4.1 Atoms and isotopes

Question 1

Atomic structure

Answer 1

Atoms are the building blocks of all matter.
They are incredibly small, with a radius of only 1 × 10-10 m.
This means that about one hundred million atoms could fit side by side across your thumbnail.
Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus.
The radius of the nucleus is over 10,000 times smaller than the whole atom, but it contains almost all of the mass of the atom.

Question 2

Proton

Answer 2

Protons have a relative charge of +1.
They are found in the nucleus.
An element’s atomic number is the number of protons it possesses.
All atoms of the same element have the same number of protons.

Question 3

Neutron

Answer 3

Neutrons have a relative charge of 0 - they are neutral.
Like protons, they are found in the nucleus.

Question 4

Electron

Answer 4

Electrons have a relative charge of -1.
They are found in fixed orbits around the nucleus.
In any atom, the total number of negative electrons equals the number of positive protons, meaning atoms have no overall electric charge.

Question 5

Electronic arrangement

Answer 5

Electrons are arranged in shells around a nucleus. Each shell has a different energy level. When an atom absorbs or emits electromagnetic radiation, its electron arrangements can change.

Question 6

Atomic number

Answer 6

The number of protons in an atom is called its atomic number (it can also be called the proton number).
Elements in the periodic table are ordered by their atomic number.
Therefore, the number of protons determines which element an atom is.

Question 7

Mass number

Answer 7

The total number of particles in the nucleus of an atom is called its mass number.
The mass number is the number of protons and neutrons in the atom.
The number of neutrons can be found by subtracting the atomic number from the mass number.

Question 8

Isotopes

Answer 8

Although the number of protons in a particular element is always the same, the number of neutrons can be different.
Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons.
Isotopes occur naturally, but some are more rare than others.
For example, about 2 in every 10,000 Hydrogen atoms is Deuterium.
Tritium is even more rare (about 1 in every billion billion hydrogen atoms).

Question 9

Positive ions

Answer 9

Electrons in the outer energy level can be knocked out from an atom.
This can happen in a number of ways:
When objects are rubbed together, electrons can be removed by friction.
When electrons absorb electromagnetic radiation they can gain enough energy to leave the atom.
From chemical reactions.
When one or more electrons are removed from an atom, it becomes positively charged.

Question 10

History of an atom - 1 - John Dalton

Answer 10

1800’s - John Dalton suggests that each of the elements are made from just one type of atom: tiny spheres that could not be divided.

Question 11

History of an atom - 2 - JJ Thomson

Answer 11

1897 - J Thomson discovers the electron and proposes the plum pudding model: the atom is a ball of positive charge and the negative charge are embedded in it (like blueberries in a blueberry muffin).

Question 12

History of an atom - 3 - Ernest Rutherford

Answer 12

1911 - Ernest Rutherford fired alpha particles at a piece of very thin gold foil(about 10,000 atoms thick).
Thomson’s plum pudding model predicted that…
All alpha particles… passed straight through
However what was observed was:
Most alpha particles… passed straight through
A very few alpha particles… were deflected by more than 90 degrees
Most of the atom is empty space
All the positive charge and most of the mass is concentrated in a small volume (the nucleus)

Question 13

History of an atom - 4 - Niels Bohr

Answer 13

1913 - Niels Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances The theoretical calculations of Bohr agreed with experimental observations.

Question 14

History of an atom - 5 - James Chadwick and others

Answer 14

1920-1932 - Later experiments led to the idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge. The name protons was given to these particles.

About 20 years after scientists had accepted that atoms have nuclei, in 1932 James Chadwick carried out an experiment which provided evidence for neutral particles in the nucleus. These became known as neutrons. The discovery of neutrons resulted in a model of the atom which was pretty close to the modern-day version.