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The Atom
LOs
Describe the structure and
properties of the atom
Explain the history of atomic models
Nuclear symbol
Nuclear symbol
12
27
C
Al
6
13
Periodic table symbol
Periodic table symbol
35.5
24.3
Cl
Mg
17
6
What is…
1) The atomic number of an element?
2) The mass number?
3) How do you establish the number of neutrons?
4) What is the relationship between protons and
neutrons?
What can you say about this species?
2+
Mg
The subatomic particles
Draw a table to show the properties of the
protons, neutrons and electrons.
Protons
Neutrons
Electrons
Let’s get back to magnesium
24
25
26
Mg
Mg
Mg
12
12
12
What are these 3 called?
How are the 3 atoms similar?
How are the 3 atoms different?
Isotopes
Atoms with the same number of protons, but a
different number of neutrons.
Same chemical properties, but WHY?
Different physical properties, WHY?
Exam Questions
Atomic Models
Produce a flow diagram explaining
how the model of the atom has
evolved over time
• Include the scientists name
• How the model is better than the previous model
• Evaluate how successful each model is (one
sentence)
Group discussion
Plenary
Can you explain atoms, their subatomic particles and
their properties?
Can you quickly calculate the number of protons neutrons
and electrons in an atom from the nuclear symbol?
Can you explain what an isotope is, and what this means
for chemical and physical properties?
Can you evaluate the different atomic models?
Starter: Linking and Synthesis
How can you link Carbon-12 to the Bohr model
of the nucleus? (3 marks)
Homework
What you should understand already
1) The 5 different models of the atom, and who
proposed them
2) What you can work out from a nuclear
symbol
3) What isotopes are, and what you can say
about the chemical and physical properties
Relative isotopic mass
Definition:
The relative isotopic mass is the mass of an
isotope of an element on a scale where an atom
of carbon-12 is 12.
u = unified atomic mass unit
Relative atomic mass
Definition:
The relative atomic mass (Ar) is the average
mass of an atom of an element on a scale where
an atom of carbon-12 is 12.
Relative molecular mass or
Relative formula mass
Definition:
The relative molecular mass (Mr) is the average
mass of a molecule or formula unit on a scale
where an atom of carbon-12 is 12.
Relative Masses
Why is the relative mass of magnesium 24.3?
Question) The relative abundance of atoms of
magnesium are 84% 24Mg, 2% 25Mg and 14%
26Mg. What is the relative atomic mass.
Relative Masses (2)
Why is the relative mass of copper 63.5?
Masses and Moles
• Use p10 and 11 to identify the key ideas you think
are important.
My ideas:
Then we will discuss as a group and add or remove
the key ideas:
Group ideas:
Key equations
Exam Questions
Empirical Formula and Balancing
equations
Plenary
1. Do you know the 3 definitions for the relative
mass of atoms, isotopes and molecules.
2. Do you know what relative abundance means,
and can you calculate the relative atomic mass
in under 2 minutes?
3. Do you know what a mole is, what Avagadro’s
constant is, and what links the relative mass and
the mass of one mole.
Weighing Atoms
It is very difficult to weigh individual atoms. Why?
Relative atomic masses
To make life easier, scientists decided to compare
masses against Carbon. We say carbon has a mass of
12, and elements are either lighter or heavier.
This value is called the Relative Atomic Mass (Ar).
For example:
Hydrogen is much lighter than carbon: H (Ar) = 1
Mercury is much heavier than carbon: Hg (Ar) = 197
Relative Formula Mass
The relative atomic mass is for each element in the
periodic table.
The relative formula mass is for compounds.
Relative formula mass (Mr)
E.g. CaCO3 – Calcium Carbonate
Mr = Ca + C + 3xO
= 40 + 12 + (3 x 16)
= 100
How do we know that…
• Sodium Chloride is NaCl
• Glucose is C6H12O6
• Aluminium sulfate is Al2(SO4)3
Empirical formula
This is a method to work out the simplest formula for
compounds. You need to perform an experiment to
work this out.
If 10g of calcium, reacts with 17.8g of chlorine,
what is the empirical formula?
IN PAIRS – complete 1 each
Question 1
9g of aluminium reacts with
35.5g of chlorine. What is
the empirical formula of
aluminium chloride?
Question 2
28g of iron reacts with 16g
of oxygen. What is the
empirical formula of iron
oxide?
Alternative Question
Compound Y is made up of only carbon,
hydrogen, and oxygen atoms. The percentage
composition is 59% carbon, and 33% oxygen.
What is the empirical formula?
Empirical formula
Use the empirical formula from the previous
question. (also called the formula mass).
Molecular formula
The Mr of the compound is 200g. What is the
molecular formula?s
Empirical Formula Practical
1)
Put a crucible and lid on a top
pan balance, and record the
mass.
Mass (g)
Crucible and lid
2)
3)
4)
5)
Add 5 magnesium strips into
the crucible, and record the
mass again.
Place the crucible on a clay
triangle, and heat strongly with
a Bunsen burner for 5 minutes.
Put a heat proof mat on the top
pan balance, and zero the
balance.
Place the hot crucible on the
heat proof mat, and record the
final mass.
Crucible, lid and
magnesium (before
heating)
Mass of magnesium
Crucible, lid and
magnesium (after
heating)
Mass of oxygen that
reacted
Using your results
Your results should look something like:
Mass of magnesium = 0.12g
Mass of oxygen = 0.08g
Using the Ar(Mg) = 24 and Ar(O) = 16,
calculate the formula. Empirical formula.
Moles…continued
LOs
1) To be able to perform mole calculations with
solid masses
2) To be able to perform mole calculations with
solutions
3) To be able to perform mole calculations with
gases
Atoms and moles
1) How many atoms are there in 2.1 moles of lithium?
2) A sample contains 3.4 x 1022. How many moles do
you have?
Solids and moles
1) A sample contains 45.7g of ethanol (C2H5OH).
How many moles do you have?
2) What is the mass of 0.45 moles of magnesium?
Solutions and moles
Concentration
1) What is the number of moles in 45cm3 of
0.25 mol dm-3 solution?
2) What is the concentration of a solution, when 0.34
moles of compound is dissolved in 500cm3 of water?
Gases and moles
1 mole of ANY gas has a
volume of 24dm3 at room
temperature and pressure
(RTP).
1) How many moles of O2 are present in 56dm3 at
RTP?
2) What is the volume of 0.46 moles of hydrogen
gas at RTP?
Pulling triangles together
Standard Solutions
1)
Decide on the concentration you need.
2)
Check the volume on the flask.
3)
Calculate the number of moles you
need to make the concentration (conc
triangle)
4)
Calculate how many grams this is.
5)
Add the sample to the flask
6)
Fill to around 80% of the flask with
distilled water.
7)
Thoroughly mix by inverting. Then top
up to the line.
Your turn
Produce a 0.25 mol dm-3 of sodium chloride.
Salts
Learning Objectives
• Describe the reaction of an acid with
carbonates, bases and alkalis to form a salt
• Explain that salts are formed, when the H+ of
an acid is replaced by the metal or NH+
• Explain that a base readily accepts H+ ions
(examples include OH- and NH3)
What is an acid?
• Around 1810 Humphrey Davy discovered that
hydrochloric acid is made of only hydrogen
and chlorine
• This led to it being established that hydrogen
was common to all acids
• An early definition of an acid states that they
have at least one hydrogen atom that can be
replaced by a metal atom
+
H
Bronsted-Lowry Theory
• Stated that an acid is a proton donor and a
base is a proton acceptor.
• For example
• HCl + NH3
Cl- + NH4+
• Note acids and bases can only react in pairs:
one acid and one base
A salt is…
• when the H+ of an acid is replaced by the metal
• 1. Mg(s) + 2HCl (aq)  MgCl2(aq) + H2(g)
OR
• when the H+ of an acid is replaced by the NH+
• Nitric Acid + Ammonium Hydroxide:
• HNO3 + NH4OH  NH4NO3 + H2O
Chemical reactions
Hydrochloric acid (HCl)
Sulphuric acid (H2SO4)
Citric acid (C6H8O7)
Magnesium oxide (MgO)
Sodium Hydroxide (NaOH)
What is the formula for
aluminium oxide?
Salts part 2
LOs
C grade – know the standard acid base equations
B grade – be able to calculate the formula of a
hydrated compound
A grade – be able to balance all equations and
include correct state symbols
You need to know standard acid base
reactions (GCSE stuff)
1) metal + acid  salt + hydrogen
2) Base + acid  salt + water
3) Carbonate + acid  salt + water + carbon dioxide
Ionic Equations
Similar to full equations, but focus on the
important ions in a reaction.
MgO + HCl  MgCl2 + H2O
Warning: you can also have ½ numbers in
equations at AS:
C2H6 + O2  CO2 + H2O
Working out the formula of salts
Hydrated Compounds
Some definitions
• Anhydrous – literally means without water
• Hydrated – Hydration occurs when water
reacts with a compound producing a single
chemical product.
• Water of crystallisation – Many salts form dry
crystals containing molecules of water. This
water which does not wet the crystals is
known as water of crystallisation.
• Some compounds contain H2O in their structure. These compounds are called hydrates.
• This is different from (aq) because the H2O is
part of the molecule (not just surrounding it).
• The H2O can usually be removed if heated.
• A dot separates water: e.g. CuSO4•5H2O is
copper(II) sulfate pentahydrate.
• A greek prefix indicates the # of H2O groups.
The method
1) Record the mass of the hydrated salt.
2) Record the mass of the anhydrous salt.
3) Calculate the mass of water (subtract
anhydrous from hydrated)
4) Using Mr of salt and water, calculate mole
ratio of salt to water.
Example
Na2SO4.xH20
Worked Example
• Q1 – A student found that a sample of
hydrated cobalt (II) chloride contained 5.2g of
anhydrous cobalt (II) chloride and 4.32g of
water. Calculate the formula of hydrated
cobalt (II) chloride (Hint cobalt chloride =
CoCl2)
Question 2
Q2 – 13.9g of hydrated iron (II) sulphate crystals
were heated, driving off the water of
crystallisation, leaving 7.6 of anhydrous iron (II)
sulphate. Calculate the formula of hydrated iron
(II) sulphate. (Hint Iron sulphate = FeSO4)
• Q3 – Hydrated barium chloride is heated. Its mass
decreases from 30.5g to 26.0g. What is the
formula of the hydrated salt?
• Q4 – Hydrated sodium carbonate, Na2CO3·xH2O
was found to contain 62.9% water by mass.
Calculate the value of x
• Q5 – Potash alum has the formula
K2SO4·Al2(SO4)3·yH2O, calculate y, the number of
moles of water of crystallisation given that the
hydrated salt contains 45.57% water by mass
Answers
•
•
•
•
•
Q1 – CoCl2·6H2O
Q2 – FeSO4·7H2O
Q3 – BaCl2·2H2O
Q4 – 10
Q5 - 24