The structure of the atom

In a nutshell

Atoms are tiny blocks of matter that make up everything! Every atom consists of a positively charged nucleus surrounded by negatively charged electrons. The mass and size of the nucleus is much larger than the electrons. The model of the atom has changed over time due to scientific discoveries.

The atomic model over time

'Atomos'

The Greeks had the idea of everything being made of smaller parts. In the $$5th$$​ century BC, Democritus thought that if matter was divided enough times it will eventually get to tiny, indivisible blocks of matter, which form everything. He called these atoms after 'atomos', meaning indivisible. 

Plum pudding model

A new model, suggested by J.J. Thomson, described atoms as spheres of positive charge with small negatively charged electrons in them. This is called the plum pudding model as the electrons are similar to plums in a plum pudding. 

Tip: Nowadays plum puddings aren't really known. Instead, think of the electrons as chocolate chips in a spherical chocolate chip muffin!

1. Positively-charged "pudding" 2. Negatively-charged "plums" (electrons)

Alpha scattering experiment

The plum pudding model was disproven in $$1909$$​ by Rutherford's gold foil experiment. In this experiment he fired a beam of positively charged particles (alpha particles) at a piece of thin gold foil. Thomson's model would suggest that they all either pass through or be slightly deflected.

However, Rutherford found that some particles were deflected more than expected and a small number were deflected back the way they came!

Nuclear model

This experiment led to the first nuclear model of the atom. Rutherford realised that as some particles were deflected back, most of the mass had to be concentrated in a dense nucleus at the centre. This must have an overall positive charge as it repelled some of the alpha particles. As only a few particles were deflected at all, nearly all the atom must be empty space.

1. Plum-pudding model 2. Nuclear model

Bohr model

In $$1913$$​, Niels Bohr improved the model by adding electron energy levels. This is the idea that electrons orbit the nucleus at different distances. 

Only certain amounts of electrons can orbit each energy level. Up to two electrons can occupy the first energy level, and up to eight electrons can occupy the second and third energy level.

Electrons fill up the energy levels starting from the first energy level and increasing from there.

1. Positive nucleus 2. The first (lowest) energy level contains up to two electrons 3. The second energy level contains up to eight electrons 4. The third energy level (the outer shell in this case) holds up to eight electrons 5. Empty space constitutes $$99\%$$​ of the atom

The atom

Rutherford later discovered that the positively-charged nucleus could be divided into even smaller, positively-charged particles, called protons. Scientists then saw evidence for another particle inside the nucleus, one with a similar mass to the proton, but neutral in charge (zero charge). This was later called the neutron. 

A typical atom has a diameter of the order of magnitude $$10^{-10}\,m$$. A typical molecule has a diameter of the order of magnitude $$10^{-9}\,m$$.​

1. Protons 2. Neutrons 3. Electrons

Nucleons

A nucleon is a particle inside the nucleus. Atomic nuclei consist of protons and neutrons, so protons and neutrons are nucleons. Electrons are not nucleons since they are outside the nucleus.

The key properties of the proton, neutron and electron are summarised in the table:

Atoms are neutrally charged because they have the same number of protons and electrons. Atoms are written as $${^{X}_{Y}A}$$​ , where $$A$$​ is the element label, $$X$$ is the mass number and $$Y$$ is the atomic number.

The atomic number is the number of protons in an atom. This number defines the atom; an atom with an atomic number of $$1$$ is always hydrogen; an atomic number $$2$$​ is always helium. 

The number of electrons in a neutral atom is equal to the atomic number.

The mass number is the number of nucleons (protons and neutrons) in the atom. This is equal to the mass of the atom since electron mass is negligible. This is sometimes called the nucleon number, since it is equal to the number of nucleons in the atom. 

Example

A helium atom $${^{4}_{2}He}$$​ consists of two protons and two neutrons (so four nucleons in total) being orbited by two electrons at a distance (in the lowest energy level).