Periodicity, Group 2 And 7

Question 1

What’s the trend in atomic radius across period 3

Answer 1

The atomic radius decreases across period 3.
This is bc the increasing nuclear charge across the period, drawing outer electrons (same level/diff sub shells) closer to nucleus

Question 2

What’s the trend in melting point across period 3

Answer 2

The graph showing melting point across the period can be split into 4

1. The melting points of metallic elements increase
   - from Na to Al number of outer electrons increase
   - so more electrons can be delocalised (= greater attraction
   Between positive ions and delocalised electrons)
   -
   - sizes of ions decreases from Na+ to Al3+
   - again leading to greater attraction

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2. Melting point of silicon is very high
- silicon has a giant covalent structure; lots of energy required to break
The strong covalent bonds

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3. The melting point of P4, S8, Cl2, and Ar are low
- Phosphorus, sulfur and chlorine are simple covalent molecules
And little energy is required to overcome the weak vdw forces
- Ar exists as atoms and very little energy required to
Overcome weak vdws between atoms

The melting points increase from P to S then decrease
- the attractions between molecules are vdws (non polar molecules)
- if a molecule has more electrons the induced dipole interactions
Will be greater, causing greater vdw between molecules.
» increasing energy required to melt

• Argon’ melting point is very low as has very weak vdws

Question 3

What is the trend in first ionisation energy in period 3

Answer 3

The first ionisation energy across period 3 generally increases.
—> this increase is caused by increase in nuclear charge
With no increased shielding
The atomic radius hence is decreasing; outer electron
Is closer to nucleus with greater charge (more energy needed to remove)

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First ionisation energies (G3 and 6)
- atoms of G3 elements show a decrease in ionisation energy
- due to division of energy level into sub shells
—> like aluminium’s outer 3p1 electronics further from nucleus
With extra shielding (less energy to ionise)

• atoms of G6 elements show a decrease in ionisation energy
• as electrons start pairing in the p subshell
• the additional repulsion between the two electrons lowers energy
  Needed to remove(decreasing ionisation energy)

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First ionisation energies G1 and G0
- G1 atoms have lowest ionisation energy in every period
- as have greatest atomic radius and lowest nuclear charge
- G0 atoms have highest
- as have smallest atomic radius and highest nuclear charge

Question 4

What are the trends down group 2

Answer 4

Atomic radius
The outer electrons is pulled in closer to nucleus
When attraction between increases, so radius = smaller
Also, increasing number of electrons = more repel
-
- as G2 descends, outer electrons is less tightly held
- as shielded from increasing nuclear charge
- which new electrons filling more energy levels
- also provide extra repulsion and outer electron
- moves further from nucleus (radius increases)

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Ionisation energy
- First ionisation energy decreases down group
- energy required to remove outer depends in attraction
Between nucleus and outer electron
- G2’s atomic radius increases
- outer electron is tightly held due to shielding
from increasing nuclear charge
- so easier to remove; ionisation energy decreases

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Melting point
Generally melting points of the G2 metals decrease as go down
- G2 elements exist as giant metallic strucs
- a regular arrangement of positive ions
Surrounded by a sea of delocalised electrons
- metallic bond is attraction between the layers
Greater strength of metallic bond, higher the melting point
-
- number of delocalised electrons, metal ion charge and ion size
- are proportional to metallic bond strength
&raquo_space; G2 has the same 2+ charge and number of delo. Per atom
- as group descends, radius increases
- attraction between ion and delo electrons decreases
So strength of metallic bond decreases and melting point decreases

Question 5

What are reactions of G2 metals with water

Answer 5

Be doesn’t react with water

Mg reacts very slowly with water; readily with steam
Mg + 2H2O = Mg(OH)2 + H2 (water)
Mg + H2O = MgO + H2O (steam)
» MgO instead of OH with steam as OH is unstable
at higher temperatures,
thermally decomposes to give O and H2O

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Ca, Sr and Ba react with cold water to produce metal OH and H2
Reactivity increases as you descend group
Eg. Ca + H2O = Ca(OH)2 + H2

Ca reacts w water releasing bubbles of H2 in exothermic
-> white ppt Ca(OH)2 forms and remaining solution’s alkaline
—> since ppt is slightly soluble in water
Reactions with Sr and Ba are similar
As they get more reactive, amt of ppt decreases as more soluble
Down group.

Question 6

What’s the solubility if G2 sulfates in water

Answer 6

They become less soluble down the group
Mg2+ no ppt
Ca2+ thin white ppt
Sr2+ white ppt
Ba2+ thick white ppt

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PRACTICAL use of Ba salts to test for sulfate

Add unknown solution
- add dilute HCl (removes other ions like CO32-
- add aq BaCl2 solution
- if white ppt formed, sulfate ions present
Simplest ionic eq: Ba2+ + SO42- = BaSO4

Question 7

What are selected uses of G2 elements and their compounds (Mg)

Answer 7

Magnesium
- Mg is used in extraction of titanium
» titanium has applications in aerospace, marine and motor
Vehicle industries due to very high corrosion resistance
» very difficult to extract titanium from its ores
- in theory C can be used to extract from oxide but
titanium forms a brittle carbide
-
- but can be extracted using Mg
- TiO2 is converted to the chloride - subsequently
Reduced to Ti by reaction with Mg
TiCl4 + 2Mg = Ti + 2MgCl2

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Magnesium Hydroxide
- sparingly soluble; sold as a suspension in water
- (in this form, known as milk of magnesia)
- taken to alleviate constipation and (as antacid)
To neutralise excess acid in the gut
- solutions are slightly alkaline due to low solubility
Not irritating the oesophagus

Question 8

What are selected uses of G2 elements and their compounds (Ca)

Answer 8

Calcium hydroxide
- in solid form is known as slaked lime , neutralising acidic soil
- pH of soil affects its physical , chemical, and biological properties
> affecting crop yield
-
- as soil pH decreases, soluble Al3+ and Mn ion lvls increase
Becoming toxic (restrict root growth, curling and black spot on leaf)
- low pH restrict bacterial growth - wch help leguminous plants fix
N2 from air
—>reducing soil acidity prevents all these effects and increases availability
Of other important nutrients (P4)
—> BUT soil texture improves, allowing plants with small seedlings
An opportunity to break through soil surface

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Calcium Oxide
- neutralises sulfur dioxide - may be produced as a byproduct
Of fossil fuel combustion (prevents acid rain)
—> CaCo3 can also be used

Question 9

What are selected uses of G2 elements and their compounds (BaSO4)

Answer 9

Barium sulfate
- can be eaten as part of a ‘barium meal’
- barium is good at absorbing x rays, so when BaSO4
Reaches gut, the outline of gut can be located with X-rays
- although barium ions are toxic, this is harmless as BaSO4
Is insoluble and not absorbed into blood