Atoms and elements

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Everything is made from atoms, including you. Atoms are tiny particles that are far too small to see, even with a
microscope. If people were the same size as atoms, the entire population of the world would fit into a box about a
thousandth of a millimeter across!

                                   We usually                    We often draw
                                   imagine atoms                 atoms as
                                   as being like                 circles.
                                   tiny balls.

There are over a hundred different types of atom. A substance that contains just one type of atom is called an element.
Because there are over a hundred different types of atoms, there are over a hundred different elements.

Lead and gold are elements. They only contain lead or gold atoms. You cannot change one element into another
element, or anything simpler, using chemical reactions. This is why any attempts to turn lead into gold are doomed to
        Most Common Elements in Earth’s Rocks
     Element          Symbol          Number of Protons            An element is a substance made from one type
Oxygen                   O                     8                      of atom. It cannot be changed into another
Silicon                  Si                    14                     element or anything simpler using chemical
Aluminum                 Al                    13                     reactions.
Iron                    Fe                     26                  An atom is the smallest particle of an element
Calcium                 Ca                     20                     you can get.
Sodium                  Na                     11
                                                               We will be looking more closely at rocks and minerals
Potassium                K                     19
                                                               next months!
Magnesium               Mg                     12
Titanium                 Ti                    22
Hydrogen                 H                     1

Chemical symbols
Each element is given its own chemical symbol. This is
usually one or two letters long, but sometimes three letters
are used. Every chemical symbol starts with a capital letter,
with the second or third letters written in lower case. For
example, Mg is the correct symbol for magnesium, but mg,
mG and MG are wrong.

Take care to write chemical symbols correctly.

Sometimes it is easy to tell which element a symbol stands for. For example, O stands for oxygen and Li stands for
lithium. Sometimes it is not easy to tell which element a symbol stands for. This is because the symbol comes from a
name for the element that is not an English word. For example, W stands for tungsten (from the word wolfram) and Na
stands for sodium (from the word natrium). However, the same chemical symbols are used all over the world, no
matter which language is spoken.

The periodic table
All the different elements are arranged in a chart called the
periodic table.
     They are arranged so that similar elements are found in vertical columns, called groups.
     The horizontal rows are called periods.
     The metals are on the left and the non-metals are on the right.
     One non-metal, hydrogen, is often put in the middle.

                                                                        Only elements are found in the periodic table.
Atoms and molecules
Remember that an element is a substance made from only one type of atom. The atoms of some elements do not join
up with each other. The element Helium, an unreactive gas used in party balloons, is like this (A). The atoms of most
other elements do join up with each other. They form molecules. Molecules consist of two or more atoms joined
together. Some molecules consist of pairs of atoms. Hydrogen and oxygen are like this (B). Some molecules consist
of lots of atoms joined together. Sulphur is like this (C).

               A. Helium                                       B.                                 C. A sulphur
               atoms do                                        Hydrogen                           molecule
               not join up                                     and oxygen                         contains eight
               with each                                       atoms pair                         atoms joined
               other.                                          up to form                         together.

The atoms of the different elements can join together in chemical reactions to form compounds. For example,
hydrogen and oxygen are elements. They react together to form water, a compound. There are countless different
ways for the elements to join together, and millions of compounds are known.

Properties of compounds
The properties of compounds are usually very different from the properties of the elements they contain. For example,
hydrogen and oxygen are both gases at room temperature, but water is a liquid.

The table compares the properties of iron and sulphur (the two elements), and iron sulphide (the compound).

The atoms in a compound are chemically joined together by strong forces called bonds. You can only separate the
elements in a compound using another chemical reaction. Separation methods like filtering and distilling or using
tweezers will not do this.

Compounds form when atoms join together in new ways in chemical reactions.

                                                                  Remember that a compound is a substance made
                                                                  up from two or more elements, chemically joined
                                                                  together. This means that compounds will always
                                                                  exist as molecules, not separate atoms. The
diagrams show some molecules of common compounds.

Molecules of three common compounds.

A mixture is made from different substances that are not chemically joined together - imagine M&Ms which can be
mixed together in a packet, but which are not chemically joined to each other.

The different substances in a mixture can be separated from each other without needing a chemical reaction, in the
way that different M&Ms can be picked out and put into separate piles.

A packet of sweets contains a mixture of different colors

Mixture and compounds
Mixtures have different properties from compounds. The table summarizes these differences.

An example
Remember that iron and sulphur react together when
they are heated to make a compound called iron
sulphide. What are the differences between a mixture of
iron and sulphur, and iron sulphide? Here are some of
    The mixture can contain more or less iron, but
       iron sulphide always contains equal amounts of
       iron and sulphur.
    The iron and sulphur atoms are not joined
       together in the mixture, but they are joined
       together in iron sulphide.
    The iron and sulphur still behave like iron and
       sulphur in the mixture, but iron sulphide has
       different properties from the iron and sulphur it
    You can separate the iron from the mixture using
       a magnet but this does not work for iron

Homogeneous Mixture
Solutions are homogeneous mixtures or types of mixtures that appear to be a single substance.
Homogeneous means the same throughout. The particles in this type of mixture are evenly distributed.
Salt water is a solution. Soda is a solution. Steel is also a solution! Steel (a mixture of carbon and iron)
and brass (a mixture of zinc and copper) are called alloys or solid solutions. Solutions are always

                                      Heterogeneous Mixture
                                      These mixtures have larger parts that are different from one another and
                                      are unevenly mixed. Sand is an example. A toy box full of toys is also
an example. There are two types of special heterogeneous mixtures called suspensions and colloids.
Suspensions are mixtures that have large particles. Think “Snow Globe!” The particles will settle out and
collect. You can filter out the particles in a suspension. Colloids have tiny particles that stay dispersed in
the liquid. Milk is a colloid; so is mayonnaise and whipped cream! You can not filter a colloid.

Can you recognize elements, compounds and mixtures?

  * An element contains just one type of atom.
  * A compound contains two or more types of atom joined together.
  * A mixture contains two or more different substances that are not joined together.
  * The different substances in a mixture can be elements or compounds.

The table shows some examples.

Using the periodic table (in your planner), write the chemical symbol for the following elements.
      1. ______ iron                                        4. ______ oxygen
      2. ______ carbon                                      5. ______ copper
      3. ______ hydrogen                                    6. ______ silver

  Using the periodic table, write the name of the element from the chemical symbol.
     7. ______ He                                         10. ______ Na
     8. ______ N                                          11. ______ F
     9. ______ Mg                                         12. ______ Li

  Name the elements contained in the following compounds.
    13. ammonia (NH3)                ____________, ____________
    14. ethanol (C2H6O)              ____________, ____________, ____________
    15. sodium chloride (NaCl)       ____________, ____________

  Classify each of these samples of matter as an element (E), a compound (C), a solution (S), a
  heterogeneous mixture (H), or a colloid (D).
     16. ______ Chlorine
     17. ______ H2O (water)
     18. ______ Dirt
     19. ______ Sugar water
     20. ______ Milk
     21. ______ Vegetable soup
     22. ______ Oxygen
     23. ______ Mayonnaise
     24. ______ NaHCO3 (Baking Soda)
     25. ______ NaCl (Salt) and water

  Label each model as a compound, element, or mixture.

  26._________         27._________       28._________           29._________

  30. Describe the difference between a mixture and a pure substance.

  31. What is the difference between a colloid and a suspension?

32. What is the difference between an element and a compound?

33. Is there any way to separate a homogeneous mixture? As an example, describe whether it’s
possible to separate salt water. If it is possible, explain how. If it’s not possible, explain why not.

34. Why is it easier to separate a heterogeneous mixture than a homogeneous mixture? Explain.

Define the following:
   35. Alloy
   36. Atom
   37. Colloid
   38. Compound
   39. Element
   40. Heterogeneous mixture
   41. Homogeneous mixture
   42. Matter
   43. Mixture
   44. Solution