Atoms

Level 1

The basics of matter

The matter around us – like air, water or our body – is made up of molecules. But looking closer, we discover that these molecules are themselves made of even smaller particles: atoms.

Imagine a piece of sugar being cut in half, then in half again, indefinitely. At the end of the process, we would obtain atoms: the fundamental "building blocks" of all matter.

Dalton's model: spheres to represent atoms

At the beginning of the 19th century, John Dalton proposed a simple model: each atom is a small solid sphere. When several of these spheres combine, they form a molecule.

   Examples:

A water molecule = 2 hydrogen atoms + 1 oxygen atom

Modèle eau

A carbon dioxide molecule = 1 carbon atom + 2 oxygen atoms

Modèle dioxyde de Carbone

  The colours used to represent atoms in models are not real, but they help to distinguish them visually.

The 4 star atoms of living things

Out of more than 100 different types of atoms, 4 are very common in biological molecules:

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)
  • Nitrogen (N)

Each is represented by a chemical symbol: a capital letter, sometimes followed by a lowercase letter.

Chemical formulas

Thus with the symbols of each atom, we can write the chemical formula of the molecule to give its composition. We write the symbol of the atom followed, as a subscript, by the number of atoms present in the molecule.

  Note that if there is only one atom, we do not write the 1 to simplify the notation.

  Example

 For water, which has 2 hydrogen atoms and one oxygen atom. This gives us  $H_2O$

For carbon dioxide, which has 2 oxygen atoms and 1 carbon atom, the formula is  : $CO_2$

Molecule Builder

Available atoms

H
Hydrogen
1 bond max
O
Oxygen
2 bonds max
C
Carbon
4 bonds max
N
Nitrogen
3 bonds max
💡 Predefined molecules
H
O
C
N
🔬 Construction area

Drag atoms here to create a molecule
🗑️
What do you think the chemical formula is?

 
 

A bit of history

The word "atom" comes from the Ancient Greek atomos, which means "indivisible". Ancient Greeks had already imagined this idea around 400 BC. It was then forgotten for centuries, then validated by scientific experiments in the 19th century.

Rutherford's experiment: a revolutionary model

At the beginning of the 20th century, the physicist Ernest Rutherford performed a landmark experiment. He bombarded a thin gold foil with particles. To his great surprise, most passed through the foil, but some were deflected or bounced back. This means that the atom is not a solid sphere!

Rutherford's model

An atom consists of:

  • a nucleus, which is very small, positively charged, and where almost all the mass is concentrated,
  • an electron cloud containing electrons, which are very light and negatively charged, moving around the nucleus,

An atom is therefore mainly made up of empty space between the nucleus and the electrons: this is called a lacunar structure.

Balance of charges: electrical neutrality

An atom is overall electrically neutral:

  • the number of positive charges (protons) = the number of negative charges (electrons)

  If an atom loses or gains electrons, it becomes an ion (positive or negative).

+6
Nucleus charge
+6
Electron charges
−6
Electrically neutral atom

The nucleus: protons and neutrons

The nucleus is made of nucleons:

  • protons, positively charged,
  • neutrons, without charge, which stabilize the nucleus.

Atomic notation: 

An atom is written in the following way:

$^A_ZX$

  • X: symbol of the atom
  • Z: Atomic number = number of protons
  • A: Mass number = number of nucleons (protons + neutrons)

  Example: For $^{12}_6C$ the atom is composed of:

  • 6 protons because Z = 6
  • 6 neutrons because A = 12 ➞ $12nucleons - 6protons = 6neutrons$  
  • 6 electrons because an atom is neutral, there are as many electrons (- charged) as protons (+ charged) 
    ➞$ 6 (charge +) + 6(charges -) = \varnothing $
Building atoms:

The periodic table

The periodic table is attributed to Dmitri Mendeleev, who proposed a classification of elements based on the recurrence of chemical properties. Certain atoms have similar properties and are grouped together.

Thus, certain columns or blocks correspond to families of elements, all having similar chemical properties. 

  Example:

  • noble gases in the last column are all very unreactive and generally gaseous.

  • alkali metals in the first column, which tend to lose 1 single electron and are very reactive (especially with water).

    Atoms are ordered by increasing atomic number. 

Click to access the interactive version

 

The Bohr model

Bohr proposed that electrons are arranged in shells around the nucleus:

  • Shell 1:  up to 2 electrons
  • Shell 2:  up to 8 electrons
  • Shell 3:  up to 8 electrons (or 18 electrons for larger atoms)

Electrons fill the shells closest to the nucleus first (from 1 to 3).

Stabilized or reactive?

An atom is stable when its outer shell is full. A stable atom tends to react less; otherwise, it seeks to react to stabilize itself:

  • it will form bonds with other atoms and form molecules.
  • it will gain or lose electrons to form an ion.

 Examples:

  • Carbon (Z=6): $(1)^2(2)^4$ ➞ unstable (mainly forms molecules)
  • Neon (Z=10): $(1)^2(2)^8$ ➞ stable because the outer shell is complete.

Electron subshell

Each shell is divided into subshells in which the electrons are distributed.

There are 4 types of subshells:

  • s: the simplest, holding a maximum of 2 electrons.
  • p: holding a maximum of 6 electrons
  • d: holding a maximum of 10 electrons
  • f: holding a maximum of 14 electrons.

Klechkowski's rule

To fill subshells in the correct order, Klechkowski's rule is used

Examples:

  • Carbon (Z=6): $1s^2 2s^2 2p^2$ 
  • Neon (Z=10): $1s^2 2s^2 2p^6$