Section 4.3

The nucleus

Particle Location
Electron
In the space surrounding nucleus
Charge
1-
Relative
Mass
(amu)
Actual
Mass
(g)
0
(1/1840)
9.11 x 10 -28
Proton Nucleus 1+ 1
1.673 x 10 -
24
Neutron Nucleus 0 1
1.675 x 10 -
24

–What makes an atom of oxygen an oxygen atom?

• An element’s atomic number (Z) represents the number of protons in its nucleus
• The number of protons in an atom’s nucleus determines the identity of the atom
– If an atom has 9 protons it’s fluorine, if it has
20 it’s calcium
• In a neutral atom:
– number of protons = number of electrons, so: atomic number = # protons = # electrons

Atomic number (Z): number of protons
Hydrogen
1
H
1.008
Element name
Symbol: one or two letters
Atomic mass: weighted average of element’s isotopes masses
*Elements are arranged according to atomic number

– An element’s atomic number determines its position on the periodic table
• The periodic table is organized left-to-right, topto-bottom by increasing atomic number

TAKE A LOOK AT A PERIODIC TABLE:
• Complete the following table:
Element
Number of protons
Number of electrons sodium (Na)
Copper (Cu)
Boron (B)
66
14

• An atom’s mass number (A) represents the total number of neutrons and protons in the nucleus of an atom
Mass number = # protons + # neutrons or
Mass number = atomic number + # neutrons
• How do you determine the number of neutrons in an atom?
# neutrons = mass number (A) - atomic number (Z)

• It is always true that:
– A carbon atom has 6 protons in its nucleus
– A neutral carbon atom has 6 electrons
• A carbon atom also contains neutrons, but unlike electrons and protons the number of neutrons can change from carbon atom to carbon atom
• Some carbon atoms have 6 neutrons, others have 7 neutrons and still others have 8 neutrons

• Dalton’s atomic theory states that all atoms of a given element are identical. This is mostly true
• Atoms of the same element can differ in the number of neutrons
• most elements have two or more isotopes
• Isotopes are atoms of the same element with different numbers of neutrons (and therefore different masses)
– atoms have the same atomic number but different mass numbers

• To distinguish one isotope from another an atom’s mass number is added after the element’s name:
– carbon-12; carbon-14; uranium-235
• Remember the mass number of an isotope represents the sum of the neutrons and protons in the nucleus protons neutrons electrons
Potassium-39 Potassium-40 Potassium-41
19
20
19
19
21
19
19
22
19

isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium) protons electrons
1 1
1
1
1
1 neutrons
0
1
2

MASS NUMBER = # PROTONS + # NEUTRONS
A
SYMBOL
ATOMIC NUMBER = # PROTONS
Z
X

56
26
Fe
26 protons
26 electrons
30 neutrons

symbol
atomic number mass number number of protons number of electrons number of neutrons
Fe 56
60
Al
144
102
59
27
45 45
31

92
235 U
92
238 U uranium-235 uranium-238 used for nuclear reactors and atomic bombs

• You have an atom each of two isotopes of carbon:
– What is the same about the two atoms?
– What is different about the two atoms?
– How can we tell the two atoms apart?

• Isotopes with more neutrons have greater mass, but are chemically the same (under normal conditions)
• The number of protons determines the identity of the element and electrons are responsible for an element’s chemical properties

Element name
Hydrogen
1 Atomic number (Z): number of protons H
1.008
Symbol: one or two letters
Average atomic mass: weighted average of element’s isotopes masses
*Elements are arranged according to atomic number

• The mass of an atom depends on the number of electrons, protons and neutrons it contains
• because atoms are extremely small particles (even the smallest speck of dust can contain
10,000,000,000,000,000 atoms) it’s impractical to measure the mass of atoms in grams

Particle
Electron
Proton
Neutron
Actual Mass (g)
9.11 x 10 -28
1.673 x 10 -24
1.675 x 10 -24

MEASURING ATOMIC MASS
• Because the actual masses of protons and neutrons are very small chemists have developed a way to measure atoms based on the Carbon-12 atom as the standard
• Instead of grams, the unit we use is the Atomic Mass Unit
(amu)
– Carbon-12 is exactly 12 amu
– 1 amu is exactly 1/12 of the mass of a carbon-12 atom
• Protons and neutrons have a mass slightly greater than 1 amu
Particle Mass (amu)
Electron 0.000549
Proton
Neutron
1.007276
1.008665

• When one consults the periodic table, the atomic mass of carbon is not 12.00 amu but 12.011 amu
• This is because most elements have more than one naturally occurring isotope and the atomic mass on the periodic table is the weighted average of the mass of each of the element’s isotopes

AVERAGE ATOMIC MASS
• In nature elements exist as a mixture of their isotopes, for example chlorine:
• 75% chlorine-35
• 25% chlorine-37
• Atomic mass of chlorine is 35.453 amu
• So the average atomic mass is calculated by taking the weighted average of the isotopes’ masses
– This is why an element’s atomic mass is not a whole number

Isotope
Mass of isotope
Abundance of isotope
(%) carbon-
12 carbon-
13
12.00000 amu
13.00335 amu mass x abundance
98.90%
1.10% carbon-12: 12.000 x 0.9890 = carbon-13: 13.00335 x 0.0110 =
11.868 amu
+ 0.143 amu
12.011 amu

PRACTICE: CALCULATE ATOMIC MASS
FOR ELEMENT X. IDENTIFY THE
ELEMENT
Isotope
X-6
X-7
Mass (amu) % abundance
6.015
7.016
7.5%
92.5%

Isotope
6 X
7 X
Mass (amu) % abundance
6.015
7.016
7.5%
92.5%
1. Mass contribution = (mass)(% abundance)
6 X Mass contribution = (6.015)(.075) = 0.451 amu
7 X Mass contribution = (7.016)(.925) = 6.490 amu
2. Sum of the mass contributions:
0.451 amu + 6.490 amu = 6.941 amu
Which element is this?