Download atomic number

AS Chemistry
Lesson 1 atomic structure
Sunday, 30 April 2017
Lesson objectives
Atomic Structure
• (a) describe protons, neutrons and electrons in terms of
relative charge and relative mass;
• (b) describe the distribution of mass and charge within an
atom;
• (c) describe the contribution of protons and neutrons to the
nucleus of an atom, in terms of atomic (proton) number
and mass (nucleon) number;
• (d) deduce the numbers of protons, neutrons and electrons
in:
– (i) an atom given its atomic and mass number,
– (ii) an ion given its atomic number, mass number and ionic charge;
• (e) explain the term isotopes as atoms of an element with
different numbers of neutrons and different masses;
Remember this is a model. It is the
nuclear model of an atom.
ELECTRON – negative (-1), mass
1/2000 (so small compared to the
nucleus we say it is negligible).
Nucleus
contains
protons and
neutrons
Occupy shells around the nucleus
+
NEUTRON –
neutral (0),
same mass as
proton (“1”)
+
-
Structure of the atom
PROTON –
positive (+1),
same mass as
neutron (“1”)
Draw a labelled diagram for
the structure of the atom.
This can be any atom or
just a generic diagram.
Essential information
Subatomic
particle
Mass
Charge
Location
Subatomic
particle
Relative mass
Relative charge
Proton
1
+1
Neutron
1
0
electron, e-
1/2000
-1
1.
2.
3.
4.
5.
Some GCSE revision questions
The number of protons that an element has
determines what atoms it contains. For example, all
carbon atoms have six protons.
In any atom, which particle is present in the same
number as the number of protons?
Using the properties of the subatomic particles
explain why is there no overall charge on an atom?
Using the properties of the subatomic particles
justify where most of the mass of an atom is
found.
Using the structure of an atom determine where
most of the matter (material) in an atom is
distributed.
Why is the overall charge of the nucleus positive?
Most of the atom is empty space!
If you imagine an atom being the size of Wembley
stadium, the nucleus would be about the size of a football
on the centre spot.
The electrons would be two peas flying around the whole
stadium. The rest of it: emptiness.
What is the structure of an atom?
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.
In an atom the number of positive protons is the same as the
number of negative electrons.
Nucleus contains protons
and neutrons, overall
charge is positive.
Most of an atoms mass is
found here.
Most of the atom is empty
space.
Electrons give the atom it’s
overall size and shape.
An atom has no overall charge because the number of
positive protons is the same as the number of
negative electrons.
Distribution of mass in an atom
• The nucleus of an atom takes up almost no
space
• It is tiny but it makes up most of the mass of
the atom
• This is because it contains protons and
neutrons which both have a mass of 1, the
electrons orbit the nucleus and have
negligible mass
• It contains positive protons and neutral
neutrons – so it overall it is positive
Distribution of matter in atoms
• The rest of the atom is mostly empty
space
• There are negative electrons going around
the nucleus really fast. They give the atom
its overall size
• The mass of an electron is negligible
compared to a proton or neutron, this
means you can usually ignore it.
Isotopes
The number of protons is what determines
which element it is.
8 protons = oxygen
2 protons = helium
But the number of neutrons can vary.
If the neutrons vary then what else will
vary?
If neutrons vary then so does the atoms….
….mass!
Isotopes
Definition:
• Isotopes are atoms of the same element with
different numbers of neutrons.
Isotopes of an element will have:
•
–
–
–
•
different masses
the same number of protons and electrons
different numbers of neutrons
Isotopes of an element react in exactly the same
way because reactions involve electrons, neutrons
have no effect on chemical reactions.
Mass number (nucleon number)
total number of
protons and neutrons
in the nucleus
A
Z
X
Element symbol
Symbol for
an isotope
Atomic (proton) number
1) Number of protons in the nucleus – defines the element
2)All atoms of the same element have the same number of
protons
Chlorine -35 and -37 are examples of isotopes
Different mass numbers mean different numbers of neutrons
35
17
18 neutrons
(35 – 17)
Cl
37
17
Cl
20 neutrons
(37 – 17)
The atomic numbers are the same. Both isotopes have 17
electrons and 17 protons
Remember
Number of neutrons = mass number – atomic number
1. It is the number and arrangement of
electrons that decides the chemical
properties of an element. Isotopes have
the same configuration of electrons, so
they have the same chemical properties.
2. Isotopes of an element do have slightly
different physical properties such as
density and rates of diffusion, which is
dependant on the mass of particles.
Isotopes of Carbon
Carbon is a mixture of three isotopes, for
each of them complete the table:
Isotope
Mass number
Atomic Number
No. of neutrons
12
6
C
C
13
6
C
14
6
Isotope questions (actual exam ones)
1. Hydrogen, deuterium and tritium are all isotopes
of each other.
a) Identify one similarity and one difference
between these isotopes.
b) Deuterium can be written as 2H. Determine the
number of protons, neutrons and electrons in a
neutral deuterium atom.
c) Write a nuclear symbol for tritium, given that it
has 2 neutrons.
2. A certain atom X has one less proton and two
more neutrons than K. What are its atomic
number and mass number?
3. Explain in terms of sub atom particles, why C-12
and C-14 are isotopes
Ions
What is an ion?
It is an atom that is charged.
How does this happen?
Either by gaining or losing one or more
electrons.
Definition
An ion is a positively or negatively charged
atom or (covalently bonded) group of atoms
(a molecular ion)
Ions
• Ions have different numbers of protons
and electrons
• Negative ions have more electrons than
protons
e.g. Br- the negative charge means that
there’s one more electron than there are
protons. Br has 35 protons, so it must
have 36 electrons. The overall charge is
+35 – 36 = -1
• Positive ions have fewer electrons than
protons
e.g. Mg 2+ charge means that there’s 2
fewer electrons than there are protons.
Mg has 12 protons, so Mg 2+ must have 10
electrons. The overall charge
= +12 – 10 = +2
Ions
Complete the table:
Ion
Mass number, A
Atomic Number, Z
No. of protons
No. of neutrons
No. of electrons
Overall charge
23
11
Na+
35
17
Cl-
Ions questions
a) How many protons, neutrons and
electrons are in the following atoms and
ions?
b) Which are atoms and ions? How do you
know?
1) 173Li 2) 2411Na 3) 199F 4) 5526Fe
5) 3919 K+ 6) 199 F- 7) 3920 Ca 2+ 8) 178 O 2-