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Transcript
1.9 understand that atoms consist of a central
nucleus, composed of protons and neutrons,
surrounded by electrons, orbiting in shells
1.10 recall the relative mass and relative charge
of a proton, neutron and electron
1.11 understand the terms atomic number, mass
number, isotopes and relative atomic mass (Ar)
John Dalton had the first ideas about the existence
of atoms over 200 years ago.
However, it is only relatively recently that special
microscopes (called electron microscopes) have
been invented that can actually ‘see’ atoms.
Electron microscopes produce images similar to
this one. What could it be showing?
The grey blobs are individual lead atoms.
Atoms are very small – they are about
0.00000001 cm wide.
Think about the thickness of a crisp.
The number of atoms you would
need to stack up to make the
thickness of a crisp, is approximately
the same number of crisps you would
need to stack up to make the height
of Mount Everest!
That’s roughly 7 million crisps!
For some time, people thought that atoms were the smallest particles and could not
be broken into anything smaller.
Scientists now know that atoms are actually made from even smaller particles. There
are three types:
proton
neutron
How are these particles arranged
inside the atom?
electron
Protons, neutrons and electrons
are not evenly distributed in an
atom.
The protons and neutrons exist in a dense
core at the centre of the atom. This is
called the nucleus.
The electrons are spread out
around the edge of the atom. They
orbit the nucleus in layers called
shells.
There are two properties of protons, neutrons and electrons that are especially
important:
 mass
 electrical charge.
Particle
Mass
Charge
proton
1
neutron
1
electron
almost 0
+1
0
-1
The atoms of an element contain equal numbers of protons and electrons and so have
no overall charge.
• Protons (p+)
–
–
–
1
+ electrical charge
mass = 1.672623 x 10-24 g
relative mass = 1.007 atomic mass units (amu) but we can round to
• Electrons (e-)
–
–
negative electrical charge
relative mass = 0.0005 amu
but we can round to 0
• Neutrons (no)
– no electrical charge
–
mass = 1.009 amu but we can round to 1
• Have a look at your Periodic table…
• We will study the actual elements later BUT
• Check out all the numbers!!
All atoms of the same element have
the same number of protons in the
nucleus, Z
13
Al
26.981
Atomic number
Atom symbol
AVERAGE Atomic Mass
• C atom with 6 protons and 6 neutrons is the
mass standard
• = 12 atomic mass units
• Mass Number (A)
= # protons + # neutrons
• NOT on the periodic table…(it is the AVERAGE
atomic mass on the table)
• A boron atom can have
5 p + 5 n = 10 amu
A =
A
10
Z
5
B
 Show the name of the element, a hyphen, and the
mass number in hyphen notation
sodium-23
 Show the mass number and atomic number in
nuclear symbol form
mass number
23 Na
atomic number
11
• Protons: Atomic Number (from periodic table)
• Neutrons: Mass Number minus the number of
protons (mass number is protons and neutrons
because the mass of electrons is negligible)
• Electrons:
– If it’s an atom, the protons and electrons must be the SAME
so that it is has a net charge of zero (equal numbers of + and
-)
• Using your play-doh to make the molecular model for:
• Boron
P:
5
N:
6
E:
5
• Helium
P:
2
N:
2
E:
2
3
N:
4
E:
3
• Lithium
P:
• Carbon
• Oxygen