C2 - The Periodic Table

Question 1

Organising elements in the early 1800s

Answer 1

Elements are categorised by:
• physical and chemical properties
• atomic weight
Atomic numbers were not discovered then as sub atomic particles were not discovered
The only thing they could measure with atomic weight, and so the known elements are arranged in order of atomic weight . When this was done, a periodic pattern was noticed in the properties of the elements.
Early periodic tables were not complete and some elements placed in the wrong group. This is because elements were placed in the order of atomic weight and did not take into account their properties.

Question 2

Dmitri Mendeleev

Answer 2

• 1869 - overcame some problems of early periodic tables by taking 50 known elements and arranging them into his Table of Elements (with some gaps)
• put elements mainly in order of atomic weight but did switch that order if the properties meant it should be changed
• gaps were left so that elements with similar properties stayed in the same group
• some of these gaps indicated unknown elements - Mendeleev could guess what their properties would be
• made great predictions about germanium

Question 3

Discovery of isotopes

Answer 3

The discovery of isotopes in the early 20th century confirmed that Mendeleev was correct to not place element in a strict order of atomic weight, but to also take account of their properties. Isotopes of the same element have different masses that have the same chemical properties so occupy the same position on the periodic table.

Question 4

Properties of alkali metals

Answer 4

• all have one electron in their outer shell which makes them very reactive and gives them similar properties
• alkali metals are soft and have low density
• as you go down, they become more reactive because the outer electron is more easily lost
• as you go down, there are lower melting and boiling points
• as you go down they have a higher relative atomic mass
• form 1+ ions
• react to form ionic compounds - generally white solids that dissolve in water to form colourless solutions
• when put in water - they react vigorously
• produce hydrogen gas - amount of energy given out when they react increases down the group
• also form hydroxides that dissolve in water to give alkaline solutions
• reaction with chlorine produces a salt
• can react with oxygen to form a metal oxide

Question 5

Halogens - Group 7 elements

Answer 5

• non metals with coloured vapours
• as you go down the group:
- less reactive
- higher BP and MP
- higher relative atomic masses
• can form molecular compounds - covalently bond with other non-metals to achieve a full outer shell. All have simple molecular structures
• form 1- ions called halides when they bond with metals - compounds that form have ionic structures
• more reactive halogens will displace (the salt of) less reactive ones

Question 6

How do you explain trends in the periodic table?

Answer 6

in terms of the attraction between electrons in the outermost shell and the nucleus.
• This electrostatic attraction depends on:
the distance between the
outermost electrons and the nucleus
the number of occupied inner shells (energy levels)
of electrons, which provide a shielding effect
- the size of the positive charge on the nucleus (called the nuclear charge).

The increased nuclear charge, due to extra protons in the nucleus, going down a group is outweighed by the other two factors.
• Therefore electrons are easier for the larger atoms to lose going down a group, and harder for them to gain going down a group.