Understanding Atoms
Year 9 Science
Contents
• Atomic structure
• Bohr shell theory
• Periodic table
Video:
• Atomic Structure and the periodic table
Recall from last year’s science that
Elements are…
Pure substances that cannot be broken down into
other chemicals.
Gold
Mercury
Carbon
These substances are made up of only one type of
atom
(A substance of an element will consist of millions of
atoms but they will all be the same atom).
What is an ATOM?
• All matter in the universe is made up of
atoms.
• It is the simplest part of an element.
Atomic structure
All atoms have the same basic
structure.
• A CENTRAL region called
the NUCLEUS.
• An OUTER region known
as the ELECTRON SHELLS
Inside the nucleus
The NUCLEUS contains TWO TYPES of
subatomic particles:
Electron shells
•PROTONS (which carry a positive
charge)
•NEUTRONS (which carry no charge)
+
Nucleus
-
Atomic Mass Units (AMU)
• Protons and neutrons weigh about the same
1.66054x10-24g or
(0.00000000000000000000000166054g)
• Because this number is so small, Scientist
call this number one atomic mass unit
(amu).
Electron shells
The ELECTRON SHELLS contain only ONE TYPE of
subatomic particle:
•ELECTRONS (which carry a negative charge).
•They are so small that they can be considered to have
a negligible mass (so small it can be ignored).
Electrons
Electron shells
Nucleus
-
What are the three subatomic
particles?
Summary of subatomic particles
Source: Chapter 2 Understanding Atoms. In Heinemann Science Links 3 (VELS
Edition) 2006, pp.27
Atomic Number
 The number of protons in an atom’s nucleus is
known as its Atomic Number.
 This number is different for each different type of
atom.
 Each element (in the periodic table) is assigned an
atomic number.
Atomic Number
Atomic Number Question:
How many protons do these elements have?
Number of Electrons
• The number of electrons found in an atom will be
the same as the number of protons (atomic
number) that the atom’s nucleus contains.
• Protons carry a positive (+) charge &
electrons carry a negative (-) charge.
+
-
• Therefore the numbers of each particle in an atom
have to be the same to keep the charges (+/-)
balanced.
Number of electrons Question:
How many electrons do these elements have?
Mass Number
• The total number of protons AND neutrons
found in an atom’s nucleus is known as its
Mass number.
• Mass number is expressed in atomic mass
units. (amu)
• This number is not shown on all Periodic
Tables.
Mass Number Question:
• Helium has a mass number of 4. If the atomic
number of Helium is 2; How many protons and
neutrons does Helium have?
Mass Number = No. of protons + neutron
4 =2+2
Therefore Helium has 2 protons and 2 neutrons
• Boron has a mass number of 11: Boron atoms
have an atomic number of 5; How many neutrons
does Boron have?
Mass Number = No. of protons + neutron
11 = 5 + 6
Therefore Boron has 5 protons and 6 neutrons
Mass Number
• Superscript and subscript numbers are attached to the
symbol of an element to indicate its atomic and mass
numbers.
Example,
Mass No.
X
Atomic No.
12
4
C
6
23
He
2
Na
11
Carbon (C) has an atomic number of 6 & a mass number of 12.
Helium (He) has an atomic number of 2 & a mass number of 4
Sodium (Na) has an atomic number of 11 & a mass number of 23.
• The mass number generally appears above the atomic
number, and is usually the larger of the two.
Mass Number Question:
Chlorine (Cl)
35
Cl
17
What is the atomic number and mass
number of chlorine?
Isotopes
• Isotopes are atoms of the same elements
with different numbers of neutrons.
Example: Carbon
12
C
6
13
N=6
P=6
Normal Carbon Atom
C
N=7
P=6
6
Carbon Atom Isotope
Atomic Mass Versus Mass Number
• Atomic mass is the weighted average of the mass numbers
of isotopes of the element.
• The Atomic mass of an atom (NOT the mass number) is
usually expressed as a number of grams of that element
that contain 6 x 1023 atoms of that element. (600 000 000
000 000 000 000 000)
• This number is usually on all Periodic tables.
• The atomic mass is usually very close (if not identical) to the
atom’s mass number.
Element
has
___________
Mass Number
____________
which is
which is
which is
_____________
Average mass of
mixture of its
___________
Number of
protons
which equals
which contain
____________
Same number of
____________
Different number
of ___________
Summary
Protons- carry a positive (+) charge, found in nucleus
Neutrons- carry no charge, found in nucleus
Electrons- carry a negative (-) charge, found in
electron shells
Atomic number = number of protons
Mass number = number of protons + neutrons
No. of electrons in neutral atom = atomic number
Isotopes- are atoms of the same elements with
different numbers of neutrons
Questions
• Heinemann Science Links 32 pp. 32
Questions 2- 4
Bohr Shell Model
• Bohr proposed that electrons are found within specific regions
of an atom called shells.
• Electrons in an atom are arranged in shells
•
The first shell - found closest to the nucleus can hold only
two electrons.
• The second shell can hold eight
electrons
3 2 1
• The third shell can hold eighteen electrons.
• The further out shells can hold progressively more electrons.
• Bohr also noticed that electrons tend to be
naturally found in shells
of the lowest energy.
• The lowest energy shell is the first shell.
Therefore the first shell always fills up first
before the second shell.
References
• 2006. Chapter 2: Understanding Atoms. In
Heinemann Science Links 3 (VELS Edition).
• http://www.colorado.edu/physics/2000/period
ic_table/atomic_mass.html
Components of an atom
Nucleus
electron
neutron
proton
Electron
Shells
• Then one oxygen atom weighs eight times as
much as two hydrogens.
Source:http://www.qrg.northwestern.edu/projects/vss/doc
s/Propulsion/1-what-is-an-atom.html