GCSE Chemistry "Atomic Theory, Bonding and Structure"
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Course
Chemistry
Institution
Well detailed notes covering all of the Atomic Theory, Ionic and covalent bonding (with dot and cross diagrams) as required for the GCSE/IGCSE Chemistry exam.
ATOMIC THEORY & STRUCTURE
Elements are made up of tiny particles called atoms.
Each atom is made up of subatomic particles called protons, neutrons and electrons.
The modern theory of atomic structure is known as the nuclear model. This model suggests
that the atom consist of:
A very small and extremely dense region called the nucleus.
A cloud of negative-electrically charged particles around the nucleus.
The nucleus is very small when compared to the rest of the whole atom. This model also
suggests that the atom consists of three smaller subatomic particles which are protons,
neutrons and electrons.
These particles are extremely small and light, such that their mass cannot be
measured in conventional units such as kilograms or grams.
Atomic mass units (a.m.u) are used instead.
Protons, neutrons and electrons
Protons are found in the nucleus of every atom and carry a positive charge (i.e their charge
is +1). The mass of each proton is taken as 1 a.m.u.
Neutrons are also found in the nucleus of atoms (except that of hydrogen). They are
uncharged (i.ie their charge us 0) and have approximately the same mass of protons.
Unlike proton and neutrons which are found in the nucleus of every atom, electrons are found
orbiting the nucleus in orbital paths called shells. Each electron carries a unit of negative
charge (i.e has a charge of -1), and has a mass of about 1/1837 of a proton. It is therefore
taken to have a mass of 0 or ‘negligible’.
Structure of an atom
K.G Page 1 of 28
,The table below summarizes the properties of subatomic particles.
Subatomic particles Relative mass Charge
Proton 1 +1
Neutron 1 0 (neutral)
Electron Negligible – can be considered 0 -1
Atomic number and mass number
The nuclear symbol of an atom tells you its atomic (proton) number and its atomic mass.
The atomic number (or proton number) is the number of protons in the nucleus of an atom.
The symbol for this number is Z. It is also the number of electrons present in an atom and
determines the position of the element on the Periodic Table.
ATOMIC NUMBER = NUMBER OF PROTONS = NUMBER OF ELECTRONS
The atomic mass (mass number) of an atom is the number of protons and neutrons in the
nucleus. The symbol for this number is A.
Therefore, the atomic mass minus the atomic number gives the number of neutrons of an
atom.
ATOMIC MASS = NUMBER OF PROTONS (ATOMIC NUMBER) + NUMBER OF NEUTRONS
NUMBER OF NEUTRONS = ATOMIC MASS – ATOMIC NUMBER
In the Periodic Table, the atomic number is presented at the bottom of an element, while the
atomic mass is presented on top of an element; as shown below.
Diagram showing the notations used on the Periodic Table
K.G Page 2 of 28
, For example, consider the following element:
Sodium has an atomic mass of 23 and an atomic number of 11.
With this, we can conclude that sodium has:
11 protons
11 electrons
12 neutrons (23-11)
Isotopes
Whereas all the atoms of a particular element contain the same number of protons, the
number of neutrons may vary; i.e. atoms of one element have the same Z but may have a
different A. These particular types of atoms are called isotopes. All elements have isotopes.
Isotopes are atoms of the same element that contain the same number of protons and
electrons but a different number of neutrons.
Therefore, it can be said that isotopes have the same atomic number but different atomic
mass.
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the
mass number.
A very popular example of isotopes are those of hydrogen; hydrogen-1, hydrogen-2 and
hydrogen-3.
Coincidentally, these isotopes have their own name; protium, deuterium and tritium
respectively.
K.G Page 3 of 28
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