Understanding the structure of the atom is the basis of all chemistry. It is important to recall the history of the atom, understand the structure of the atom and to understand the concept of isotopes.
History of the atom
Throughout time, there have been various proposed models for the atom. Today, we use the Bohr model.
Discovery
Model
John Dalton, 1803
Dalton proposed that all matter is made up of atoms. Dalton's model considered atoms to be a solid spherical model, lacking any subatomic particles.
JJ Thomson, 1904
JJ Thomson discovered the electron. This lead to the idea of the 'plum pudding' model, proposing the atom to have a net positive charge with negative electrons trapped within.
Niels Bohr, 1913
Ideas from Ernest Rutherford, Neils Bohr and James Chadwick all contribute to the current model of the atom. The 'Bohr model', a neutral atom with electrons found in shells, orbiting a central positive nucleus (of protons and neutrons).
Atomic structure
Atoms consist of protons, neutrons and electrons. Protons and neutrons are located in the nucleus, which is responsible for most of the mass of an atom. Electrons are located in orbitals, which are responsible for most of the volume of an atom.
Subatomic particle
RELATIVE CHARGE
RELATIVE MASS
Proton
+1
1
Neutron
0
1
Electron
−1
18361
Note: The relative mass of an electron is also readily seen as 0.0005 which is equivalent to 18361
Atomic number and mass number
Any element from The Periodic Table can be written symbolically with their atomic number and mass number.
ZAE
E
An element in The Periodic Table.
Z
The atomic number - the number of protons in the nucleus of the atom.
A
The mass number - the total number of protons and neutron in the nucleus of the atom.
Isotopes
Isotopes are atoms of the same element with the same number of protons but different number of neutrons. Isotopes therefore have the same atomic numbers but different mass numbers.
Example
Cl−35andCl−37are an exemplar pair of isotopes.
Subatomic particles
The number of each subatomic particle can be calculated using the atomic and mass numbers.
procedure
1.
Use the atomic number to find the number of protons. This is also equivalent to the number of electrons.
2.
Use the mass number to find the number of neutrons. This is calculated by subtracting the atomic number from the mass number.
Example
Find the number of protons, neutrons and electrons in the isotopes of chlorine−35 and chlorine−37.
isotope
symbol
protons
electrons
neutrons
Cl−35
1735Cl
17
17
35−17=18
Cl−37
1737Cl
17
17
37−17=20
Note: The physical properties of isotopes may differ. Physical properties depend on the mass of the atom and since isotopes have different numbers of neutrons, the mass differs.
