The Eye - Muscles of the Iris

There are two muscles in the iris: the circular muscle and the radial muscle, the diagram below shows both of them

Courtesy of http://georgi-georgiev.com/

These muscles control the iris, and incase you've forgotten, the iris controls how much light is let into the pupil.

The two muscles work antagonistically, meaning that when one constricts the other relaxes and when one relaxes the other constricts.

What happens in low light concentrations (dark)?

The pupil will widen when it is dark, allowing more light into the eye.

The radial muscle fibres constrict, widening the eye and the circular muscle relaxes due to the constriction of the radial muscle.

What happens in high light concentrations (light)?

The pupil will narrow when its light, allowing less light into the eye.

The circular muscle fibres constrict, reducing the pupil size and the radial muscle relaxes due to the constriction of the circular muscle.

Please note that this is a reflex.

Read about RODS AND CONES next!

The Eye - Rods and Cones

There are hundreds of millions of rods and cones covering the surface of the retina of the eye.


Cones

Cones detect colours.
There are around 7 million cones in the human eye.
There are three types of cones, each type detects a different colour, red, green or blue.
When all three types of cones are stimulated equally, we see white.


Rods

Rods detect shapes.
Rods help us see at night, when there isn't enough light for the cones to work.
There are around 120 million rods in the human eye.

Courtesy of http://littlewebgiants.com

You don't have to know what rods and cones look like for the exam, but it might be helpful to see what they look like.

Look at the FOVEA next!

The Eye - Fovea

The fovea is the area in the retina where the cones are concentrated. It is also the part of the eye where images are examined in detail.

Objects within a 2° angle of the eye are the only objects that can be seen in detail. By scanning across scenes, the eye builds up an accurate picture of the object or scene.

Image courtesy of www.studyblue.com

Group 0 - The Noble Gases

What are the noble gases?

The noble gases are helium, neon, argon, krypton, xenon and radon (radioactive).


Properties of the noble gases

• They are all non metals
• They were discovered after Mendeleev had invented the periodic table
• Colourless gases
• Exists as monatomic gases, e.g. Ar, Ne, He etc. 
• Very unreactive
• Also called Group VIII
• Very steady electron structures that are difficult to change
• Other elements try to get these electron configurations by under going chemical reactions
• Their outer electron shells are full

Compounds of helium, neon and argon have never been found, but recently, compounds of xenon and krypton with fluorine and oxygen have been found, e.g. XeF.

Uses of argon, neon and helium

Neon is used in lasers and neon lights.

Argon gas is the gas used in lightbulbs that stops the tungsten filament reacting with air.

Helium is used in balloons and medical procedures, e.g. MRI machines.

You can separate helium from natural gas by liquefying the other elements.

You can also obtain the other noble gases by the fractional distillation of liquid air.

Group VII - The Halogens

What are the elements?

The elements in Group VII are fluorine, chlorine, bromine, iodine and astatine (radioactive).
Chlorine, bromine and iodine are the only elements in this group that you need to have an in depth knowledge of.



Properties of chlorine, bromine and iodine

• They exist as diatomic molecules, e.g Br₂
• Their colour darkens as you move down the group - chlorine is pale green, bromine is red-brown and iodine is purple-black
• They form molecular compounds with other non-metals, e.g. HCl
• As you move down the group they gradually change state, chlorine is a gas, bromine is a liquid and iodine is a solid.
• When they react with hydrogen they produce hydrogen halides, for example HCl,  and hydrogen halides produce acidic solutions when dissolved in water, for example HCl becomes hydrochloric acid
• When they react with metals they produce ionic metal halides, 2FeCl₃ (iron(III) chloride) is an ionic metal halide

Electronic structure and reactivity

The reactivity decreases as you go down the group, this is unlike what happens in Groups I and II, the order of reactivity is below:

Image courtesy of http://isbchem1.pbworks.com

When the halogens react, they gain an extra electron. This is because they have 7 electrons in their outer shell (we know this because they're in Group VII) so to fill their outer shell it's easiest to gain 1 electron, rather than loose 7. When that 1 extra electron, they then have the electron configuration of a noble gas.

The reason chlorine is more reactive than iodine is because chlorine is a smaller atom, so the electrons in the outer shell are held in closer to the nucleus. As you go down the group, the outer electron shell will get further away and will be held less securely.

Uses of fluorine, chlorine, bromine and iodine

Florine - used for toothpaste and in drinking water

Chlorine - kills bacteria in drinking water, an ingredient in PVC plastic and is in bleach

Bromine - used in medicines, fire retardants and disinfectants

Iodine - used in medicines, disinfectants and is also used as a photographic chemical

Group 1 - The Alkali Metals

What are the elements?

The elements in Group I are:

• Lithium
• Sodium
• Potassium
• Rubidium
• Caesium 
• Francium (radioactive)

For IGCSE you need to be aware of all of these metals, but you only need to know the details about the first three.

All of these metals have to be kept under oil, because they're very reactive, so they can't come into contact with air or water.

Properties of lithium, sodium and potassium:

• Soft metals
• Good conductors of electricity
• Low densities
• Low melting points
• When freshly cut by a knife, they have a shiny surface
• Burn in air, forming white oxides, the white oxides form alkaline solutions of the metal hydroxide when dissolved in water
• React vigorously with water, making an alkaline solution of the metal hydroxide and hydrogen gas
• React vigorously with halogens (e.g. chlorine) forming metal halides (e.g. sodium chloride)
• Potassium reacts the most with water, then sodium and lithium reacts the least with water

Gradual changes (like the one above) are called trends, trends allow chemists to predict things about elements that they have not yet observed.

The further down the group you go, the more reactive the metals are, so obviously francium is the most reactive. For more information on this look at the post on The Periodic Table 

When the Group I elements react, they loose an electron, becoming more stable because their electron configuration is now the same as that of a noble gas. 

The Periodic Table and Electronic Structure

History

The Periodic Table was invented in 1969 by the Russian scientist Dmitri Mendeleev (at that time, Professor of Chemistry at St. Petersburg University).

He arranged the 63 elements (all that were known at that time) by increasing atomic mass, leaving gaps where he predicted that elements, that were undiscovered at that time, would fit into the table.

Mendeleev arranged is so that elements that have similar properties are in the same groups.

This is the Periodic Table supplied by www.cie.org.uk
So this is probably the most reliable one you can find!

Key Terms


Groups = the vertical columns in the Periodic Table

Periods = the horizontal rows in the Periodic Table

Modern Periodic Table

There are currently 118 known elements!

Elements are arranged by proton number in the modern Periodic Table and elements with similar priorities are still in the same groups.


Groups:

There are eight groups of elements:

• Group I
• Group II
• Group III
• Group VI
• Group V
• Group VI
• Group VII
• Group 0 (sometimes known as Group VIII)

Group I is known as the alkali metals, Group II as the alkaline earth metals, Group VII as the halogens and Group 0 as the inert gases or noble gases.

The big block of elements between Groups II and III are the transition metals.

Periods:

The periods are numbered 1-7, going down the periodic table.

Across each period the properties of the elements change:

Courtesy of Bryan Earl and Doug Wilford's
Cambridge IGCSE Chemistry Third Edition

Metals and non-metals:

The bold line that starts beneath boron divides the Periodic Table into two parts.

The elements on the left of the line are metals and the elements on the right are non-metals.

Metalloids is the name given to the elements that are on this dividing line, they are metals that have properties of both metals and non-metals.

Electronic Structure and The Periodic Table

The group that an element is in determines how many electrons it has in it's outer shell.

N.B. This does not apply to the elements in Group 0, they have either 2 or 8 electrons

So the elements in Group I have 1 electron in their outer energy level and the elements in Group II have 2 electrons in their outer energy level, and so on and so on.

As you move down a group, the metallic characteristics of the elements increases, this happens because the outer energy shell becomes further away from the nucleus, as do the electrons in it. So there's less attraction between the outer energy shell's electrons and the nucleus, due to distance, so the electrons in the outer shell are easier to loose.
This doesn't happen in Group VII, in this group the reactivity DECREASES as you go down the group.