ATOM: the smallest particle of an element that retains the identity of the element in a chemical reaction
Just how big is an atom?Copper is an example of an
element; if you were to grind a pure copper penny down to its individual atoms you would have approximately 2.4 x 1022
atoms. The earth has only about 6.9 x 109 people.
There are about 100 million Cu atoms in 1 cm
5 nm gold nanoparticles
The Atom
• Atoms are divisible (can be broken down)
• They are made up of 3 primary subatomic particles:
ProtonsNeutronsElectrons
• All atoms consist of 2 regions:Nucleus
Electron Cloud
Structure of the Atom
Protons, p+
• Located in nucleus• Positive charge, mass of 1
Neutrons, no
• Located in nucleus• No charge, mass of 1
Electrons, e-
• Located in the electron cloud• Negative charge, mass of 0
Structure of the Atom
e0
1-
n1
0
p1
1+
• Remember that UNLIKE electrical charges ATTRACT each other, and LIKE electrical charges REPEL. – The positive nucleus ATTRACTS the
negatively charged electrons – The charge on the electron balances the
charge on the proton.
• What does this mean for our atom?– An atom will contain equal # p+ and e-
– Atoms are electrically neutral (charge of zero)
Electrical Charges
We characterize a nucleus by its Atomic Number (Z) and its Mass Number (A).
Hydrogen1
H1.008
Name of Element
Atomic Symbol
Atomic Number
Atomic Mass (Weight)
Periodic Faceplate
ATOMIC NUMBER (Z): indicates number of protons in the nucleus of the atom
Atomic number identifies an element. Atoms of the same elements have the same number of protons.Atoms of different elements have different numbers of protons.
Atomic Number
MASS NUMBER (A): total number of nucleons in an atom (total # p+ and n0)
Mass # = p+ + n0
Since electrons have practically no mass and are located outside the nucleus, the entire mass of an atom can be considered to be located in the nucleus.
Mass Number
Mass Number
Element Mass # (A)
# of protons
# of neutrons
Carbon-12 12 6 6
Phosphorus-35 35 15 20
Gold-197 197 79 118
ISOTOPE- atom of the same element having different mass due to different number of neutrons
• Isotopes of a particular element ALL have the same number of protons but different numbers of neutrons.
• Isotopes are chemically the same.
Isotopes
Isotopes
Isotope p+ n0 e- Nucleus
Hydrogen–1
(protium) 1 0 1
Hydrogen-2
(deuterium) 1 1 1
Hydrogen-3
(tritium) 1 2 1
• Isotopes of hydrogen are unusual in that they have distinct names.
• Identifying an isotope requires knowing both the name or atomic number of the element and the mass of the isotope.
• Isotopes are usually identified by specifying their mass number.
Naming Isotopes
hyphen notation : mass number is written after the name of the element
e.g. uranium-235 U-235
nuclear symbol : shows the composition of a nucleus
ZA
XMass Number Atomic
SymbolAtomic Number
Naming Isotopes
# p+ = Atomic Number
# n0 = Mass # - Atomic #
# e- = # p+ (if neutral atom)
What about if the atom is not neutral???We form an ION.
ION: atom or group of atoms with an overall positive or negative charge
Determining the Number of p+ no e-
Cation (positive ion) = # e- decreases … loses e-
Ions
I’ve lost my CAT ION.Are you sure?
Yes, I’m positive.
PARTICLE LOCATION CHARGEREL.MASS SYMBOL CHANGE?
Proton
Neutron
Electron e0
1-
n1
0
p1
1+Nucleus
Nucleus
Outside thenucleus (in e-
cloud)-1
0
+1 1
1
0
p +
n0
e-
New atom
Mass(Isotope)
Charge(ion)
Summary of Atomic Structure