Arrangement of Elements
In the periodic table, the elements are arranged in order of proton number, also called atomic number. This is the number of positive protons. This is shown at the base of an element in the periodic table.
The letter(s) in the middle of an element within the periodic table is the element symbol or element formula.
The number at the top of an element in the periodic table is called the relative atomic mass.
Each row is a period. Every period starts with a very reactive metal in group 1.
Every period ends with an unreactive gas in group 8.
Relative atomic mass and Group 1 metals
Relative atomic mass is a comparative measure of the mass of one atom of an element.
Eg. Magnesium has a RAM of 24 (as shown by the number below the element symbol for Magnesium in the periodic table), whereas Carbon has a RAM of 12, meaning that magnesium atoms are twice as heavy as carbon atoms.
Each column within the period table is called a group, with the first column being group 1, and goes up by 1 each time up until group 8 (or it is sometimes called group 0, but 8 may be easier to remember). Each group has it's own properties.
Group 1 - This group is called the Alkali Metals - they are very reactive, good conductors of heat and electricity, have high boiling points, low melting points, low densities and are soft.
The metals in group 1 become more reactive as you go down the group.
Group 1 Metals (contiunued)
The alkali metals are so reactive with reactive that they have to be kept under oil to stop them reacting with oxygen or moisture in the air.
They are called alkali metals because they react with water to form alkaline solutions.
Reactions with water example (they react very vigorously):
lithium + water ---> lithium hydroxide and hydrogen
Reactions with chlorine example:
sodium + chlorine ---> sodium chloride
lithium + chlorine ---> lithium chloride
(they form chlorides, which are all salts) [the term 'salts' covers all compounds of metals and non-metals]
Group 7 non-metals
Group 7 in the periodic table are called the halogens, which literally means 'salt-former', as they form salts when they combine with metals.
The halogens are very reactive, are bleaching agents, have low melting and boiling points and are molecular (so the atoms are joined in pairs: Cl₂, Br₂, I₂). The forces between the molecules are weak, and so it is easy to seperate them and turn the halogens into gases.
The halogens become darker in colour as you move down the group, and also become less vigorous down the group.
Metal + Halogen reaction example:
sodium (alkali metal) + chlorine (halogen) ---> sodium chloride (common salt).
Examples of Halogens
- Chlorine is a dense, pale-green gas that is smelly and poisonous, and occurs as chorides (especially sodium chloride in the sea)
- Bromine is a deep red liquid with red-brown vapour that is smelly and poisonous, and occurs as bromides (especially magnesium bromide in the sea)
- Iodine is a grey solid with purple vapour that is smelly and poisonous and occurs as iodides in some rocks and in seaweed.
The mass of an atom is concentrated in a tiny, central nucleus. The nucleus consists of protons and neutrons. The protons have a positive electric charge, whilst the neutrons are uncharged (neutral).
Around the nucleus are the electrons. The electrons are negatively charged. The mass of an electron is so small that it can often be ignored. In an atom, the number of electrons is always equal to the number of protons, meaning the overall atom is uncharged.
Electron = negative charge (1-)
Proton = positive charge (1+)
Neutron = Neutral (no charge)
Electrons are contained in shells around the nucleus. These shells can also be called energy levels. The number of shells and the number of electrons in the outer shell varies from one element to another.
The first shell can hold a maximum of 2 electrons.
The second shell can hold 8 electrons.
The third shell holds 8 electrons.
The fourth shell holds 18 electrons.
Every time a shell is filled, a new period (row) starts on the periodic table.
Electronic Structure (continued)
Because the periodic table tells you the proton number of each element, and number of protons = number of electrons, you can work out how many shells a particular atom has and how the electrons are distributed within them.
eg. Neon has a proton number of 10, meaning it also has 10 electrons. This means that it has two shells, with 2 electrons in the first shell (as this is how many the first shell can hold), and 8 electrons in the second shell. Both shells are filled fully, and this makes neon atoms stable. You would write this electon arrangement as '2,8'.
The number of electrons in the outer shell of any atom is the same its the group number in the periodic table.
The number of shells any atom has is the same as its row/period number in the periodic table.
When the atoms of some metals are heated, they give off coloured light. Each metal gives off a different colour and so it can be used to identify them.
Example flame colours of Group 1 elements:
Lithium (Li) RED
Sodium (Na) YELLOW
Potassium (K) LILAC
A prism can be used to split non-visible light and form a spectrum, each element having it's own distinctive spectrum.
Spectroscopy has led to the discovery of rubidium, caesium, and helium.
When a metal reacts with a non-metal, each metal atom loses an electron from it's outer shell, giving it a positive charge (as there are now more protons than electrons, and protons have a positive charge). It is now not an atom, but an ion.
For example, when sodium (Na) which has 11 electrons, loses one of them, it becomes a positively charged sodium ion. It's electron arrangement is now the same as a neon atom (2,8), but it is not the same thing, because it is positively charged.
The formula for a sodium ion is Na⁺.
If an metal atom loses two electrons from it's outer shell (eg. magnesium), it becomes an ion with the charge of 2⁺ (eg. Mg2⁺).
When a metal reacts with a non-metal, each non-metal atom gains the number of electrons needed to fill it's outer shell. The atom gains negative electrons, but still has the same number of positive protons, so the ion is negatively charged.
For example, when fluorine atom (F) which has 9 electrons, gains an electron, it becomes a negatively charged fluorine ion. It's electron arrangement is now alsothe same as a neon atom (2,8), but is not an atom but a negatlively charged ion.
The formula for a sodium ion is F⁻
There are strong electrostatic forces of attraction between two oppositely-charged ions in an ionic compound, called an ionic bond.
Positively charged metal ions attract negatively charged non-metal ions attract each other and hold each other together in a compound.
If, for example, a metal ion had a charge of 2⁺, you would either need a 2⁻ ion to attract it or two 1⁻ ions to attract to it in order to balance out the forces.
Oppositely charged ions arrange in a regular way to form giant ionic lattices. Ionic compounds often form crystals as a result.
Properties of Ionic Compounds
Ionic compounds have high melting and boiling points, because ionic bonds are very strong and a lot of energy is needed to break them.
Ionic compounds also conduct electricity when liquid, because when the ions are dissolved or melted the ions are free to move and the electrons can flow to produce an electric current.
Example of an ionic compound:
Sodium Chloride (NaCl) - 800 degree melting point, conductive when melted or dissolved
When heated and melted, the ions in ionic compounds move so much that they cannot stay in a regular arrangement and can conduct electricity.
When ionic compounds are dissolved, the ions can move independently and so can conduct electricity too.
Ionic theory - Electrolysis
During electrolysis, the compound which is being split back into its elements is heated and made molten to allow the ions to move independently.
Two electrodes (conducting rods) are connected to a power supply, with the negative electrode (cathode) connected to the negative terminal of the power supply and the positive electrode (anode) connected to the positive terminal.
The ionic compound is molten which means the positive ions can move towards the negative electrode, and the negative ions will move towards the positive electrode. The negative ions lose their extra electron(s) as they travel up the positive electrode, whilst the positive ions gain back electron(s) as they travel up the negative electrode, returning them to to their original atoms.