The atom: history, structure and isotopes

In a nutshell

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.

Chemistry; Atomic structure and the periodic table; KS5 Year 12; The atom: history, structure and isotopes

Subatomic particle	RELATIVE CHARGE	RELATIVE MASS
Proton	$+1$	$1$
Neutron	$0$	$1$
Electron	$-1$	$\frac{1}{1836}$

Note: The relative mass of an electron is also readily seen as $0.0005$ which is equivalent to $\frac{1}{1836}$ 

Atomic number and mass number

Any element from The Periodic Table can be written symbolically with their atomic number and mass number.

^A_Z \large E

$E$	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

$Cl-35$ and $Cl-37$ are 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
$Cl-35$	$^{35}_{17} Cl$	$17$	$17$	$35 - 17 = 18$
$Cl-37$	$^{37}_{17} Cl$	$17$	$17$	$37 - 17 = 20$

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.