Chemistry - Chemical Patterns (C4)

Dmitri Mendeleev

Periodic Table is based on Dmitri Mendeleev's ideas. 

He arranged elements into groups (columns) and periods (rows) based on their RAMs (relative atomic masses) and patterns in their properties.

Mendeleev left gaps for undiscovered elements & predicted properties for missing elements.

Other scientists

• Johann Dobereiner noticed 'traids' that linked patterns of RAMs for 3 elements.
• John Newlands noticed 'octaves' pattern, where every 8th element had similar properties.

Scientists rejected their ideas because most elements didn't fit their 'patterns'.

When new elements were discovered, they fitted Mendeleev's ideas.

Line Spectrum

When elements are heated they emit coloured flames. The coloured light can be split into a line spectrum which is unique to each element.

Helium was discovered when chemists looked at the line spectrum from the sun. (Due to the development of spectroscopy).

Atoms

Atoms have a nucleus containing protons and neutrons.

Electrons travel on the outside of the atom in shells.

• Proton: positive charge
• Electron: negative charge
• Neutron: no charge

Proton number and electron number are the same.

Number of protons + number of neutrons = relative atomic mass (RAM)

Electrons are arrange in shells around the nucleus.

First shell: closest to nucleus and holds two electrons.

Second & third shell: further away from nucleus and hold eight electrons.

Examples:

• Oxygen has a proton number of 8 so the electron arrangement would be 2, 6
• Potassium has a proton number of 19 so the electron arrangement would be 2, 8, 8, 1

Electrons on different shells have different energy levels.

The closer the electron shell is to the nucleus, the lower the energy level.

Elements in the Periodic Table are arranged by their proton numbers.

• Period: horizontal row.
• Group: vertical coloumn.

The number of electrons in the outer shell increases across a period.

Properties such as melting points change across a period. They are called trends.

Elements in Group 0 have full electron shells and are inert (very unreactive).

Elements with up to 3 electrons on the outer shell are metals.

Elements with 5 or more electrons on the outer shell are non-metals.

REMEMBER

• Group number = number of electrons on outer shell.
• Number of protons = number of electrons.
• Number of protons + number of neutrons = relative atomic mass (RAM).

Group 1 is called the Alkali Metals.

All elements in group 1 have 1 electron on the outer shell. (Because they're in group 1).

Group 1 elements are soft metals so they can easily be cut with a knife. 

The surface which is cut is shiny but it tarnishes easily in moist air by reacting with oxygen.

All elements in group 1 form ions with a +1 charge. (They lose an electron)

Reactivity increases as you go down the group because the outer electron is the easiest to lose as it is furthest away from the nucleus.

Group 1 elements all react with water. The reaction gets more violent as you move down the group because they are more reactive.

The reaction

In the reaction between group 1 elements and water, hydrogen gas is formed which 'pops' when it's lit.

The reaction also makes a metal hydroxide which is an alkali and turns the pH indicator blue.

The equation

Metal + water  metal hydroxide + hydrogen

HINT: Use M to stand for any of the group 1 metals so it would be: 2M(s) + 2H O(l)   H (g) + 2MOH(aq)

Example: 2Na(s) + 2H O(l)   H (g) + 2NaOH(aq)

Safety

Group 1 metals are flammable and their hydroxides are corrosive. When handling group 1 metals, they should be kept away from water and naked flames.

Sodium reacts vigorously with chlorine to give a yellow flame and makes a white solid (sodium chloride OR 2NaCl)

As you go down the group, the reactions get faster because it's more reactive down the group.

Word and symbol equations for reaction between sodium and chlorine are:

• Sodium + Chlorine   Sodium chloride
• 2Na(s) + Cl (g)   2NaCl(s)

Chlorine

• At room temperature it's a pale green gas.
• The colour of the gas is pale green.

Bromine

• At room temperature it's a red-brown liquid.
• The colour of the gas is reddish-brown.

Iodine

• At room temperature it's a dark grey liquid.
• The colour of the gas is purple.

The halogens all contain diatomic molecules. (Each molecule has 2 atoms joined together).

As you go down the group, the melting and boiling points increase.

Group 7 elements are corrosive and toxic and need to be used in a fume cupboard.

Halogens react with alkali metals and other metals such as iron (Fe) to form metal halides. For example:

• Metal (iron) + Halogen (chlorine) = Metal halide (iron chloride)
• Metal (sodium) + Halogen (bromine) = Metal halide (sodium bromide)
• Metal (potassium) + Halogen (iodine) = Metal halide (potassium iodide)

Halogens are less reactive as you go down the group.

Displacement reactions happen when a more reactive halogen takes the place of a less reactive halogen in a compound.

All halogen atoms have 7 electrons in their outer shell.

EXTRA: Chlorine is more reactive than bromine & displaces it from potassium bromide solution.

Compounds made from group 1 and 7 elements such as Sodium Chloride are solids with high melting points. (Because they are towards the top of the group).

Sodium (Na) has to lose an electron because it's in group 1 so it forms a positive charged ion.

Chlorine (Cl) has to gain an electron because it's in group 7 so it forms a negative charged ion.

They are ionic compounds because they contain charged particles or ions and are arranged in a regular pattern called a crystal lattice.

Ionic compounds are soluble in water and conduct electricity when they are melted. 

When an ionic crystal melts/dissolves in water, the ions are free to move.

An ionic compound does not conduct electricity when it is a solid because the ions have to move to the electrodes to complete the circuit.

An atom of a Group 1 element loses one electron to become a positive ion.

An atom of a Group 7 element gains one electron to become a negative ion.

All Group 1 metals become ions with +1 charge.

All Group 7 metals become ions with -1 charge.

In the formula for an ionic compound the number of positive & negative charges balance. For example, Na+ and Cl- have one positive & one negative charge so they cancel out to make NaCl.

You can work backwords from the formula to work out the charge on the ions.

For example, CaCl  has 2 charges from the two Cl- ions.

So the Ca ion has a charge of Ca +