3.2 Periodic Trends

You should know that orbitals are simply areas of space around nuclei where electrons are likely to be.

But they could be further away, so how do we know where the edge of an atom is?

For non-metallic elements that form molecules we can measure the average distance between nuclei and divide this by 2

This is called the Covalent radius - but only applies to covalent molecules.

For other elements we measure the distance between nuclei in the solid state and divide this by two – the van der Waals’ radius.

• • Learn the trend down groups and across periods

1. Why should atoms get larger as we go down a group?

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2. Why should atoms get smaller as we go across a period?

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Learn the definition and be able to write an equation

“The First Ionisation Energy is the minimum energy needed to remove 1 mole of electrons from one mole of gaseous atoms in their ground state.”

Learn the trend in IE across Periods and down Groups

1. Why should IE get smaller as we go down a group?

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2. Why should IE get (generally) larger as we go across a period?

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1. Explain the drops in Ionisation Energy from He-->Li, Ne -->Na, Ar-->K,

(Same reason each time!)

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2. Explain the drops in Ionisation Energy from Be-->B, Mg -->Al

(Same reason each time!)

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3. Explain the drops in Ionisation Energy from N-->O, P -->S

(Same reason each time!)

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v Learn the definition and be able to write an equation for the first electron affinity

“The first electron affinity is the energy change when one mole of gaseous atoms gain one mole of electrons to become 1 mole of singly charged negative ions”

Learn the definition for electronegativity

“Electronegativity is the relative attraction of an atom for the bond-pair in a covalent bond”

Why do electronegativities decrease down each group?

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Why should electronegativity increase across each period?

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Most elements can form oxides.

Metal Oxides – react with acids. Non-Metal Oxides – react with alkalis.

This allows us to classify elements as metals or non-metals.

• Balance the following equations:

Na₂O_(s) + HCl_(aq) --> NaCl_(aq) + H₂O_(l)

CaO_(s) + HNO_3(aq) --> Ca(NO₃)_2(aq) + H₂O_(l)

SO_{3 (g)} + NaOH_(aq) --> Na₂SO_4(aq) + H₂O_(l)

CO_{2 (g)} + Ca(OH)_2(aq) --> CaCO_3(s) + H₂O_(l)

SiO_{2 (g)} + NaOH_(aq) --> Na₂SiO_3(aq) + H₂O_(l)

An Oxide that can behave as both an acid and a base is Amphoteric

If an amphoteric substance can also donate and accept H⁺ ions it is said to be Amphiprotic – most but not all amphoteric substances are amphiprotic.

v Balance & the reactions of Aluminium Oxide with NaOH & HCl

Al₂O_3(s) + HCl_(aq) --> AlCl_3(aq) + H₂O_(l)

Al₂O_3(s) + NaOH_(aq) --> NaAl(OH)_4(aq)

1. Which Halogen is the most reactive?

2. Explain the trend in reactivity in Halogens.

3. Complete and balance the chemical equation for the reaction between:

a. Cl_2(g) + Na_(s) -->

b. Br_2(g) + K_(s) -->

c. F_2(g) + Cs_(s) -->

More reactive halogens will displace less reactive Halides from solution.

Eg Bubbling Fluorine into a Chloride solution will cause displacement

F_2(g) + 2Cl^-_(aq) à Cl_2(aq) + 2F^-_(aq)

But bubbling Chluorine into a Fluoride solution will not cause displacement

Cl_2(g) + 2F^-_(aq) à NO REACTION

You should be able to suggest which of these mixtures will cause a reaction and what you should observe, if anything.

• Write Ionic Equations for the mixtures that result in reactions

• Why can smaller halogens remove electrons from larger Halides?