top of page

Atomic structure

Electrons, protons and neutrons

An atom is made up of a central nucleus surrounded by electrons. In the middle of an atom there is a very small and dense nucleus where virtually all of the mass is concentrated. This contains two types of particles called protons and neutrons. A third type of particle called an electron is found outside the nucleus and these are negatively charged.

The charge on a proton is positive and the charge on a neutron is neutral so that the overall charge on the nucleus is positive. The relative charge of a proton is +1 and the relative charge on an electron is -1. The number of protons are equal to the number of electrons in any atom so that the overall charge of an atom is neutral. 

original?tenant=vbu-digital

A good way to remember the charge on sub-atomic particles is that:

  • Protons are Positive
  • Neutrons are Neutral
  • So electrons must be negative

Atomic number and atomic mass

Atoms of a particular element have the same number of protons and the number of protons is called the atomic number (or proton number). Both protons and neutrons have mass and each have a relative mass of 1. The number of protons plus the number of neutrons is called the atomic mass (or mass number). 

The following table summarises some important points covered:

Particle      Location       Relative charge      Relative mass

​

Proton       Nucleus        +1                            1

Neutron     Nucleus         0                             1

Electron     Shells           -1                             0 or 1/1836

The atomic number of an element is the number of protons but it is also equal to the number of electron since these two particles are equal in number and opposite in charge and therefore cancel each other out resulting in a neutral atom. 

Using atomic number and atomic mass the number of neutrons can be calculated as follows:

number of neutrons = atomic mass – atomic number

Electron shells or energy levels

 hat depends on their distance from the nucleus. The shell closest to the nucleus has the lowest energy level. Electrons will occupy the lowest energy level first before they fill subsequent higher energy levels. Diagrams can be drawn to show the electron arrangement as shown in figure 1 that represents a carbon atom with 2 electrons in the first shell and 4 electrons in the second shell. When showing electron arrangements of atoms on a diagram the nucleus does not have to be included as in figure 1.

Elements in the same group have the same number of electrons in their highest energy level (outer electrons). When atoms of an element have the same number of electrons in the outside shell they react in a similar way and therefore have similar chemical

​

​

 

properties.

There is a limit to the number of electrons an energy level (or shell) can hold:

​

   · The first shell (lowest energy level) can hold only 2 electrons

   · The second energy level can hold up to 8 electrons

   · The third energy level can also hold up to 8 electrons, 

after which the fourth will begin to filling atoms to show the number and arrangement of electrons the numbers of electrons in each energy level can simply be written to give the electronic structure or electronic configuration. Again looking at figure 1, the atom of carbon can clearly be seen to have two electrons in the first energy level and 4 in the second energy level so its electronic configuration is 2, 4.

Sodium has an atomic number of 11 this means that it has 11 protons but it must also have electrons to cancel out the charge on the 11 protons. The first 2 electrons will fill the first shell, the next 8 electrons will fill the second shell and finally the remaining 1 electron will be held by the third shell. This means that sodium has 1 electron in its outer shell and this is why it is placed in group 1. Its electronic configuration is 2, 8, 1.

Exercise 1

1. What is the electronic configuration of magnesium (Mg)?

2. Why is it placed in group 2 in the periodic table?

Diagrams to show the electron arrangement of atoms

original?tenant=vbu-digital

3. What is different about the electronic structure of helium compared to the atoms of other elements shown here?

Exercise 2

1. Use the following words to complete the sentences below.

energy,   nucleus,  group,   outermost,   shells

An atom has a central .............. around which electrons are arranged in ............... (energy levels). The electrons further away from the nucleus are of higher ............... than those close to the nucleus. All elements in the same ............... of the periodic table have the same number of electrons in their ..................... shell.

2. Describe the relationship between elements that are in the same group in the periodic table.

3. Using the periodic table draw the arrangement of electrons in the following atoms providing the electronic structure:

a. Li

b. Be

c. B

d. C

4. Using the periodic table draw the arrangement of electrons in the following atoms providing the electronic structure:

a. Na

b. Mg

c. Al

d. Si

5. How does the electronic structure change in the atoms of elements in question 3 compared to question 4?

bottom of page