Distinguishing Between Atoms • Atomic Number: equals the number of protons in the nucleus of an atom of that element • Atoms are electrically neutral, therefore the number of electrons must equal the number of protons • How is the number of electrons for a neutral atom of a given element related to the atomic number of that element? – The atomic number is equal to the number of electrons
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Distinguishing Between Atoms Atomic Number: equals the number of protons in the nucleus of an atom of that element Atoms are electrically neutral, therefore.
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Distinguishing Between Atoms
• Atomic Number: equals the number of protons in the nucleus of an atom of that element
• Atoms are electrically neutral, therefore the number of electrons must equal the number of protons
• How is the number of electrons for a neutral atom of a given element related to the atomic number of that element?– The atomic number is equal to the number of
electrons
Atoms of the First Ten Elements
Name Symbol Atomic Number
Composition of the Nucleus
Mass Number Number of electronsProtons
Neutrons
Hydrogen
H 1 1 0 1 1
Helium He 2 2 2 4 2
Lithium Li 3 3 4 7 3
Beryllium
Be 4 4 5 9 4
Boron B 5 5 6 11 5
Carbon C 6 6 6 12 6
Nitrogen N 7 7 7 14 7
Oxygen O 8 8 8 16 8
Fluorine F 9 9 10 19 9
Neon Ne 10 10 10 20 10
Element block
Review• The element Nitrogen
has an atomic number of what?
• How many protons and how many electrons are in a neutral nitrogen atom?
• 7
• 7, number of protons equals the number of electrons
How many protons and electrons are in each of the following atoms?
Mass Number: the total number of protons and neutrons in an atom
• Mass # = protons + neutrons
• Example: Oxygen
• 16 = 8 + n
• n = 8
The following is shorthand for writing the # of protons and neutrons, you don’t need to write the # of electrons because they are the same as the # of protons
The large "X" represents where you will find the atom's elemental symbol. The mass number, which is given the symbol "A", is located in the upper left-hand corner. The atomic number (also called nuclear charge), which is given the symbol "Z", is found in the lower left-hand corner.
Number of neutrons = mass number − atomic number
• In the case of neon (Ne):
Number of neutrons = 20- 10 = 10
• In the case of carbon (C):
Number of neutrons = 12- 6 = 6
Number of neutrons = mass number − atomic number
More Examples
Elemental Symbol
# of Protons (equal to Z) 3 9
# of Neutrons(equal to A-Z) 4 10
# of electrons (equal to Z modified by
charge number)3 9
How many protons, electrons, and neutrons are in the following atoms?
Atomic Number
Mass Number
(a) Beryllium (Be)
4 9
(b) Neon (Ne) 10 20
(a) Sodium (Na) 11 23
How many neutrons are in each atom?
16 O
8
• 8• 16• 24
32 S
16
• 32• 48• 16
Express the composition of the following atom in shorthand form
Carbon-12
• 12 C 6
• 6 C 12
• 18 C 12
Fluorine-19
• 9 F 19
• 28 F 9
• 28 F 19
• 19 F 9
Isotopes: atoms that have the same number of protons but different numbers of neutrons
• Isotopes have different mass numbers because they have different numbers of neutrons
• They are chemically alike because they have identical numbers of protons & electrons, which are the subatomic particles responsible for chemical behavior.
Example: Hydrogen has three isotopes; each isotope has 1
proton in its nucleus.
Two isotopes of carbon are carbon 12 and carbon 13. Write the symbol for each isotope using superscripts and subscripts to represent mass #
and atomic number
Carbon-12 Carbon-13
___ ___
___C ___C
Determine the number of protons, neutrons, and electrons for each of the following
isotopes of Neon Neon-20
• 22• 10• 11
• Neon-21
• 21• 10• 11
Neon-22
• 22• 10• 12
Atomic MassProperties of Subatomic Particles
Particle Symbol Relative electrical charge
Relative mass(Mass of proton
=1)
Actual mass (g)
Electron e- 1- 1/1840 9.11x10-28
Proton P+ 1+ 1 1.67x10-24
Neutron n◦ 0 1 1.67x10-24
Actual mass of a proton or a neutron is very small (1.67 x10-24 g)Since about the 1920s, it has been possible to determine these masses (protons & neutrons) by using a mass spectrometer
• This is impractical to work with, so a reference isotope as a standard is used. The chosen isotope is C-12. It was assigned a mass of EXACTLY 12 atomic mass units or amu
• Carbon-12 has 6 protons and 6 neutrons, making up its mass
• Since the mass of an atom mainly depends on the number of protons and neutrons, shouldn’t the atomic mass of an element be a whole number? That’s usually not the case, Cl is 35.453 amu
• The reason for this involves the relative abundance of the naturally occurring isotopes of the element. Most occur as a mixture of two or more isotopes
• Each isotope has a fixed mass and a natural percent abundance
The atomic mass is a weighted average of all naturally occurring isotopes
Therefore, the Atomic Mass is a weighted average mass of the atoms in a naturally occurring sample of the element
ReviewWhich isotope of copper is more abundant:
copper – 63 or copper – 65? (the atomic mass of copper is 63.546 amu)
• Copper-63
• Boron has two isotopes: boron-10 and boron-11, which is more abundant
(atomic mass is 10.81 amu)?
• Boron -11
• Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.009 amu (11X) has a relative abundance of 80.09%. Calculate the atomic mass of this element.
• 10X = 10.012 x 0.1991 = 1.993 amu
• 11X = 11.009 x 0.8009 = 8.817 amu
10.810 amu
• What is this element?
• Boron
Calculate the atomic mass of Bromine. The two isotopes of bromine have atomic masses and relative abundances of 78.92 amu (50.69%) and 80.92 amu (49.31%)