Download Element A pure substance made of only one type of atom which

Element
Atom
Molecule
Compound
Mixture
Solution
Solute
Solvent
Saturated solution
A pure substance made of only one type of atom which cannot be broken
down into smaller or simpler substances by chemical means (examples: O, He,
Fe, Ag)
The basic unit of a chemical element.
A molecule consists of two or more atoms chemically bound together (it can
be two or more atoms of the same element, or two different elements).
Examples: H2O, CO2
A molecule that consists of two different elements
A combination of two or more substances that are not chemically bound, i.e.
they can be easily separated by physical means (such as boiling, filtering, etc.)
A liquid mixture of two or more substances (which may be gases, solids or
liquids).
The substance that is dissolved in a solution
The liquid of a solution, in which the solute is dissolved.
A solution that has as much solid dissolved in it as it can possibly contain.
DIFFERENCE BETWEEN PHYSICAL AND CHEMICAL CHANGES
PHYSICAL
Usually reversible (a change in form, shape or
state)
No new substance is formed
CHEMICAL
Usually non-reversible
One or more new substances are formed
Atoms consist of electrons that surround a nucleus. The nucleus contains protons and neutrons.
NEUtrons are NEUtral (have a neutral charge)
PROtons are POsitively charged (have a charge of +1)
Electrons are negatively charged (have a charge of -1)
The mass number (aka nucleon number) is the sum of the
protons and neutrons:
Mass number = no. of protons + no. of neutrons
The atomic number is determined by the number of protons:
Atomic number = no. of protons.
In a neutral atom, the number of electrons is the same as the
number of protons:
no. of protons = no. of electrons
Atomic number = no. of electrons
Therefore, lithium has:
 a mass number of 7.
 three protons
 three elecrons
 four neutrons
The protons and neutrons are much heavier (over 2000 times) than the electron, which weighs
almost nothing. The protons and neutrons are located in the nucleus of the atom; the electrons
orbit around the nucleus.
proton number = the number of protons an atom has
nucleon number = another term for atomic mass/mass number
The modern periodic table
is a chart containing the
elements arranged in order
(from left to right) of
increasing atomic number
(aka proton number). A
vertical column, called
a group, contains elements
with similar properties. A
horizontal row is called
a period.
Atoms are orbited by electrons in shells. The first shell, which is closest to the nucleus, can hold
two electrons. Each shell thereafter can hold up to eight electrons. Electrons in the outer shell
of an atom (i.e. electrons that can bond with other atoms) are called valency electrons.
THE FOLLOWING INFORMATION IS TRUE ONLY FOR THE FIRST 20 ELEMENTS IN THE PERIODIC
TABLE (I.E. UNTIL CALCIUM):
The group (column) that an atom is in tells us how many electrons the atom has in its
outermost shell. The period (row) that an atom is in tells us how many electronic shells the
atom has.
Neutral atoms have the same number of electrons as protons; therefore, the proton number
also tells us how many electrons the atom has altogether. For example:
 the proton number/atomic number of sulphur is 16. Therefore, we know that it has
sixteen electrons altogether.
 The innermost shell can only hold two electrons. This leaves fourteen electrons.
 The next shell can hold eight electrons. This leaves six electrons.
 The last shell can also hold eight, but holds six as there are only six left.
 There are three shells altogether (which is why sulphur is in period 3).
 There are six electrons in the outermost shell (which is why sulphur is in group VI).
Atoms are always trying to end up with a full outer shell, through either gaining or getting rid of
electrons. The elements in group I are very reactive, as they only have to get rid of one electron
to have a full outer shell (if they get rid of all the electrons on a shell, the shell disappears). The
elements in group 7 are also reactive, as they only need to gain one electron to have a full outer
shell. Elements in group 8 (or group 0), however, are inert (=non-reactive), because they already
have a full outer shell, and can’t react with any other elements.