Transcript Introduction to Atoms

Introduction to
Atoms – Chapter 11
Section 2 – The Atom
pp. 318 - 324
DEFINITIONS

amu – the SI Unit used to express the
masses of the particles of an atom

Atomic number – the number of protons in
the nucleus of an atom
DEFINITIONS
Mass number – the sum of the particles in
the nucleus (protons + neutrons)
 Atomic mass – the weighted average of
the masses of all the naturally occurring
isotopes of an element

PARTS OF AN ATOM
Parts of the Where
Size
atom
Found?
Proton
In nucleus 1.7 ×10-24 g
of atom
= 1 amu
Neutron
Electron
Charge (if
any)
positive
In nucleus Same size
Neutral (no
of atom
as proton = 1 charge)
amu
Outside
‹ 1 amu
negative
nucleus in is almost
electron
zero
clouds
HELIUM ATOM
shell
proton
+
electron
N
N
+
-
neutron
ISOTOPES
 Isotopes
of an element share the
same physical and chemical
properties.
 How
can you tell one isotope from
another? You can identify each
isotope by its mass number.
Isotopes
Isotopes are atoms that have the same
number of protons but have different
numbers of neutrons.
 The proton # never changes for an
element, only the number of neutrons or
electrons can change.

Isotope Notation
Chlorine-35
 Cl-35
 35 is the mass number

Na-23
 Sodium-23
 23 is the mass number

Ions

Ions are charged particles.

An atom that loses one or more electrons
becomes a positively charged ion.

An atom that gains one or more electrons
becomes a negatively charged ion.
FORCES AT WORK IN ATOMS
1.
2.
Gravity – because the masses of
particles in atoms is small, the force of
gravity within atoms is very small.
Electromagnetic force – Particles with
the same electric charge repel; particles
with opposite charges attract. Protons
and electrons are attracted to each other
because they have opposite charges.
FORCES AT WORK IN ATOMS
3. Strong Force – the force that hold
protons in the nucleus together. The
strong force only acts on small distances
4. Weak Force – is an important force in
radioactive atoms. In certain unstable
atoms a neutron can change into a
proton and an electron. This force plays
a key role in this change.