how do atoms differ? section 4.3. review: the atom the nucleus
TRANSCRIPT
REVIEW
Particle Location ChargeRelative Mass (amu)
Actual Mass
(g)
Electron
In the space
surrounding nucleus
1-0
(1/1840)
9.11 x 10-
28
Proton Nucleus 1+ 11.673 x
10-24
Neutron Nucleus 0 11.675 x
10-24
If all atoms are composed of protons, neutrons and
electrons, how do elements differ?
–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)
THE MODERN PERIODIC TABLE
*Elements are arranged according to atomic number
HHydrogen
1
1.008
Atomic number (Z): number of protons
Element name
Symbol: one or two letters
Atomic mass: weighted average of element’s isotopes masses
– An element’s atomic number determines its position on the periodic table• The periodic table is organized left-to-
right, top-to-bottom by increasing atomic number
TAKE A LOOK AT A PERIODIC TABLE:
• Complete the following table:
ElementNumber of
protonsNumber of electrons
sodium (Na)
Copper (Cu)
Boron (B)
66
14
MASS NUMBER (A)• An atom’s mass number (A) represents the
total number of neutrons and protons in the nucleus of an atomMass number = # protons + # neutrons
orMass 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
ISOTOPES• 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
IDENTIFYING ISOTOPES• 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 nucleusPotassium-
39Potassium-
40Potassium
-41
protons 19 19 19
neutrons
20 21 22
electrons
19 19 19
isotopeproton
selectrons neutrons
Hydrogen–1 (protium)
1 1 0
Hydrogen-2(deuterium)
1 1 1
Hydrogen-3(tritium)
1 1 2
WRITING SYMBOLS FOR ATOMS
A
XZ
MASS NUMBER = # PROTONS + # NEUTRONS
SYMBOL
ATOMIC NUMBER = # PROTONS
PRACTICE: FILL IN THE BLANKS
symbol
atomic number
mass numbe
r
number of
protons
number of
electrons
number of
neutrons
Fe 56
60 144
102 45 45
59 31
Al 27
TURN TO THE PERSON NEXT TO YOU:
• 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?
IMPORTANT TO NOTE:
• 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
THE MODERN PERIODIC TABLE
*Elements are arranged according to atomic number
HHydrogen
1
1.008
Atomic number (Z): number of protons
Element name
Symbol: one or two letters
Average atomic mass: weighted average of element’s isotopes masses
ATOMIC MASS
• 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
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 1.007276
Neutron 1.008665
AVERAGE ATOMIC MASS• 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
AVERAGE ATOMIC MASS OF CARBON
IsotopeMass of isotope
Abundance of isotope
(%)
carbon-12
12.00000 amu
98.90%
carbon-13
13.00335 amu
1.10%mass x abundance
carbon-12: 12.000 x 0.9890 = 11.868 amu
carbon-13: 13.00335 x 0.0110 = + 0.143 amu
12.011 amu
PRACTICE: CALCULATE ATOMIC MASS FOR ELEMENT X. IDENTIFY
THE ELEMENT
Isotope Mass (amu)%
abundance
X-6 6.015 7.5%
X-7 7.016 92.5%