Periodicity - Unit 2, Section 1

Question 1

how is the periodic table arranged?

Answer 1

• Dmitri Mendeleev developed the periodic table in the 1800s
  -It is arranged in periods and groups by atomic number.
  -The period and group of an element gives you information about the number of electrons and electron shells that an element has.

Question 2

Elements and periods

Answer 2

All the elements within a period have the same number of electron shells.

Question 3

Elements and groups

Answer 3

All the elements within a group have the same number of electrons in their outer shell so they have similar properties.

The group number tells you the number of electrons in the outer shell.

Question 4

What does periodicity involve?

Answer 4

To do with the trends in physical and chemical properties of elements across the periodic table. E.g: Atomic radius, melting point, boiling point and ionisation energy.

Question 5

Atomic radius

Answer 5

• decreases across a period
• as the number of protons increases, the positive charge of the nucleus increases.
• This means electrons are pulled closer to the nucleus, making the atomic radius smaller.
  -The extra electrons that the elements gain across a period are added to the outer energy level, so they don’t provide any extra shielding effect.

Question 6

Melting points across Period 3

Answer 6

melting points increase from sodium to silicon but then generally decrease from silicon to argon.

Question 7

Sodium, Magnesium and Aluminium

Answer 7

• are all metals
  -melting points increase across the period because the bonds get stronger. This is because the metal ions have an increasing positive charge, an increasing number of delocalised electrons and a decreasing radius.

Question 8

Silicon

Answer 8

• macromolecular with a tetrahedral structure - strong covalent bonds link all its atoms together.
• a lot of energy is needed to break these bonds so silicon has a high melting point.

Question 9

phosphorus, sulfur, chlorine and argon

Answer 9

Phosphorus, sulfur and chlorine are all molecular substances. Their melting points depend upon the strength of the van der Waals forces between the molecules. These forces are weak and easily overcome so these elements have low melting points.

Sulfur is the biggest molecule so it has got a higher melting point than phosphorus or chlorine as its van der Waals forces will be stronger, due to being more of them.

Argon has a very low melting point because it exists as individuals atoms, resulting in very weak van der Waals forces.

Question 10

First ionisation energy

Answer 10

the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 moles of gaseous 1+ ions.

there’s a general increase as you go across period 3. This is because of the increasing attraction between the outer shell electrons and the nucleus, due to the number of protons increasing.