The nuclear model of the atom

In a nutshell

In $1911$ Rutherford conducted an experiment which grounded the modern view of the structure of the atom. He discovered that an atom is mostly empty space with a small positive nucleus. The nucleus is composed of two nucleons, protons with charge $+e$ and uncharged neutrons.

Rutherford's experiment

Rutherford's experiment was conducted in $1911$ and consisted in firing alpha particles to a thin metal foil. He found that most of the particles passed straight through while only some were deflected and a few of them were even sent back.

The apparatus

The experiment consisted in a beam of alpha particles being targeted towards a piece of gold foil in a vacuum. The alpha particles were scattered by the foil, which was only a few atomic layers thick, and detected by a fluorescent zinc sulfide screen. This screen was mounted on a microscope that could move around the set up by about $180\degree$ and would produce a speck of light for every particle hitting it.

Physics; Particle physics; KS5 Year 12; The nuclear model of the atom

1.	Alpha particle source
2.	Alpha particle beam
3.	Gold foil
4.	Fluorescent screen
5.	Microscope
6.	Observer
7.	Evacuated container

Thus, Rutherford was able to count the number of scattered alpha particles for an angle $\theta$ per minute.

Observations and conclusions

Rutherford made two significant observations that led him to make two conclusions about the structure of the atom:

observation	conclusion
Most of the particles went straight through. Only about $1$ in $2000$ were scattered.	The atom is mostly empty with most of its mass concentrated in a nucleus.
About $1$ in $10000$ particles were deflected by more than $90\degree$ .	The nucleus has a positive charge.

Physics; Particle physics; KS5 Year 12; The nuclear model of the atom

1.	Alpha particle source
2.	Gold nuclei scattering the particles

Since alpha particles are positive the nucleus must also be positive for the particles to be deflected by such high angles, which also means that this experiment can be modelled using Coulomb's law. By doing this Rutherford found that the nucleus was roughly $10^{-14}\ m$ in size, a good estimate given the true value of around $10^{-15}\ m$ .

At the time, the leading theory for the atomic structure was Thomson's plum pudding model, in which the atom was composed of a solid positively charged sphere with negative electrons embedded into it. This experiment disproved it and provided support for the new nuclear model, where the atom consists of a nucleus roughly $10^{-15}\ m$ in size and electrons orbit around it.

The nucleus

The nucleus of an atom consists of protons with charge $+e$ , where $e=1.6\times10^{-19}\ C$ , and neutrons with charge $0$ ; these are also known as nucleons.

In an atom electrons with charge $-e$ orbit around it and the overall charge is neutral when the number of protons and electrons is the same.

The nucleus of an atom can be represented as:

$^A_ZX$

symbol	definition
$X$	Chemical symbol of the element.
$A$	Nucleon number- the total number of protons and neutrons.
$Z$	Proton/Atomic number- number of protons.
$N$	Neutron number- number of neutrons, can be found with $N=A-Z$ .

Atoms that are the same element (same $Z$ ) but have differing numbers of neutrons (different $N$ ) are called isotopes.

Example

Given the neutral isotope uranium- $235$ , write down the number of neutrons and electrons in it.

First, by looking at a periodic table write down all the values:

$A=235\newline Z=92$ 

Next, write down the equation you need:

$N=A-Z$

Substitute values into the equation:

$N=235-92$ 

Work out the answer:

$N=143$ 

Now remember that for a neutral atom the number of electrons is equal to $Z$ , hence:

The number of neutrons and electrons in a neutral uranium- $235$ atom is $\underline{143}$ and $\underline{92}$ respectively.