The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

The table shows the properties of these three sub-atomic particles:

ParticleRelative massRelative charge Proton 1 +1 Neutron 1 0 Electron Almost zero –1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:

• if an atom loses one or more electrons, it becomes a positively charged ion
• if an atom gains one or more electrons, it becomes a negatively charged ion

• Atoms contain three sub-atomic particles called protons, neutrons and electrons

The Nuclear Model- this is a structure of a atom

• Limestone is mainly calcium carbonate, CaCO₃, which when heated breaks down to form calcium oxide and carbon dioxide. Calcium oxide reacts with water to produce calcium hydroxide. Limestone and its products have many uses, including being used to make cement, mortar and concrete.

• Thermal decomposition

  Calcium carbonate breaks down when heated strongly. This reaction is called thermal decomposition. Here are the equations for the thermal decomposition of calcium carbonate:

  calcium carbonatecalcium oxide + carbon dioxide

  CaCO₃CaO + CO₂

  Other metal carbonates decompose in the same way, including:

  • sodium carbonate
  • magnesium carbonate
  • copper carbonate

• An Antibody is a dead or an inactive pathogen

Immunity

Once inside the body, pathogens reproduce. Viruses reproduce inside cells and damage them, while escaping to infect more cells. Bacteria produce toxins - poisons. Cell damage and toxins cause the symptoms of infectious diseases.

Once pathogens enter the body, the immune system destroys them. White blood cells are important components of the immune system.

White blood cells

White blood cells can:

• engulf pathogens and destroy them
• produce antibodies to destroy pathogens
• produce antitoxins that neutralise the toxins released by pathogens

Pathogens contain certain chemicals that are foreign to the body, called antigens. White blood cells - lymphocytes - carry antibodies - proteins that have a chemical 'fit' to a certain antigen. When a white blood cell with the appropriate antibody meets the antigen, it reproduces quickly and makes many copies of the antibody that neutralises the pathogen.

• Pathogens are microorganisms that cause disease. The body has several defence mechanisms to prevent pathogens from entering the body and reproducing there. The immune system can destroy pathogens that manage to enter the body. New medical treatments and drugs must be tested before their use.
•  pathigens are organisms that cause disease. They include microorganisms such as bacteria, viruses, fungi and protozoa.

• Bacteria are microscopic organisms that come in many shapes and sizes. But even the largest ones are only 10 micrometres long - 1 micrometre = 1 millionth of a metre.Bacteria cause diseases such as cholera.

• Viruses are many times smaller than bacteria. They consist of a fragment of genetic material inside a protective protein coat.Viruses cause diseases such as influenza - flu.

• Once you have been infected with a particular pathogen and produced antibodies against it, some of the white blood cells remain. If you become infected again with the same pathogen, these white blood cells reproduce very rapidly and the pathogen is destroyed. This is active immunity. Sometimes you may be treated for infection by an injection of certain antibodies from someone else. This is passive immunity.

Sankey diagrams

• Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.

• The energy transfer to light energy is the useful transfer. The rest is ‘wasted’ - it is eventually transferred to the surroundings, making them warmer. This ‘wasted’ energy eventually becomes so spread out that it becomes less useful.

• Heat can be transferred from place to place by conduction, convection and radiation. Dark matt surfaces are better at absorbing heat energy than light shiny surfaces. Heat energy can be lost from homes in many different ways and there are ways of reducing these heat losses.
• There are several different types of energy, and these can be transferred from one type to another. Energy transfer diagrams show the energy transfers in a process. More efficient devices transfer the energy supplied to them into a greater proportion of useful energy than less efficient devices do