periodicity

Question 1

what are the three trends we need to know about in periodicity?

Answer 1

-atomic radius
-ionisation energy
-electronegativity

Question 2

what is the trend for atomic radius across a period?

Answer 2

decreases because nuclear charge increases and there is similar shielding so nuclear attraction increases

Question 3

what is the trend for atomic radius down a group?

Answer 3

increases because increased shielding due to increased number of electron shells so nuclear attraction decreases.

Question 4

how do the size of cations (+) compare to the size of their atoms?

Answer 4

smaller than their atoms

because they have lost an electron

but have the same nuclear charge

so the nuclear attraction is shared between fewer electrons

so electrons pulled in more closely so smaller ion.

Question 5

how does the size of an anion (-) compare to the size of the atom?

Answer 5

larger than their atoms

because they have gained an electron

but same nuclear charge

so nuclear attraction is shared over more electrons and so is weaker

so electrons pulled in less so the ion is bigger

Question 6

what is the definition of ionisation energy?

Answer 6

the energy required to remove one mole of electrons from one mole of atoms in their gaseous state

Question 7

what is the trend for ionisation energy across a period?

Answer 7

increases because

Similar shielding

but more protons (greater nuclear charge)

so greater nuclear attraction between nucleus and outer electrons

so harder to remove an electron

Question 8

what is the trend for ionisation energy down a group?

Answer 8

decreases because

number of electrons shells increases so increased shielding

so the nuclear attraction between nucleus and outer electrons is weaker

so it is easier to remove an electron

Question 9

why is there a small drop in IE from Mg to Al?

Answer 9

in Al, outer electron is being removed from the 3p orbital instead of 3s

3p electron higher in energy and also slightly shielded by 3s electrons so requires less energy to remove

Question 10

why is there a small drop in IE from P to S?

Answer 10

with sulfur there are 4 electrons in the 3p sub shell and the 4th is starting to pair electrons in the first 3p orbital

when the second electron is added to a 3p orbital there is slight repulsion between the two negatively charged electrons which makes the second electron easier to remove

Question 11

what do the exceptions for trends in ionisation energy prove?

Answer 11

-that elements are in the correct groups
-existence of sub shells

Question 12

use ionisation energy to explain why there is decreased reactivity in group 1 (the alkali metals)?

Answer 12

IE decreases down the group because
-increased shielding
-decreased nuclear attraction
-increased atomic radius

Question 13

what is electronegativity?

Answer 13

the ability of an atom to attract a pair of electrons towards itself in a covalent bond

Question 14

what is the most electronegative element?

Answer 14

fluorine

Question 15

why does electronegativity increase across a period?

Answer 15

increased nuclear charge
similar shielding
so increased nuclear attraction

Question 16

why does electronegativity decrease down a group?

Answer 16

increased shielding (atomic radius increases) so weaker nuclear attraction

Question 17

use trends in electronegativity to explain why reactivity decreases down group 7 (halogens)?

Answer 17

electronegativity decreases because
-increased shielding
-so weaker nuclear attraction