAL ATOM
Properties of Subatomic Particles
Particle
Relative
Symbol Location Charge
mass
Actual mass
(g)
1–
1
1840
9.11×10-28
Nucleus
1+
1
1.673×10-24
nucleus
0
1
1.675×10-24
Electron
e-
Around
nucleus
Proton
p+
Neutron
n0
ATOMIC NUMBER
11
Na
22.990
Sodium
• REPRESENTS THE NUMBER OF
PROTONS IN AN ATOM
• ELEMENTS ARRANGED BY
INCREASING ATOMIC NUMBER
• UNIQUE TO A GIVEN ELEMENT
• SINCE ALL ATOMS ARE
NEUTRAL, ATOMIC NUMBER
ALSO TELLS US THE NUMBER
OF ELECTRONS IN AN ATOM
• USUALLY FOUND AT THE TOP
OF THE ELEMENT’S SYMBOL
LEARNING CHECK 1
State the number of protons for atoms of each of the
following:
A. Nitrogen (N)
1) 5 protons
2) 7 protons 3) 14 protons
B. Sulfur (S)
1) 32 protons
2) 16 protons 3) 6 protons
C. Barium (Ba)
1) 137 protons 2) 81 protons 3) 56 protons
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MASS NUMBER
11
• THE TOTAL NUMBER OF PROTONS AND
NEUTRONS IN THE NUCLEUS OF AN ATOM
Na
• MASS NUMBER – ATOMIC NUMBER = # NEUTRONS
22.990
Sodium
• ROUND THIS NUMBER TO THE NEXT WHOLE
NUMBER TO DETERMINE THE MASS NUMBER
Example: Sodium (Na)
Mass Number =
Atomic Number =
23
- 11
______
12 neutrons
PRACTICE FROM THE PERIODIC TABLE
Complete the following chart:
Name
Symbol
Mass
Number
#
#
#
protons neutrons electrons
Lithium
Mg
26
16
ISOTOPES
• DALTON THOUGHT THAT…..
– Atoms could not be
divided(indivisible)….atoms can be divided
– All atoms of an element are IDENTICAL….not
really!!
– Atoms of an element have same number of p+
and e-, but the number of n0 may be different
ISOTOPES
• ISOTOPES ARE…
– Atoms of the same element with the same
number of protons, but different numbers
of neutrons.
– Atoms of the same element (same atomic
number) with different mass numbers
THE ISOTOPES OF HYDROGEN
All atoms have the same number of
protons (p+=1)…….
….but different number of neutrons in
the nucleus (which means these
isotopes will have different mass
numbers!!)
ISOTOPIC NOTATION
A
Z
X
A = mass number
(the total number of protons + neutrons)
Z = atomic number
(the total number of protons)
X = element symbol
READING ISOTOPIC NOTATION
46
21
Sc
46 = mass number
(the total number of protons (21) + neutrons (25))
21 = atomic number
(the total number of protons (21))
Sc = element symbol
(In a neutral atom, the number of electrons (21) is
equal to the number of protons)
ANOTHER WAY OF WRITING AN
ISOTOPE
46
21
Sc or Scandium-46
Name of the element followed by the mass number
(This format does not include the atomic number)
PRACTICE
16
O
8
8 p+
8n
8 e-
31
P
65
Zn
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
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PRACTICE
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
3) 34
2) Ca
3) Se
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
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17
ISOTOPES
• Most elements found as a mixture of
isotopes
• Relative abundance of each isotope is
constant
• Example: Potassium in bananas
– 93.25% of K atoms have 20 n0
– 6.7302% of K atoms have 22 n0
– 0.0117% of K atoms have 21 n0
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ISOTOPES
• Isotopes of an element differ in mass
• The more neutrons in an isotope, the
greater the mass
• Still, isotopes of an element have the same
chemical behavior
• Why? Chemical behavior depends on the
number of e-, not n0 or p+
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CALCULATING ATOMIC MASS
Percent abundance of isotopes
Mass of each isotope of that element
Average Atomic Mass=
mass isotope1(percent abundance1) + mass isotope2(percent abundance2) + …
100
100
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ATOMIC MASS OF MAGNESIUM
Isotopes
Mass of Isotope
24Mg
=
24.0 amu
Abundance
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Average atomic mass of Mg = 24.3 amu
Mg
24.3
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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 atomic mass of gallium?
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SOLUTION AT7
Ga-69 (%/100)
68.9 amu x 60.2 =
41.5 amu for Gallium-69
100
Ga-71 (%/100)
70.9 amu x 39.8
= 28.2 amu for Gallium-71
100
Atomic mass Ga
=
69.7 amu
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