Structure of the Atom

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

Atomic Number

Element Atomic # (Z) # of protons

Carbon (C) 6 6

Phosphorus (P) 15 15

Gold (Au) 79 79

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-

Ions

A N(egative) ION

Anion (negative ion) = # e- increases … gains 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

PRACTICE: Determining the Number of p+ no e-

8

18O

33

75As

15

31P