Fundamental ideas in chemistry

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Elements on the Periodic Table

Metals and non-metals

The metals are shown on the left of the periodic table, and the non-metals are shown on the right. The dividing line between metals and non-metals is shown in red on the table below. You can see that most of the elements are metals.

Chemical symbols

The atoms of each element are represented by a chemical symbol. This usually consists of one or two different letters, but sometimes three letters are used for newly discovered elements. For example, O represents an oxygen atom, and Na represents a sodium atom.

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Atoms consist of electrons surrounding a nucleus that contains protons and neutrons.

Neutrons are neutral, but protons and electrons are electrically charged: protons have a relative charge of +1 and electrons have a relative charge of -1.

Atomic structure

All substances are made from tiny particles called atoms. An atom has a small central nucleus made up of smaller sub-atomic particles called protons and neutrons. The nucleus is surrounded by even smaller sub-atomic particles called electrons.

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Atomic Structure

the proton and neutron are within the nucleus which is within the centre of the atom, the elctrons are on the edges of the atom (

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Periodic Table

The periodic table, with non-metals on the left and metals on the right (

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Atomic number and mass number

Atomic number and mass number

The atomic number of an atom is the number of protons it contains. All the atoms of a particular element have the same atomic number (number of protons). The atoms of different elements have different numbers of protons. For example, all oxygen atoms have 8 protons and all sodium atoms have 11 protons.

The mass number of an atom is the total number of protons and neutrons it contains. The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger.

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Full chemical symbols

You need to be able to calculate the number of each sub-atomic particle in an atom if you are given its atomic number and its mass number. The full chemical symbol for an element shows its mass number at the top, and its atomic number at the bottom.

Cl 35, 17 (

This symbol tells you that the chlorine atom has 17 protons. It will also have 17 electrons, because the number of protons and electrons in an atom is the same.

The symbol also tells you that the total number of protons and neutrons in the chlorine atom is 35. Note that you can work out the number of neutrons from the mass number and atomic number.

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Electronic Structure

Electronic structure

The electrons in an atom occupy energy levels. These are also called shells. Each electron in an atom is found in a particular energy level. The lowest energy level (innermost shell) fills with electrons first. Each energy level can only hold a certain number of electrons before it becomes full. The first energy level can hold a maximum of two electrons, the second energy level a maximum of eight, and so on.

The electronic structure of an atom is written using numbers to represent the electrons in each energy level. For example, for sodium this is 2,8,1

Energy level or shell                        Maximum number of electrons first 2 second 8 third 8

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Working out an element's electronic structure

Working out an element's electronic structure

Here is how to use the periodic table to work out an electronic structure:

  1. Find the element in the periodic table. Work out which period (row) it is in, and draw that number of circles around the nucleus.
  2. Work out which group the element is in and draw that number of electrons in the outer circle – with eight for Group 0 elements – except helium.
  3. Fill the other circles with as many electrons as needed. Remember – two in the first circle, and eight in the second and third circles.
  4. Finally, check that the number of electrons is the same as the atomic number.
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Periodic Table (in depth)

Elements in the same group in the periodic table have similar chemical properties. This is because their atoms have the same number of electrons in the highest occupied energy level. Group 1 elements are reactive metals called the alkali metals. Group 0 elements are unreactive non-metals called the noble gases.


A vertical column of elements in the periodic table is a group. The elements in a group have similar chemical properties to each other. For example, group 1 contains sodium and other very reactive metals, while group 7 contains chlorine and other very reactive non-metals. Group 0 (also known as group 8 or group 18) contains helium and other very unreactive non-metals.

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Image of the Periodic Table

Group 1 - alkali metals, group 7 - halogens, group 0 - noble gases. Transition metals are between group 2 and 3.  (

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Reactions of group 1 elements with water

Reactions of group 1 elements with water

Potassium reacts with a lilac flame

Lithium, sodium and potassium all react vigorously with water to form a metal hydroxide and hydrogen:

metal + water → metal hydroxide + hydrogen

The metal hydroxides are strong alkalis.

The group 1 elements need to be stored under oil to prevent them reacting with oxygen and water vapour in the air.

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Reactions of group 1 elements with oxygen

Reactions of group 1 elements with oxygen

Lithium, sodium and potassium are easily cut with a blade. The freshly cut surfaces are silvery and shiny, but quickly turn dull as the metal reacts with oxygen in the air.

The group 1 metals react vigorously with oxygen to form metal oxides.

Lithium burns with a red flame, sodium with a yellow-orange flame, and potassium burns with a lilac flame.

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Chemical Reactions

When elements react, their atoms join with other atoms to form compounds. Chemical bonds form when this happens, which involves atoms transferring or sharing electrons.

Reactions and compounds

New substances are formed by chemical reactions.

When elements react together to form compounds their atoms join to other atoms using chemical bonds. For example, iron and sulfur react together to form a compound called iron sulfide.

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Ionic bonds

Chemical bonds involve electrons from the reacting atoms. Compounds formed from metals and non-metals consist of ions. Ions are charged particles that form when atoms (or clusters of atoms) lose or gain electrons:

  • metal atoms lose electrons to form positively charged ions
  • non-metal atoms gain electrons to form negatively charged ions

The ionic bond is the force of attraction between the oppositely charged ions. This animation shows how ions form when sodium atoms react with chlorine atoms to form sodium chloride.

Covalent bonds

Compounds formed from non-metals consist of molecules. The atoms in a molecule are joined together by covalent bonds. These bonds form when atoms share pairs of electrons.

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Chemical Reactions

The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound. For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the 2 is written as a subscript, so Na2O would be wrong.

Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3:

  • a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
  • a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H
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Copper and oxygen reaction: getting a balanced equ

Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.

copper + oxygen → copper oxide

Copper and oxygen are the reactants because they are on the left of the arrow. Copper oxide is the product because it is on the right of the arrow.

Cu + O2 → CuO

There are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.

Here is the balanced symbol equation:

2Cu + O2 → 2CuO

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