Page 4 - Curriculum Visions Dynamic Book

Introduction
An element is a substance that cannot be broken down into simpler substances by any known means.There are at least 118 elements, of which 92 occur in stable, natural forms on Earth, and 26 can be made in the laboratory and may possibly occur naturally elsewhere in the universe. These elements are the fundamental materials from which everything in the universe is made. Most elements are metals, but 16 are described as non-metals and 9 as metalloids.
Atoms
Elements are composed of atoms.A piece of sulphur is made entirely of sulphur atoms, and a lump of iron is made entirely of iron atoms.When
in a pure state, an element is described as elemental.
Elements can be combined in a chemical reaction to form compounds, or they can be blended to form mixtures. There are millions of possible compounds that can be made.
Each element is represented by a symbol, such as S for sulphur, Fe for iron, or Ca
for calcium, and so on.
Atoms are made up of three kinds of particles. In the centre, or nucleus, there are two kinds, protons and neutrons. Surrounding the nucleus are electrons (often visualised as orbiting, much like planets around the Sun).
The number of protons in each element is unique and is called its atomic number.
An atom of iron, for example, has 26 protons in its nucleus, and so its atomic number is 26.The number of protons, in turn, is equal to the number of electrons surrounding the nucleus. Iron has 26 electrons.
Once the atomic number is known, all of the elements can be organised in order of increasing atomic number to make the Periodic Table (see pages 6-7).
The electrons that are present in an atom are racing around the nucleus. Each electron can be thought of as
moving in a path that is a fixed distance from the nucleus.
The chemical properties of an element depend on the number of electrons in the outermost region – called a shell – of the atom.
Scientists can draw the pattern of the electrons in what is called a shell diagram. The diagrams can be shown as a series of rings.
Scientists know that the innermost shell is very stable when it holds two electrons, the second shell is stable when it contains eight, the third shell
is stable as an outer shell when it has eight, and so on.The shell diagrams for all of the elements are shown on the pages of this set of books.
The number of protons is always equal to the number of electrons in an atom.
Abundance of the elements
Element % of Universe
Hydrogen 87
Helium 12 Oxygen 0.06 Carbon 0.03 Neon 0.02 Nitrogen 0.008 Silicon 0.003 Iron 0.002 Sulphur 0.002 Argon 0.0004 Magnesium 0.0003 Aluminium 0.0002 Calcium 0.0001 Sodium 0.0001 Phosphorus 0.00003 Potassium 0.000007
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