Chapter 5
Atoms: Building Blocks of Matter
Evidence
Direct evidence is when you do something to gather
the evidence
Examples are
Doing an experiment
Reading a book
Indirect evidence is when you do not see or touch the
subject
Examples are
Hearing from someone else
The Greek Model
All elements could be seen and touched
No element was smaller then what could be seen
The four elements are fire, earth, wind and water
Democritus
Called the Greek model (over 2000 years ago)
Named the smallest piece of matter the atom
From the Greek word atomos meaning not to be
cut
Atomists – the philosophers who agreed w/
Democritus
Hypothesis – small hard particles made up of same
material in all shapes and sizes, never ending
amount, always moving, capable of joining
together
Dalton’s Model
John Dalton (Early 1800’s)
Atomic Theory
All elements are composed of atoms, atoms are
indivisible and indestructible particles
Atoms of the same element are exactly alike
Atoms of different elements are different
Compounds are formed by joining of atoms of
two or more elements
Became foundation of atomic theory
Thomson’s Model
J.J. Thomson (1897)
Discovered that a particle smaller then the
atom had to exist
These were negatively charged particles electron
Plum Pudding model
Atom was made up of a pudding like positively
charged material throughout which negatively
charged electrons were scattered
Thomson’s Model
Raisin Pudding
Rutherford
Ernest Rutherford (1911)
Conducted an experiment in which some positive
particles went through gold foil and others
bounced off
Proposed that atoms have a small positively
charged center - nucleus
All the positively charged particles are found in
the nucleus and all the negatively charged
electrons were scattered outside the nucleus
around the edges
Rutherford’s Model
Bohr Model
Niels Bohr (1913)
Wanted to know where Rutherford’s electrons were
located – believed that the negative and positive
particles were held together because of their
attraction
Bohr placed each electron in a specific energy level
Electrons move around in definite orbits around the
nucleus – these orbits are located certain distances
from the nucleus
Bohr’s Model
Wave Model
Wave mechanics – today’s atomic theory is based
from wave mechanics
Electrons – do not move about the atom in a definite
pattern - it is actually impossible to predict the
location of the electrons
An atom has a small positively charged nucleus
surrounded by a large region where electrons are
making the atom neutral – this region is mostly
empty space
Atom
Composed of subatomic particles – particles
smaller than an atom
Proton – positively charged particles
Neutron – no charge
Electrons – negative charge
Nucleus – center of the atom
Modern Atom
Nucleus
The core of the atom
Contains 99.9 percent of the mass of the atom but is
smaller then the entire atom
Protons and neutrons located in the nucleus
Unit of measurement for subatomic particles is
Atomic Mass Unit - amu
Atomic Number
The number of protons in the nucleus is called the
atomic number
The atomic number identifies the element
This number will never change
Mass
All atoms have a mass number
The mass number of atom is the sum of the protons and
neutrons
The number of protons is the atomic number so if you
have the mass of an element you can find the number of
neutrons that make up the mass
Uranium 235 – proton number is 92 – neutron number is 143
Mass number 9, atomic number 4 – what is the # of neutrons
Atomic Mass – the average mass of all isotopes that occur
naturally for that element
Isotopes
Atoms of the same substance that have the same
number of protons but different numbers of
neutrons are called isotopes.
name
symbol A
nitrogen-12 isotope
nitrogen-13 isotope
nitrogen-14 isotope
nitrogen-15 isotope
Z
12
13
14
15
neutrons
7
5
7
6
7
7
7
Electrons
Electrons are negatively charged and have an amu of
.0006
The exact location of a electron is not known
The location in which the electron can be found is the
electron cloud
The electrons seem to be locked into a certain area
based on its energy level
Energy Levels
Represents the likely location for an electron in the
electron cloud
Electrons with the lowest energy are found in levels near
the nucleus
The higher the energy of the electron the farther away
from the nucleus
Each energy level can only hold a certain number of
electrons
Properties of elements depend on number of electrons in
outer energy level.
Quark
Current theory states that the different parts of the
element are made up of quarks
There are a number of different kinds of quarks
Quarks have properties called flavor and color –
there are six different flavors and three different
colors
Forces within an atom
Four forces account for the behavior of an atom
Electromagnetic force – either attracts or repels
particles – electrons are kept in orbit by this force
Strong Force – opposes the electromagnetic force –
“glues” protons together
Weak force – responsible for radioactive decay
Gravity – weakest force – not entirely known how it
affects an atom
Hydrogen
Atomic number of hydrogen is 1
One proton in the nucleus
no neutrons
Review
1. Carbon 14 and carbon 12 – what is the difference?
2. What is the mass number of an element with 16
protons, 17 neutrons, and 16 electrons?
3. If an elements atomic number is 12, what does its
nucleus contain?
4. How many neutrons are in an element with a
atomic number of 4 and a mass number of 9?
Questions
Go back through your notes and if
there is anything you are unsure of
ask now!!!!!!!!