The Atom
An atom consists of a
• nucleus
–(of protons and neutrons)
• electrons in space about the nucleus.
Electron cloud
Nucleus
• An atom is the smallest particle of an
element that has the chemical
properties of the element.
• The atom is extremely small. One teaspoon
of water has 3 times as many atoms as the
Atlantic Ocean has teaspoons of water.
• If a large sports stadium were an atom, a
marble would represent the nucleus.
ATOMIC COMPOSITION
• Protons (p+)
– + electrical charge
– mass = 1.672623 x 10-24 g
– relative mass = 1.007 atomic mass units (amu)
but we can round to 1 for mass.
• Electrons (e-)
–
negative electrical charge
– relative mass = 0.0005 amu
but we can round to 0 for mass.
• Neutrons (no)
no electrical charge
– mass = 1.009 amu but we can round to 1 for
mass.
–
Atomic Number, Z
All atoms of the same element
have the same number of
protons in the nucleus, Z
13
Al
26.981
Atomic number
Atom symbol
AVERAGE Atomic Mass
Mass Number, A
• C-12, atom with 6 protons and 6 neutrons is
the mass standard
• = 12 atomic mass units (amu)
• Mass Number (A)
= # protons + # neutrons
• NOT on the periodic table
Isotopes –
Page 6
• Atoms of the same element (same Z)
but different mass number (A).
• Boron-10 (10B) has 5 p and 5 n
• Boron-11 (11B) has 5 p and 6 n
11B
10B
Two isotopes of sodium.
Atomic Symbols
 Hyphen notation/mass notation - show the name of
the element, a hyphen, and the mass number in
sodium-23
 Nuclear symbol - show the mass number and atomic
number in
mass number
23 Na
atomic number
11
Nuclear
Symbol
notation
Number of
protons
Number of
neutrons
Number of
electrons in
the neutral
atom
Name of
element
Hyphen
notation
65
29
86
Cu
Kr
78
6
117
46
36
Boron
Au198
Atoms vs. Ions
• Atoms are typically electrically neutral.
• Often, atoms will gain or lose electrons.
– Cation: positively charged
More p+ than e– Anion: negatively charged
More e- than p+
Hyphen
notation
Al-27
Nuclear
Symbol
notation
Number of
protons
Number of
neutrons
Number of
electrons
Atom, Cation,
or Anion
Overall charge
Atomic
Number
He-4
238
92
Li-7
U
12
36
13
36
2
2
atom
0
35
2+