The Structure of the Atom

The Structure of the Atom

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Basic Atomic Particles :

Atoms are made up of the following particles :

Protons : Protons are positively charged particles with a mass of one

atomic mass unit. They are found in the nucleus at the centre of the

atom structure.

Neutrons : Neutrons are not charged and have a mass of one atomic mass

unit. They are found, with protons at the centre of the atom.

Electrons : Electrons are negatively charged particles with a mass of

1/1846th of an atomic mass unit. They are arranged in shells around

the central nucleus.

Useful definitions for atomic structure :

Atomic number (Z) : The number of protons in the nucleus of an atom

but is also equal to the number of electrons in the same atom (because

atoms don't carry an overall charge - the positives balance the

negatives!).

Mass Number, (A) (or atomic mass) : The number of neutrons and protons

in the nucleus of one atom of the element

The atomic number and mass number of a particular element can be found

from the periodic table. The atomic number is always displayed to the

bottom left of the symbol for the element. The mass number is always

displayed to the top left of the symbol for the element.

This symbol shows that sodium atoms have 11 electrons orbiting a

nucleus that contains 11 protons and (23 - 11=) 12 neutrons.

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E.G.

Sodium has an atomic number of 11 (and so each sodium atom contains 11

electrons and 11 protons).

The structure above represents the G.C.S.E. model of the atomic

structure of sodium. Electrons orbit a central nucleus in well defined

electron shells.

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Simple Diagram of an Atom : The Sodium atom.

The first electron shell (closest to the nucleus) can contain at most

2 electrons.

The second electron shell can contain at most 8 electrons.

The third electron shell can contain, at most, 18 electrons.