nitrogen

• The nitrogen cycle refers to the movement of nitrogen between the atmosphere, organisms, and the Earth. Nitrogen fixation is the conversion of nitrogen gas into ammonia or other nitrogen compounds that plants can use.
• As you move across a period in the Periodic Table from left to right, there are a number of trends in the properties of the elements.
• The most obvious physical trend in the elements is from metals on the left to non-metals on the right.
• Sodium, magnesium, and aluminium are metallic, shiny, conduct electricity, and react with dilute acids to give hydrogen and salts.
• Silicon is a semi-metal (or metalloid) that conducts electricity to some extent, a property that is useful in making semiconductor devices.
• Phosphorus, sulfur, and chlorine are typical non-metals that do not conduct electricity and have low melting and boiling points.
• The reactions of the elements in Period 3 are all redox reactions, since every element starts with an oxidation state of zero, and, after it has reacted, ends up with a positive or a negative oxidation state.
• Sodium and magnesium are the only metal elements in Period 3 that react with cold water.
• The reaction of sodium with water is vigorous - the sodium floats on the surface of the water and fizzes rapidly, melting because of the heat energy released by the reaction.
• A strongly alkaline solution of sodium hydroxide is formed (pH 13-14) after the reaction of sodium with water.
• The reaction of magnesium with water is very slow at room temperature, only a few bubbles of hydrogen are formed after some days.
• The resulting solution after the reaction of magnesium with water is less alkaline than in the case of sodium because magnesium hydroxide is only sparingly soluble (pH around 10).
• All the elements in Period 3 (except for argon) are relatively reactive and their oxides can be prepared by direct reaction of the element with oxygen.
• The reactions of the elements in Period 3 are exothermic.
• Sodium burns brightly in air to form white sodium oxide.
• Magnesium burns brightly in air with a bright white flame, producing white magnesium oxide.
• Aluminium burns brightly in air, producing aluminium oxide.
• Aluminium powder also burns brightly in oxygen, producing aluminium oxide.
• Aluminium appears to be an unreactive metal and is used for many everyday purposes such as saucepans, garage doors, window frames, and so on.
• Powdered aluminium is being sprinkled into the flame.
• The sum of the oxidation states in Al2O3 is zero, as it is in all compounds without a charge: (2x 3)+(3x -2)= 0.
• Information is sent through the nervous system as nerve impulses, which are electrical signals that pass along nerve cells known as neurones.
• Until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive.
• This is known as the refractory period.
• A bundle of neurones is known as a nerve.
• Neurones coordinate the activities of sensory receptors, decision-making centres in the central nervous system, and effectors such as muscles and glands.
• Neurones have a long fibre known as an axon.
• The axon of some neurones is insulated by a fatty sheath with small uninsulated sections along its length, called nodes of Ranvier.
• The sheath is made of myelin, a substance made by specialised cells known as Schwann cells.
• Myelin is made when Schwann cells wrap themselves around the axon along its length.
• The presence of Schwann cells means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next.
• The 'jumping' of the electrical impulse between nodes of Ranvier is called saltatory conduction.
• Saltatory conduction speeds up the conduction of the impulse and its transfer from one cell to another.
• Neurone cell bodies contain many extensions called dendrites, which means they can connect to many other neurones and receive impulses from them, forming a network for easy communication.
• There are three main types of neurone: sensory, relay and motor.
• Sensory neurones carry impulses from receptors to the CNS (brain or spinal cord).
• Relay (intermediate) neurones are found entirely within the CNS and connect sensory and motor neurones.
• Motor neurones carry impulses from the CNS to effectors (muscles or glands).
• Each type of neurone has a slightly different structure.
• Motor neurones have a large cell body at one end, that lies within the spinal cord or brain.