a simple model

• All substances are made of atoms. An atom is the smallest part of an element that can exist.
• Atoms of each element are represented by a chemical symbol
• There are about 100 different elements. Elements are shown in the periodic table.
• Compounds are formed from elements by chemical reactions.
• Compounds contain two or more elements chemically combined
• Compounds can only be separated into elements by chemical reactions.
• A mixture consists of two or more elements or compounds not chemically combined together.
• Mixtures can be separated by physical processes such as filtration, crystallisation, simple distillation, fractional distillation and chromatography.
• filtration: separates insoluble solids from liquids
• evaporation: water is heated and evaporates, leaving salt behind
• simple distillation: separates a liquid from a solution by boiling the solution and collecting the vapour
• paper chromatography is used to separate mixtures of soluble substances
• separation techniques:

  • chromatography
  • distillation
  • evaporation
  • filtration
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided.
• The discovery of the electron led to the plum pudding model of the atom.

  The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.
• The results from the alpha particle scattering experiment led to the conclusion that the mass of an atom was concentrated at the centre (nucleus) and that the nucleus was charged. This nuclear model replaced the plum pudding model
• Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances. The theoretical calculations of Bohr agreed with experimental observations.
• Later experiments led to the idea that the positive charge of any nucleus has smaller particles each particle having the same amount of positive charge. The name proton was given to these particles.
• The experimental work of James Chadwick provided the evidence to show the existence of neutrons within the nucleus. This was about 20 years after the nucleus became an accepted scientific idea.
• In the plum pudding model, there is a positive sphere with negative charge randomly placed within the sphere. There is no empty space.

  In the nuclear model, there is a central positive, tiny nucleus, with mostly empty space and lots of negative charges a long way from the nucleus.
• proton has a relative charge of +1
• neutron has a relative charge of 0
• electron has a relative charge of -1
• proton has a relative mass of 1
• neutron has a realtive mass of 1
• electron relative mass is very small
• Atoms are very small, having a radius of about 0.1 nm (1 x 10^-10m)
• The radius of a nucleus is less than 1/10 000 of that of the atom (about 1 x 10^-14 m).
• Almost all of the mass of an atom is in the nucleus.
• Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element.
• The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
• The elements in the periodic table are arranged in order of atomic (proton) number
• elements with similar properties are in columns, known as groups.
• Elements in the same group in the periodic table have the same number of electrons in their outer shell (outer electrons) and this gives them similar chemical properties.
• Before the discovery of protons, neutrons and electrons, scientists attempted to classify the elements by arranging them in order of their atomic weights.
• Mendeleev overcame some of the problems by leaving gaps for elements that he thought had not been discovered and in some places changed the order based on atomic weights.
• Elements with properties predicted by Mendeleev were discovered and filled the gaps. Knowledge of isotopes made it possible to explain why the order based on atomic weights was not always correct.
• Elements that react to form positive ions are metals
• Elements that do not form positive ions are non-metals.
• The majority of elements are metals. Metals are found to the left and towards the bottom of the periodic table