Atomic Structure

Atoms are constructed of smaller sub-atomic particles, although only 3 make up the basic model of the atom, (the Bohr model named after Niels Bohr), it is useful to know about 4 particle in this topic.

Neutrons: Neutral particles found in the middle (nucleus) of an atom. They have a atomic mass of 1u (actually 1.009u).

Protons: Carry a positive (+1) electrical charge and are found in the nucleus of the atom. They have an atomic mass of 1 u (actually 1.008 u).

_Electrons: __Small negatively charged particles, (-1) that orbit the nucleus in rings (orbits). We treat these as having no mass. (actually 5x10-4_u).

Positron: The positively charged equivalent of an electron. It is NOT part of the Bohr model, but these particles are important in nuclear decay and radiation.

Atoms, figure 1

As electrons are considered to have no mass, all the mass of an atom is concentrated in the dense nucleus at the centre.

Size of atoms - Mass

Atoms are very small and so to use kilograms or grams to measure individual atoms would be a challenges. A hydrogen atom, for example, has a mass around1.66 x 10-27kg.

Atoms and their particles are measured in Atomic Mass Units (u for short). 1u is equal to 1/12th _the mass of a Carbon 12 atoms. 1 _u _= 1.66x10-27_kg

Carbon12 (12C) has a mass of 12 u14N has a mass of 14 u

Atomic mass is found by adding up all the protons and neutrons in an atom’s nucleus.

Size of atoms - Dimensions

Although individual atoms do vary in size from hydrogen to the larger atom like Plutonium, they are all in the order of 10-10 m across. A nucleus is around 10-14m across. Meaning a nucleus of an atom makes up only about 0.001% of the total size of the atom. However, it contains all the mass.The rest of the atom is made up of the electrons orbiting this nucleus.

Example: Atomic Structure of Carbon 12

612C The larger upper number is the atomic mass and the small number the atomic number.

Number of protons = atomic number.

Number of neutrons = atomic mass - atomic number.

Number of electrons = number of protons. (atoms are neutral so have even numbers of protons and electrons).

Carbons has:

6 protons

6 neutrons

6 electrons

Atoms, figure 2

Electron Orbits

The electrons in an atoms move around the nucleus in set orbits, these orbits can only contain a set number of electrons. Two in the first orbit closest to the nucleus, eight in the second orbit and eight in the third, giving the 2,8,8 configuration. The carbon atom in the diagram above has a total of 6 electrons, giving it a 2,4 configuration.

Note that for the exam you will not be required to know the orbital configuration beyond the first three orbits.

The electrons in the outer orbit can be promoted to a higher orbit, they move up one orbit, by absorbing energy. This takes them out of their normal energy orbit, known as their ground state and places them in an excited state. That is they have more energy than normal and this is an unstable situation. The electron will then return to its ground state, lower energy, orbit by emitting a wave of electromagnetic radiation. The wavelength, (and energy level) of the radiation will depend upon the change in energy state of the electron.

Electrons can absorb energy in this fashion from incoming radiation, including being heated. An electrical current in a wire also has a heating effect and can produce radiation in the visible light and the infrared range. This is the principle upon which an old fashioned filament light bulb works. They are very energy inefficient, because you have to use a lot of electrical energy to heat the wire in order to get it to glow and emit both infrared (heat) and the visible light.

The total energy absorbed must equal the total energy of the exchange, in accordance with the law of energy conservation. As some of the absorbed energy is used by the electron to move between the orbits, the emitted radiation will be lower in energy than that absorbed.


An isotope is a variety of an element with different numbers of neutrons. All isotopes of an element have the same number of protons and electrons. They vary in the number of neutrons. This also affects their atomic mass and their nuclear stability. Each extra neutron adds 1 u of mass.

In the periodic table carbon is normally listed as Carbon(12). This is one isotope of carbon, it is the most common and the most stable form of carbon.

Carbon has isotopes ranging from Carbon(8) to Carbon(22), all of them have 6 protons and 6 electrons.

Atoms, figure 1

Only Carbon(12), (13) and (14) are stable enough to be found in nature. The others are so unstable that they undergo nuclear decay every quickly and convert into other elements that are more stable. As Carbon(12) makes up 98.9% of the carbon on Earth, it is the one most commonly listed in the periodic table.

Example: Compare Carbon(12), Carbon(13) and Carbon(14) structures.

126_C _ Atomic Mass: 12u, Atomic Number: 6(Stable)

Protons: 6, Electrons 6, Neutrons: 6 Nucleon Number (protons plus neutrons): 12 Nuclear charge +6

136C Atomic Mass: 13u, Atomic Number: 6(Stable)

Protons: 6, Electrons 6, Neutrons: 7 Nucleon Number (protons plus neutrons): 13 Nuclear charge +6

Atoms, figure 2

146C Atomic Mass: 14u, Atomic Number: 6(Radioactive)

Protons: 6, Electrons 6, Neutrons: 8 Nucleon Number (protons plus neutrons): 14 Nuclear charge +6

Carbon(14) is stable enough to be found in nature, but does deacy over long periods of time, emitting β- (beta negative) radiation to become Nitrogen(14) a very stable isotope of nitrogen.


An atom is an electrically neutral particle, all isotopes of when they are atoms are neutral. All have an even number of positively charged protons in the nucleus and negatively charged electrons in their orbits.

To give an atom a charge the number of electrons needs to changed. This can happen during chemical reactions when electrons are transferred between atoms of different elements. This forms an ionic bond between the atoms.

__Once an atom has been charged it is known as an ion. __

Gaining electrons produces a negatively charged ion, called an anion.

Losing electrons produces a positively charged ion, called a cation.

Ions can also be formed by the removal of electrons from the outer orbit of an atom by electromagnetic radiation absorption. High energy electromagnetic waves in the ultraviolet to gamma range contain enough energy to excite an electron enough to be ejected from the atom. This produces an ion in a process known as ionisation. UV, X-rays and gamma are ionizing radiations.

Atoms, figure 1

How can an X-ray change an atom?
Your answer should include: absorbed / electron
Explanation: The x-ray can be absorbed by an electron in the outer orbit of an atom and excite the electron into a higher orbit. This will then return to its ground state orbit by emitting electromagnetic radiation. It is also possible that the energy absorbed by the electron will result in it being ejected from the atoms, this will make the atom become a positively charged ion.
What is the difference between an electron and a positron?
Your answer should include: negative / positive