Summary Sheet

Summary Sheet

Elements

Elements are the building blocks of all substances. They cannot be broken down into simpler substances by chemical means as all their atoms are identical.

Atoms

The reason that the atoms in an element are identical is that they all contain the same number of protons. Protons are positive sub-atomic particles that exist in the nucleus of an atom. The ATOMIC NUMBER for an atom tells us how many protons it contains.

Atoms also contain electrons. These are negative particles that revolve around the nucleus of an atom in orbits, shells or energy levels. See sodium. The number of electrons is always the same as the protons in a neutral atom.

Mendeleev and the Periodic Table

A scientist called Dmitri Mendeleev classified 63 known elements into the PERIODIC TABLE according to their PHYSICAL and CHEMICAL PROPERTIES and in order of their MASS NUMBERS. The mass number for an atom is quite obviously the mass of that atom. Mendeleev predicted that elements would be discovered if he couldn’t find one that would fit into one of his patterns.

The Modern Periodic Table

Nowadays elements are still classified using the periodic table. Elements are still in groups according to their physical and chemical properties but are now in order of their ATOMIC NUMBER.

Groups

It is now known that elements in the same group with similar physical and chemical properties have the SAME NUMBER OF ELECTRONS IN THEIR OUTER SHELLS. This also ties in with the group that they are in. E.g, Elements in group I such as sodium (2:8:1) and potassium (2:8:8:1) are in there as they both have 1 electron in tier outer shell. This in turn is the reason why they are both metals with low melting and boiling points (physical properties) and react rapidly with water (chemical property).

Periods

As you move across a period in the periodic table an outer shell is being filled. Once the outer shell has been filled a new period begins. The first period only contains 2 elements hydrogen and helium as the littlest inner shell only is being filled.

Patterns within groups

Elements within groups have similar physical and chemical properties. Also, as you move down a group trends can be noted. For example, in group 1 the reactivity increases down the group, the melting and boiling points decrease and density increases.

In group 7 the reactivity decreases down the group, the melting and boiling points increase and the density increases.

End of Summary Sheet