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Lesson Explainer: Atomic Model Chemistry


1. From Thomson to Chadwick

An atom consists of a nucleus with positive charges and electrons with negative charges orbiting it.

What are the experimental facts that led to the evolution of the structure of the atom towards the current model?

james chadwick
Figure 1: James Chadwick (1891–1974).

Thomson discovered the electron, Rutherford showed that matter is essentially empty, and Bohr helped explain why the electrons in Rutherford's model did not crash into the nucleus. When Rutherford demonstrated the existence of the proton, a new problem was raised: How do positively charged protons overcome the electric repulsion force that would normally drive them apart and cause the nucleus to burst?

Moreover, in 1919, some physicists had discovered a very penetrating ray by bombarding beryllium with alpha particles. However, this ray was impossible to be directly detected.

In 1932, the English physicist James Chadwick, one of Rutherford's students, proved that these penetrating rays, which had not been identified by physicists, contained a particle with no electric charge and a mass equal to that of a proton. He named it neutron. In order to account for the existence of neutrons, Chadwick improved the current model of the atom. He explained that it was the neutrons that prevented the nucleus from bursting by their presence in it.

The Rutherford atomic model modified by Bohr and improved by Chadwick is called the Rutherford-Bohr atomic model. It is still in use today and is also called the simplified atomic model. It presents the atom as a splitable entity with positive particles (protons) and neutral particles (neutrons), concentrated in a tiny and dense nucleus, and negative particles (electrons) - with a mass 2‎ ‎000 times less than that of the proton, knowing that the mass of an electron is equal to the ratio of the mass of a proton to 2‎ ‎000 - evolving on electronic layers.

The mass of the proton is equal to 1.673×10 kg. In the nucleus of an atom there are 𝑍. The mass of the neutron is equal to 1.675×10 kg. The total number of particles in the nucleus (protons and neutrons) is denoted by 𝐴. The nucleus consists of protons and neutrons. The nucleus consists of protons and neutrons according to a simple law; there must be as many protons as there are electrons in orbit around the nucleus for the electric charge of the atom to be zero.

Document: The existence of neutrons


  1. How did chemists establish such a structure and what is its interest?
  2. What is an atom made of?
  3. What does its nucleus contain?
  4. What is the uncharged particle in the nucleus called?
  5. What can we say about the masses of the proton and the neutron?
  6. Calculate the mass of an electron.
  7. Why do we say that the mass of the atom is mainly concentrated in its nucleus?

2. Conservation of the element copper

Some elements in the Universe are conserved after having undergone several transformations. We can for example demonstrate the conservation of the element copper during a sequence of chemical transformations in the laboratory of a high school.

In what form (atom, ion, or compound body) is the element copper found during these transformations?

Experiment 1: Action of nitric acid on copper

Equipment and materials

1 test tube on a stand, 5.0 mL graduated pipette fitted with a pipette filler, 50 mL beaker, 1 wooden tweezers.

0.30 g of copper metal, 1 solution of nitric acid at 5.0 mol⋅L−1.

Copper turn
Figure 2: Copper metal.


  • All chemical entities (atoms, ions, etc.) with the same atomic number 𝑍 are called chemical elements.
  • This experiment gives off a toxic gas and must therefore be carried out by the teacher and under the fume hood.


  1. Introduce 0.30 g of copper metal into the test tube.
  2. Pour a small amount of the nitric acid solution into the beaker.
  3. Using the 5.0 mL graduated pipette with a pipette filler, draw up 2.0 mL of the nitric acid solution and place it in the test tube containing the copper under the hood.


  1. What ion is responsible for the blue color in the test tube?
  2. Copy and complete the following figure using the chemical formulas involved.

Experiment 2: Formation of copper(II) hydroxide

Equipment and materials

1 test tube on a stand, 10.0 mL volumetric pipette fitted with a pipette filler, 25 mL graduated cylinder, 50 mL beaker, 100 mL beaker, 250 mL beaker, 1 filter paper, 1 funnel, 1 spatula.

A copper sulfate solution at CmolL=0.50, a sodium hydroxide solution at CmolL=1.0.

Figure 3: Formation of Cu(HO)2 precipitate.


  1. In the 100 mL beaker, introduce 10.0 mL of the copper sulfate solution.
  2. Add about 20 mL of the aqueous sodium hydroxide solution.
  3. Filter off the resulting copper(II) hydroxide precipitate.
  4. Collect the resulting copper(II) hydroxide precipitate in the test tube.

Figure 4: Filtration of copper oxide precipitate.


Copy and complete the following figure using the chemical formulae involved.

Experiment 3: Heating of copper(II) hydroxide

Equipment and materials

1 Bunsen burner, 1 wooden tweezers, 1 spatula, 1 laboratory glass bowl. Copper(II) hydroxide precipitate obtained in the test tube (experiment 2).


  1. Introduce the collected precipitate in a test tube.
  2. Light the Bunsen burner in compliance with the safety instructions.
  3. Carefully heat the test tube containing the precipitate. Stop heating as soon as the precipitate becomes black. The black solid obtained is called copper(II) oxide.

Figure 5: Heating of copper oxide.


Copy and complete the following figure using the chemical formulae involved.

Experiment 4: Action of Carbon on Copper Oxide

Equipment and materials

1 test tube, 1 Bunsen burner, 1 pair of wooden tweezers, 1 spatula, 1 square white sheet, 1 funnel. The copper(II) oxide obtained in experiment 3, carbon powder.


  1. Fold the white sheet along these diagonals.
  2. Mix without loss, on the white sheet, a spatula of copper(II) oxide powder and about half a spatula of carbon powder.
  3. Using the funnel, place the mixture in a test tube.
  4. Heat the test tube extensively until the orange-red color appears. A metal is found.


Copy and complete the following figure using the chemical formulae involved.

General conclusion

Represent all the previous transformations by completing the following cycle.

3. Analyzing the elements in the Universe

Each substance found in the Universe consists of one or more elements and in variable amounts.

How can we explain their abundance?

The Great Upheaval

big bang
Figure 6: The Big Bang.

Everything would have started with a gigantic explosion, about fifteen billion years ago. A big boom, according to the theory of the "Big Bang".

The birth of the Universe, the instant zero, remains a mystery. But a few fractions of a second after 10 s, the newborn is weak, smaller than that "•" but has a hell of an appetite for expansion! Quite feverish too, billions of billions of degrees. A considerable energy, already used to make particles. (...).

Milky Way
Figure 7: Our Galaxy: The milky way.

The first nuclei are formed from neutrons and protons; first from Hydrogen 11H, then from Deuterium 21H and Helium 42He. After 300‎ ‎000 years, about 3‎ ‎000 degrees. The electrons associate with the nuclei to form the first atoms: atoms of Hydrogen and atoms of Helium. That is when the Universe becomes transparent; this constitutes the extreme limit of observability.

Thus, during its evolution, a star first consumes Hydrogen to transform it into Helium. At higher temperatures, the fusion of Helium gives Oxygen and Carbon which, in turn, will be used to give new elements: Sodium, Neon, Phosphorus, Silicon ... and then Iron, the most stable nucleus and final point of this fusion process.

Therefore, as a star ages, it depletes increasingly in Hydrogen and enriches in heavy elements.

The Earth!

Earth before big bang
Figure 8: Planet Earth.

Then 10 billion years after the Big Bang, our sun and its planetary system were formed, around 5 billion years ago. A cloud of gas which collapses under the effect of the gravitation, in its center our Sun, all around the grains of dust which agglutinate and give rise to the planets of which the Earth. (...).

  • The crust of the Earth, with an average thickness of 7 km under the oceans, 35 km under the continents, composed of Silica and double Silicate of Aluminum and metal such as Sodium, Potassium, Calcium, Magnesium...
  • The mantle (2‎ ‎900 km) composed of ferromagnesian silicates
  • The inner core (3‎ ‎500 km) composed of Nickel, Iron and in lesser quantity, Sulfur and Oxygen. The movement of liquid iron in the inner core would be the origin of the earth's magnetic field responsible for the orientation of the compasses. (...).

Extracted from Bibliography:
Sciences et Avenir hors série n62
Sciences et Vie Junior n71

Document: Abundance of elements in the Universe


  1. State the first elements found in the Universe and then group them in a table, specifying their name and atomic number.
  2. Find in the text the two most abundant elements in the Universe and the most abundant elements in Earth.
  3. How can we explain the abundance of these elements in a star?


1. Constituents of the atom

An atom consists of a positively charged nucleus and a set of negatively charged electrons. An atom is an electrically neutral entity; it has as many positive charges as negative charges.

Figure 9: The atomic model.

1.1. The nucleus and its components

Nuclei consist of particles called nucleons which are:

  • The positively charged protons,
  • The neutrons which, as their name implies, carry no electric charge.

Charles Augustin
Figure 10: Charles-Augustin Coulomb, born on June 14, 1736 in Angoulême, and died on August 23, 1806 in Paris, was a French officer, engineer and physicist.

1.2. The electrons, the elementary charge

Atoms contain as many electrons as protons. Electrons are negatively charged. Their electric charge is opposite to that of the protons. The absolute value of the charge of an electron is denoted by 𝑒. It is called elementary charge. It is expressed in Coulomb (C). 𝑒=1.60×10C

Table 1: Charges of elementary particles.

Eletcrical Charge (C)1.60×101.60×100

Example: The carbon atom has 6 protons in its nucleus; so it has 6 electrons.

1.3. Symbolic notation of a nucleus

  • The number of protons in a nucleus is called its atomic number. It is denoted by 𝑍.
  • The total number of nucleons (protons and neutrons) is denoted by 𝐴. The number of neutrons 𝑁 of a nucleus is: 𝑁=𝐴𝑍.

The numbers 𝐴 and 𝑍 are all we need to perfectly characterize a nucleus: a nucleus is noted AZX, where X is the symbol of the element, 𝐴 the number of nucleons and 𝑍 the number of protons.

Example: 147N represents a nucleus consisting of 14 nucleons, 7 of which are protons: 𝑍=7 and 𝐴=14.

1.4. Mass of an atom

The masses of the neutron 𝑚 and of the proton 𝑚 are almost the same. They are greater than the mass of the electron 𝑚. The mass of the atom 𝑚atom containing 𝐴 nucleons is almost equal to 𝑚=𝐴×𝑚atomnucleus.

Example: Calculate the mass of the nitrogen atom 147N: 𝑚=𝑍𝑚+(𝐴𝑍)𝑚+𝑧𝑚.

Since 𝑚=𝑚=𝑚=1.6710nucleuskg, and the mass of the electrons is negligible, the approximate mass 𝑚atom of the atom can be equal to: 𝑚=𝐴×𝑚atom (where 𝑚 and 𝑚 are the mass of a neutron and the mass of a proton, respectively). 𝑚()=14×1.67×10=23.4×10atom147Nkg

Table 2: Masses of elementary particles.

Mass of particles in kg9.11×101.67×101.67×10

2. Chemical elements

2.1. Definition

All entities with the same atomic number define a chemical element. Each of them is represented by a symbol that identifies it. However, atoms of the same chemical element can have different numbers of neutrons in their nuclei, and they are called isotopes.

  • The isotopes have the same atomic number 𝑍 and therefore belong to the same element.
  • They have the same electron pattern and the same chemical properties.
  • They differ in the number 𝐴 of their nucleons.

Table 3: Symbol and atomic number of some of the most common chemical elements.

Name of the elementSymbolAtomic number 𝑍


126C; 136C and 146C are carbon isotopes. We notice that 𝑍=6 does not vary but the number of neutrons varies respectively 𝑁=6; 𝑁=7 and 𝑁=8.

2.2. Conservation of the element copper during chemical transformations

The atoms of the elements are conserved throughout the chemical reactions. They are found differently combined in the resulting compounds. None of the elements can appear or disappear during a chemical reaction.

Figure 11: Cycle of Copper Conservation.

2.3. Simple and Compound Bodies

  • A simple body is a chemical entity that contains only one element.
  • A compound body is a chemical entity that contains several elements.


compound body and simple body
Figure 12: Compound body (a), Simple body (b).

The solid CuO is a compound body (Fig. 12.a).

The solid copper, Cu is a simple body (Fig. 12.b).

Key points

Composition of the atom

  • Atoms consist of a central nucleus surrounded by moving electrons.
  • The nucleus consists of nucleons: protons and neutrons.
  • To represent the atom of an element X in symbolic notation, we write AZX.

Masses and electric charges of the atom components

Any electric charge can be expressed in terms of the elementary charge 𝑒, which is approximately 1.6010 C.

Electric Charge1.60×10C1.60×10C0 C
Mass of particles in kg9.11×101.67×101.67×10
  • An isolated atom is electrically neutral.
  • The approximate mass of the atom is: 𝑚=𝐴×𝑚atom

Chemical elements

  • A chemical element is the set of atoms whose nuclei have the same number of protons.
  • An element is characterized by its atomic number 𝑍 and by its symbol.
  • The chemical elements are conserved throughout a chemical transformation.


  • Atoms having the same atomic number 𝑍 but different numbers of nucleons 𝐴 are called isotopes.

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