Ch. 3.3 ppt. Modern Atomic Theory
Transcript Ch. 3.3 ppt. Modern Atomic Theory
Warm Up– Monday, September 8th
• Honors- Turn in your lab report over Lab #1
• All- Grab a Ch. 3.3 Notes packet from cart, answer
these questions on the front.
1. Decide whether each of the following statements
concerning atoms is true or false.
a) All of the particles in the nucleus of an atom are charged.
b) The atom is best described as a uniform sphere of matter
in which electrons are embedded.
c) The mass of the nucleus is only a very small fraction of the
mass of the entire atom.
d) The size of the nucleus is only a very small fraction of the
total size of the atom.
e) The number of neutrons in an atom must equal
the number of electrons.
Modern Atomic Theory
Atom – smallest unit of an element that retains
the properties of that element.
1) Atoms are composed of two regions and
three subatomic particles.
a) Nucleus - positively charged region
located at the center of the atom.
Very small compared to the total size of the atom.
(marble = size of nucleus / football field = size of
Composed of two particles
1. Proton = Positive particle
2. Neutron = Neutral particle
Almost all mass is located in nucleus (extremely
Nucleus held together by strong nuclear forces.
b) Electron Cloud –
Mostly empty space
Gives atom its size
Contains tiny negatively charged
particles called electrons.
2) Atoms are very small in size.
So small use picometers (1x10-12m)
Atomic radii range from 40 to 270 pm
3) Atoms have very small masses.
• Example: mass of oxygen atom
2.6 x 10-23g.
atomic mass unit (amu)
1 amu = 1.66 x 10-27g = 1/12 the
mass of a carbon-12 atom.
atoms have masses from 1-300
Comparison of Subatomic Particles
approx. 1 amu
approx. 1 amu
.0005 amu ≈ 0
4) Atoms are electrically neutral.
Although an atom contains charged
particles, the atom overall is neutral.
A proton and an electron have the same
amount of charge just opposite in sign.
Number of protons = number of electrons.
Atomic Number and Mass Number
Atomic Number (Z) –
– number of protons in the nucleus of atom of a
– whole number listed on the periodic table.
Mass Number (A) –
number of protons and neutrons in nucleus of an
– do not confuse with decimal number listed on
There are two different ways to represent atoms.
Symbol Notation –
• Consists of name of element and a number.
– Example: chlorine – 37
• Number after name is the mass number.
• Sample Problem:
Determine the number of protons, neutrons,
and electrons for a bromine-80 atom.
One Minute Paper
• You have one minute to answer these two
questions concerning today’s lesson.
– What was the most important thing you
– What is still confusing?