Science Notes Grade 7

Elements and Compounds (Notes)

 Over 2000 years ago a Greek philosopher called Democritus suggested that everything was made up of tiny pieces.
Democritus suggested that, if you could keep on cutting up a substance into smaller and smaller pieces, you would
eventually end up with a very small piece that could not be cut up any more. Democritus called his tiny pieces of
matter atoms. ‘Atom’ means ‘cannot be divided’.

 We now know that atoms really do exist. Today, we can even see some of the larger kinds of atoms, using very
special microscopes called scanning tunneling microscopes. The photograph shows the atoms in some carbon
nanotubes. (‘Nano’ means very, very small.)

 There are many different types of atoms. Scientists have discovered 94 different kinds of atoms that occur naturally
in the Universe. Another 24 kinds of atoms have been made in laboratories.

 Some substances are made up of just a single kind of atom. A substance that is made of just one kind of atom is
called an element.

 Carbon is made only of carbon atoms. Gold is made only of gold atoms. Silver is made only of silver atoms. Carbon,
gold and silver are examples of elements.

 Each type of atom has different properties. This is why different elements have different properties.
 Some substances are made up of individual atoms. For example, a piece of gold is made up of millions of individual

gold atoms. Neon – which is a gas – is made of individual neon atoms. Other substances are made up of little groups
of atoms. The atoms join together in groups of two or more.
 A group of atoms joined tightly together is called a molecule.
 Some elements are made up of molecules. For example, in oxygen the atoms are joined together in pairs. A
molecule of oxygen is made from two atoms of oxygen joined together.
 Sulfur molecules are made up of eight sulfur atoms joined together. They can be joined in slightly different ways.
 All substances are made of small particles called atoms.
 Elements are made of one type of atom.
 Molecules are particles made of two or more atoms joined together.
 We have seen that there are more than 100 different elements (including those that have been made in
laboratories). Chemists use a shorthand way of referring to them. They have given each element a symbol.
 Sometimes the symbol is the first letter of the English name of the element. For example, the symbol for oxygen is O
 Sometimes the symbol is the first letter of the English name of the element plus another letter from its name. For
example, the symbol for helium is He
 Sometimes the symbol is taken from the name of the element in another language. For example, the symbol for
sodium is Na, from the old Latin name ‘natrium’.
 The first letter of the symbol is always upper case. The second letter, if there is one, is always lower case.
 Some elements, such as copper and gold, have been known for thousands of years. Others, such as radium and
aluminium, have only been discovered more recently.
 Some elements are very abundant (common) on Earth, and some are rare. The pie chart shows the approximate
proportions of the various elements that are found in the
Earth’s crust.
 People believed that water was an element until about 1800. It
was only then that a scientist passed electricity through water
and found that it split up into two gases – hydrogen and
oxygen.
 Electricity can be used to split water into the elements
hydrogen and oxygen.
 Each element has its own chemical symbol.
 Some elements are very abundant on Earth, and other
elements are rare.

 Scientists have developed a very useful way of arranging the elements. This is called the Periodic Table.
 The full Periodic Table, containing all of the 118 known elements, is very large and complex! We will look at just the

first 20 elements.

 The Periodic Table is organised in rows and columns. The rows are called periods. The columns are called groups.
 The atoms are organised so that, as you read across each row (period) from left to right, the atoms increase in mass.

Hydrogen atoms have the smallest mass, then helium atoms, then lithium atoms.
 The Periodic Table is organised so that elements with similar properties are close together.
 The elements are arranged in the Periodic Table.
 The elements are arranged in order of the mass of their atoms.
 Metals are towards the left hand side of the table, and nonmetals on the right hand side.
 We have seen that some substances are elements. An element is made up of only one kind of atom.
 Many substances are made up of more than one kind of atom. If the different sorts of atoms are joined tightly

together, then the substance is a compound.
 The chemical term for two atoms joining tightly together is bonding. In a compound, two or more different kinds of

atoms are bonded. For example, when sodium atoms bond with chlorine atoms, they form the compound sodium
chloride.
 A compound is very different from the elements from which it is made. Once two different elements are bonded,
they completely lose the properties of the individual elements. The compound has totally new properties.
 The compound – sodium chloride – is not at all like either sodium or chlorine. You may have eaten some sodium
chloride today. Sodium chloride is common salt. You would not want to eat any sodium or chlorine, though.
 Each compound has a chemical name. The chemical name usually tells us the elements that the compound is made
from.
 There are some important rules to remember when naming compounds.
 If the compound contains a metal, then the name of the metal comes first in the name of the compound.
 If the compound contains a non-metal, the name of the nonmetal is usually changed. For example, the compound
made from sodium and chlorine is not sodium chlorine, but sodium chloride.
 When two elements form a compound the name often ends in ‘ide’.
 Some compounds contain two different elements, plus a third element – oxygen. These compounds often have
names ending with ‘ate’. For example, a compound of calcium, carbon and oxygen is called calcium carbonate.
 Copper sulfate is a compound made up of copper, sulfur and oxygen.
 Sometimes, the name of a compound tells us how many of each kind of atom are bonded together.
 Carbon dioxide is made up of molecules in which one carbon atom is joined to two oxygen atoms. ‘Di’ means ‘two’.
 Carbon monoxide is made up of molecules in which one carbon atom is joined to one oxygen atom. ‘Mon’ or ‘mono’
means ‘one’.
 A compound is formed when atoms of two or more elements bond together.

 A compound has completely different properties from the elements from which it is made.
 It is easy to decide if a substance is a compound by looking at a particle diagram or model. If there are different

kinds of atoms bonded together, then it is a compound.

 Carbon dioxide, water and methane are all compounds because their molecules are made up of different kinds of
atoms. Oxygen is an element because the atoms in the molecule are both oxygen atoms.

 Every compound has a chemical name. For example, the compound of sodium and chlorine is called sodium
chloride.

 Some compounds also have everyday names. For example, sodium chloride is also known as common salt. Every
compound also has a formula. The formula contains the symbols of the elements that are bonded together in the
compound.

 Be very careful reading the symbols of the elements. For example, you do not want to confuse the symbol for
carbon, C, with the symbol for calcium, Ca.

 The little number written below and to the right of some symbols tells you how many atoms of each element are
found in one molecule of the compound. If there is no number, that means there is just one atom of that element.

 There is one more kind of compound that you need to know about. These are the hydroxides.
 You may remember learning about sodium hydroxide and potassium hydroxide when you studied acids and alkalis.

Hydroxides are alkalis. Sodium hydroxide and potassium hydroxide are both strong alkalis. When they dissolve in
water, they form solutions that can neutralise acids.
 The formula for sodium hydroxide is NaOH. The formula for potassium hydroxide is KOH.
 Each compound has its own chemical formula.
 The formula tells you which elements the compound contains and how many atoms of each element are combined
together.