Elements and the Periodic Table

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A-level Chemistry By: Please log in to see tutor details
Subject: Chemistry » A-level Chemistry
Last updated: 06/02/2016
Tags: a level chemistry, chemistry

Did you know, that to understand the properties of elements in the periodic table, it helps to know about the element's electronic structure?

There are one hundred and eighteen elements in the periodic table. The table is arranged in vertical columns called groups and in horizontal rows, called periods. There are eighteen groups and seven periods. In every element, electrons whizz around the nucleus in distinct orbits called ‘shells’. The negatively charged electrons are kept in their shells because of attraction by positively charged protons in the nucleus.

Every period or row starts with an element in group 1 – the first column on the far left of the periodic table. The period number tells you how many electron shells the element has (see table below). Hydrogen, in period 1, has one electron and because it is a gas, not an alkali metal, we won’t talk about it anymore.

Period 2 starts with Lithium, the second element in group 1, with two electron shells. Period 3 starts with Sodium with three electron shells. Potassium starts Period 4 and has four electron shells. The other elements in group 1 are Rubidium (period 5) and Caesium (period 6).

The six metals in group 1 all react with water making an alkaline solution so they are called ‘alkali metals’. As we go down group 1 the metals react more and more violently with water. At the top of group 1, Lithium fizzes gently, giving off Hydrogen, but insufficient heat is generated to melt the metal. The next element down the group is sodium, which melts immediately and skids around the surface of the water propelled by Hydrogen gas. Potassium reacts with water even faster and burns the Hydrogen produced with a beautiful lilac flame. Rubidium reacts violently and immediately, with the metal spitting. Caesium explodes on contact with water.

When alkali metals react with water, oxidation occurs with the loss of an outer shell electron to a non-metallic element, to form a single positively charged ion. For example, Na ---> Na+ + e-.

As you go down the group, from lithium to caesium, you need to put less energy into the reaction to get a positive ion formed - the first ionisation energy is lower as you go down group 1. The outer electron is lost more readily - the positive proton cannot hold on to the outer electron as the number of electron shells increases. The extra protons in the nucleus are screened by the extra shells of electrons.

Ionisation energy is dependent on: the nucleus charge; the amount of screening by the inner electrons; and the distance between the outer electrons and the nucleus.

With each metal, as you go down the Group, the increase in the positive proton charge is exactly offset by the increase in the number of inner negative electrons - the outer electrons feel a net attraction of 1+ from the centre. However, as you go down the Group, the distance between the nucleus and the outer electrons increases and so they become easier to remove - the ionisation energy falls.

The only factor affecting the size of the atom is the number of shells of inner electrons which surround the atom. More electron shells take up more space, due to electron-electron repulsion. Therefore, the atomic radius increases in size down group 1.

With water, Lithium fizzes - Caesium bangs!

Period

Group 1 alkali metal

Symbol

Ist ionisation energy (kJ/mol)

Proton number

Electron shells

Electronic configuration

Atomic radius (pm)

2

Lithium

Li

+519

3

2

2,1

134

3

Sodium

Na

+494

11

3

2,8,1

154

4

Potassium

K

+418

19

4

2,8,8,1

196

5

Rubidium

Rb

+402

37

5

2,8,18,8,1

211

6

Caesium

Cs

+376

55

6

2,8,18,18,8,1

225

 


Chris Street MSc A-level Chemistry Tutor (Bournemouth)

About The Author

Chris Street aims to make learning about the subjects he tutors enjoyable and fun. Chris builds confidence, knowledge & exam techniques in his students so that they are more likely to achieve their target grades.




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