Atomic Structure

What is an atom?

• Atom: the smallest unit of matter

that retains the identity of the
substance

• First proposed by Democratus

Atomic Structure

• Atoms are composed of 2 regions:

• Nucleus: the center of the atom that

contains the mass of the atom

• Electron cloud: region that surrounds

the nucleus that contains most of the
space in the atom

Nucleus
Electron

Cloud

What’s in the Nucleus?

• The nucleus contains 2 of the 3

subatomic particles:
• Protons: positively charged subatomic

particles

• Neutrons: neutrally charged subatomic

particles

What’s in the Electron Cloud?

• The 3rdsubatomic particle resides

outside of the nucleus in the electron
cloud
• Electron: the subatomic particle with a

negative charge and relatively no mass

How do these particles

interact?

• Protons and neutrons live compacted

in the tiny positively charged nucleus
accounting for most of the mass of
the atom

• The negatively charged electrons are

small and have a relatively small mass
but occupy a large volume of space
outside the nucleus

How do the subatomic particles

balance each other?

• In an atom:

• The protons = the electrons

• If 20 protons are present in an atom then 20

electrons are there to balance the overall
charge of the atom—atoms are neutral

• The neutrons have no charge; therefore

they do not have to equal the number of
protons or electrons

How do we know the number of
subatomic particles in an atom?

• Atomic number: this number indicates the

number of protons in an atom
• Ex: Hydrogen’s atomic number is 1

• So hydrogen has 1 proton

• Ex: Carbon’s atomic number is 6

• So carbon has 6 protons

**The number of protons identifies the

atom.
Ex. 2 protons = He, 29 protons = Cu

• Mass number: the number of protons

and neutrons in the nucleus
• Ex: hydrogen can have a mass of 3.

Since it has 1 proton it must have 2
neutrons

• # of neutrons = mass # - atomic #

How do we know the number of
subatomic particles in an atom?

Determining the number of

protons and neutrons

• Li has a mass number of 7 and an

atomic number of 3
• Protons = 3 (same as atomic #)
• Neutrons= 7-3 = 4 (mass # - atomic #)

• Ne has a mass number of 20 and an

atomic number of 10
• Protons = 10
• Neutrons = 20 - 10= 10

What about the electrons?

• The electrons are equal to the

number of protons
• So e-= p = atomic #

• Ex: He has a mass # of 4 and an

atomic # of 2
•p+= 2
• no= 2
• e-= 2

Determine the number of
subatomic particles in the

following:

• Cl has a mass # of 35 and an atomic #

of 17
• p+= 17,

no= 18,

e-= 17

• K has a mass # of 39 and an atomic #

of 19
• P+= 19,

no= 20

e-= 19

How exactly are the particles

arranged?

• Bohr Model of the atom:

Reviewers think this could lead to misconceptions!
All of the
protons and
the neutrons

The 1string can
hold up to 2 e-

The 2ndring can
hold up to 8 e-

The 3rdring
can hold up
to 18 e-

The 4thring
and any after
can hold up to
32 e-

What does carbon look like?

Mass # = 12

atomic # = 6

p+= 6

no= 6

e-= 6

6 p and 6 n live
in the nucleus