The Atom

1
The Atom Subatomic Particles
2
Atomic Number

• atomic number
• size of nuclear charge
• number of protons in nucleus
• determines identity of element
• used to give order to periodic table
• For neutral atoms
• atomic number number of electrons

3
What is an atom?
4
Number of Protons Number of Electrons Number of
Protons Atomic Number Atomic Mass Protons
Neutrons
5
What we knew by 1932
6
Location of Subatomic Particles

• Electrons located outside nucleus
• Protons neutrons located inside nucleus
• Protons neutrons nucleons

7
Isotopes

• atoms of same element with
• different neutrons (mass is different)
• protons stays same! (atomic is same)
• Daltons theory modified in number of important
  ways
• the atom is divisible (protons, neutrons,
  electrons)
• atoms of same element can be different (different
  neutrons)

8
Mass Number

• Mass number protons neutrons

9
Notation

• 12C

6

• left superscript is mass number
• left subscript is atomic number

10
Notation
12 protons neutrons 6 protons

• 12C

6
How many neutrons in this atom?
12 6 6 neutrons
11
17O
Mass number is ? Atomic number is ? of
neutrons is ? of electrons in atom is?
8
12
Mass number is ? Atomic number is ? of
neutrons is ? of electrons in atom is?
13
Mass number is ? Atomic number is ? of
neutrons is ? of electrons in atom is?
14
Other Notations

• Might also see C-12
• number following symbol is mass number!
• Need to look up atomic number

15
Whats the difference between H-1, H-2, and H-3?

• All Hs have 1 proton
• H-1 mass 1
• 1 proton, 0 neutrons
• H-2 mass 2
• 1 proton, so it must have 1 neutron
• H-3 mass 3
• 1 proton, so it must have 2 neutrons

16
Consider U-234, U-235, U-238

• Whats the atomic number of U?
• How many protons in U?
• How many neutrons in U-234?
• How many neutrons in U-235?
• How many neutrons in U-238?
• How many electrons in U?

92
92
234 92 142
235 92 143
238 92 146
92
17
How many neutrons in Po-217?

• Whats the atomic number?
• How many protons?
• So the neutrons are

84
84
217 84 133 neutrons!
18
IONS

• Atoms electrically neutral so ...
• electrons protons
• IONS atoms that gained or lost electrons
• protons and electrons aren't anymore, so ions
  are not neutral they carry a charge (/-)?

19
IONS

• Charge of ion protons minus electrons
• (subtract the electrons since are negative)?

20
IONS

• If atom loses electrons, it has more positive
  charge than negative
• What kind of ion is it?
• If atom gains electrons, it has more negative
  charge than positive
• What kind of ion is it?

21
Charge of Ions

• Ion charge written as right superscript
• Cl-1 a chloride ion with a charge of -1
• Na1 a sodium ion with a charge of 1
• O-2 an oxygen ion with a charge of -2
• If no right superscript, its understood to be
  zero and therefore a neutral atom

22
Putting it all together

• How many protons, neutrons, electrons in each
  of the following
• 23Na1

34S-2
16
25Mg2
64Zn2
12
30
12 p, 13 n, 10 e
23
What is an amu?

• equal to the atomic mass unit
• amu ? 1/12 the mass of the C-12 atom
• C-12 is used as the reference for atomic mass

24
Atomic Weight or Mass

• Atomic mass is relative
• 1st H-1 used as standard
• 1 atom of H 1 amu
• next O-16 used as standard
• O combines with many elements
• 1 atom of O 16 amu's
• Now C-12 is the standard
• 1 atom of C-12 has a mass of exactly 12 amu's