The theory of atomic structure has changed over time

At the start of the 19th century John Dalton described atoms as solid spheres and said that different sphere made up the different elements
In 1897 J J Thompson concluded from his experiments that atoms weren’t solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles-electrons.The ''solid sphere’ idea of atomic structure had to be changed. The new theory was known as the ‘plum pudding model’.
- The plum pudding model showed the atom as a ball of positive charge with electrons stuck in it

Rutherford showed that the plum pudding model was wrong

In 1909 Ernest Rutherford and his student Ernest Marden conducted the famous alpha particle scattering experiments. They fired positively charged alpha particles at an extremely thin sheet of gold
From the plum pudding model, they were expecting the particles to pass straight through the sheet or be slightly deflected at most. This was because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom. But, whilst most of the particles did go straight through the gold sheet, some were deflected more than expected, and a small number were deflected backwards.
- So the plum pudding model couldn’t be right
Rutherford came up with an idea to explain his new evidence- the nuclear model of the atom. In this, there’s a tiny, positively charged nucleus at the centre, where most of the mass is concentrated. A ‘cloud’ of negative electrons surrounds this nucleus-so most of the atom is empty space, When alpha particles came near the concentrated, positive charge of the nucleus, they were deflected. If they were fired directly at the nucleus, they were deflected backwards. Otherwise, they passed through the empty space

Scientists realised that electrons in a ‘cloud’ around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse. Niels Bohr’s nuclear model of the atom suggested that all the electrons were contained in shells.
Bohr proposed that electrons orbit the nucleus in fixed shells and aren’t anywhere in between. Each shell is a fixed distance from the nucleus
Bohr’s theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists’ observations at the time

Further experiments by Rutherford and others showed that the nucleus can be divided into smaller particles, which each have the same charge as a hydrogen nucleus. These particles were named protons
About 20 years after scientists had accepted that atoms have nuclei, James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus which are now called neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model.

At the start of the 19th century John Dalton described atoms as solid spheres and said that different sphere made up the different elements
In 1897 J J Thompson concluded from his experiments that atoms weren’t solid spheres. His measurements of charge and mass showed that an atom must contain even smaller, negatively charged particles-electrons.The ''solid sphere’ idea of atomic structure had to be changed. The new theory was known as the ‘plum pudding model’.
- The plum pudding model showed the atom as a ball of positive charge with electrons stuck in it

In 1909 Ernest Rutherford and his student Ernest Marden conducted the famous alpha particle scattering experiments. They fired positively charged alpha particles at an extremely thin sheet of gold
From the plum pudding model, they were expecting the particles to pass straight through the sheet or be slightly deflected at most. This was because the positive charge of each atom was thought to be very spread out through the ‘pudding’ of the atom. But, whilst most of the particles did go straight through the gold sheet, some were deflected more than expected, and a small number were deflected backwards.
- So the plum pudding model couldn’t be right
Rutherford came up with an idea to explain his new evidence- the nuclear model of the atom. In this, there’s a tiny, positively charged nucleus at the centre, where most of the mass is concentrated. A ‘cloud’ of negative electrons surrounds this nucleus-so most of the atom is empty space, When alpha particles came near the concentrated, positive charge of the nucleus, they were deflected. If they were fired directly at the nucleus, they were deflected backwards. Otherwise, they passed through the empty space

Scientists realised that electrons in a ‘cloud’ around the nucleus of an atom, as Rutherford described, would be attracted to the nucleus, causing the atom to collapse. Niels Bohr’s nuclear model of the atom suggested that all the electrons were contained in shells.
Bohr proposed that electrons orbit the nucleus in fixed shells and aren’t anywhere in between. Each shell is a fixed distance from the nucleus
Bohr’s theory of atomic structure was supported by many experiments and it helped to explain lots of other scientists’ observations at the time

Further experiments by Rutherford and others showed that the nucleus can be divided into smaller particles, which each have the same charge as a hydrogen nucleus. These particles were named protons
About 20 years after scientists had accepted that atoms have nuclei, James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus which are now called neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern day accepted version, known as the nuclear model.