Electrons: negatively charged particle in an atom

orbits the nucleus
Nucleus: positively charged centre of an atom


contains most of the mass of an atom but occupies
very little space
made up of protons and neutrons
Proton: positively charged subatomic particle found in
the nucleus

number of protons determines which element it is
Neutrons: uncharged subatomic particle found in the
nucleus

have the same mass as protons
Proton
(p+)
Neutron
(n0)
Electron
(e-)
Charge
+
0
-
Location
In nucleus
In nucleus
Orbiting nucleus
Relative Mass
1
1
1/2000 (*0)
Protons and
neutrons make
up the central
nucleus
Electrons
orbit the
nucleus like
the planets
orbit the Sun
 Each
element has a unique name and symbol
 The
symbol of an element is usually
represented by a CAPITAL LETTER (followed
by a lower case letter if needed)

e.g. Na – sodium, O – oxygen, Fe - iron
Atomic number: the number of protons in an
atom’s nucleus

e.g. atomic number of
nitrogen (N) is 7

also means that it has 7 protons
 The
mass of an atom consists of the contents
of its nucleus
Mass number: the number of protons and
neutrons in an atom’s nucleus

It will always be a whole number (NO decimals!)
Atomic mass: the mass of an atom in atomic mass units
(amu; symbol= u)

e.g. the atomic mass of
nitrogen (N) is 14.01

The atomic masses given are not whole numbers
because they are the average of the masses of
its isotopes
Isotope: an atom with the same number of protons
but different number of neutrons
e.g. lithium has TWO isotopes 
Li – 6 p+, Li – 7 p+

NOTE: therefore, to
determine the mass number
of an element, ROUND the
atomic mass from the
Periodic Table

e.g. Beryllium


Atomic mass is 9.01u  round to 9 (mass number)
Atomic number is 4

Therefore, the number of neutrons is (9 - 4 = 5)
 How
many protons and neutrons does
Fluorine have?
 How
many protons and neutrons does
Sodium have?
 In
a neutral atom,
 How
many protons, neutrons and electrons
does Oxygen have?
 How
many protons, neutrons and electrons
does Lithium have?
 Is
another way to represent an elements symbol
with the mass and atomic number:

Mass Number is written to the top left of the symbol

Atomic number is written to the bottom left of the
symbol

e.g. potassium (K)
mass number
39
atomic number
19
Homework:
 Read pg. 234-235 in textbook and do
Q: #1 - 4 on pg. 240
 Complete worksheet “Subatomic
Particles Worksheet”