C2aL1 Atomic and Electronic Structure

Key Words

Atoms - the smallest particle into which an element can be broken down

Electrons - a tiny negatively charged particle that orbits the nucleus.

Electronic notation - the way the electrons are arranged in the shells.

Element - a substance that consists of one kind of atom only.

Neutrons - neutrally charged particle in the nucleus.

Nucleus - tiny part of the atom where most of the mass is found.

Particles - little bits of material.

Protons - positively charged particles in the nucleus.

Relative mass - mass compared to the mass of a proton.

Shells - where the electrons are found.

Sub atomic - particles that make up the atom.

Symbol - short hand for elements.

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Grade E

Atoms consist of protons, neutrons, and electrons, which are sub-atomic particles.  The nucleus is in the middle, and the electrons surround it.  Here is a lithium atom:

The table shows the charges and masses of the sub atomic particles:

Particle Relative mass Relative Charge
Proton 1 +1
Neutron 1 0
Electron 1/1800 -1

We show each element as a symbol:

Mass number

12              

C

6               

Atomic number

 

  • The atomic number is the number of protons.

  • The mass number is the number of protons and neutrons.

Therefore:

number of neutrons = mass number - atomic number

 

Elements are arranged in order of rising atomic number.

 

The electrons are arranged in shells.  In the lithium atom, there are two shells.  In sodium there are three shells.

 

 

This is because there are 11 protons, which are arranged as follows:

  • 2 in the first shell (the one nearest the nucleus);

  • 8 in the second shell;

  • 1 in the outer shell.

Notice that in both lithium and sodium there is one outer shell electron.  This makes their reactions similar, and they are in the same group (Group 1) of the periodic table.

Grade C

The idea of the atom comes from the Greek scientist Demokritos.  Everything was made up of little particles which he called atoms (Greek for "un-cuttable") which could not be chopped up any further.

John Dalton in the 19th Century suggested that:

  • atoms of the same element were the same, but were different to other elements;

  • matter can neither be created, nor destroyed;

  • atoms can be joined to form molecules.

In 1895 electrons were discovered.  In 1900 they thought that atoms were made of protons and electrons.  In 1911, Rutherford discovered the nucleus.  In 1932, Chadwick discovered the neutrons.  We know a lot more now as technology improves, but we don't know everything.

Magnesium has an electron arrangement like this:

Its electronic notation is 2, 8, 2.  The first shell always has two electrons in it; the second shell has 8 electrons.

Calcium is like this:

Its electronic notation is 2, 8, 8, 2.  Both calcium and magnesium have 2 electrons in the outer shell.  their reactions are quite different to sodium and lithium, but are similar to each other.  They are in Group 2 of the periodic table.

 

Grade A

In chemical reactions, only the electrons interact.  The nuclei of elements is NOT changed at all.  Nuclear reactions can occur, whereby one element is made into another.  Conditions need to be extreme for this to happen.  A lot of energy can be released.

Evidence of similarity between elements of the same group is shown by limestone, which can be made of calcium carbonate or magnesium carbonate.  The rock dolomite is calcium magnesium carbonate.  Exchange has occurred between calcium and magnesium atoms, because they are similar.