Now you have not only solved the chemical equation problem, but also proved to yourself that you can understand chemistry.

      The Periodic table

      In 1869, a Russian chemist Dmitry Mendeleev published an article in which he presented his periodic table of chemical elements. He noticed a repetition of physical and chemical properties of chemical elements when he arranged them in order of their atomic weight.

      Later, it was proved that physical and chemical properties of elements depending on their number of protons and since the number of protons determines an element́s atomic weight, the elements could be arranged in order of their atomic weight.

      A two dimensional periodic table has vertical groups and horizontal periods.

      Elements that belong to the same group have similar properties. For example, Sodium (Na) and Potassium (K) are alkaline metals that belong to the first group. These metals are so soft that they can be cut with a knife. When a small bit of sodium is placed in water, it starts dissolving and producing a colorless and odorless gas. This gas is hydrogen.

      In reaction with water, alkaline metals produce alkali, a strong base. Sodium and water produce Sodium hydroxide. (NaOH)

      Sodium belongs to the first group and the third period.

      In the 7th group of the same period we find chlorine Cl. Chlorine is a greenish poisonous gas that was used as a chemical weapon in WWI. The reaction of mixing chlorine with water produces a strong hydrochloric acid (HCl)

      If you mix sodium hydroxide and hydrochloric acid a table salt will be produced.

      Knowing to which group and period an element belongs, a chemist can tell a lot about the element́s properties.

      As you know, an atom contains three kinds of particles: positive protons, neutral neutrons and negative electrons. Protons and neutrons comprise the atomic nucleus while electrons are located at some distance from the nucleus.

      The position of the electron in the atom is described by its four quantum numbers: shell, sub-shell, orbital, spin.

      Shells or the main quantum number n can be equal to any whole number 1, 2 3… It determines the electron energy and its average distance from the nucleus.

      Subshell or angular momentum quantum number l (small L) describes the shape of an electron orbital.

      When l=0 the electrońs orbital has spherical shape that is called an S orbital

      When l=1 the electrońs orbital has a dumbbell shape and is called a p orbital.

      When l=2 the electrońs orbital is called a d orbital.

      When l=3 the electrońs orbital is called an f orbital.

      m is a magnetic quantum number. It may change from +l (small L) to -l (small L).

      As a result, there are 3 types of p orbitals (m=-1, m=0 and m=+1). Two electrons may exist on each type of p orbital. In total, 3p orbitals may have 6 electrons.

      There are 5 types of d orbitals (m=-2, m=-1, m=0, m=1, m=2), two electrons may exist on each type of d orbital. In total, 5d orbitals may have 10 electrons.

      There are 7 types of f orbitals (m=-3, m=-2, m=-1, m=0, m=1, m=2, m=3), two electrons may exist on each type of f orbital. In total, 7f orbitals may have 14 electrons.

      s – spin projection quantum number or spin of electron can be +1/2 or – 1/2.

      Imagine that you have a desk with a stack of book shelves. If you have only one book you will put it on the first shelf. You will not put it on the top shelf near the ceiling. If you have a few books, you put them where it would be easier to reach one. In an atom, the electrons start filling orbitals with the orbital that has the lowest energy if it is not in contradiction to the Pauli Exclusion Principle.

      The Pauli Exclusion Principle states that no 2 electrons in the same atom may have the same quantum numbers. For example, 2 electrons on 1S orbital in the atom of helium have opposite spins.

      When 1S orbital is filled, 2S orbital will fill next.

      The electronic configuration of a Hydrogen atom is:

      1s ¹

      Hydrogen: 1s ¹

      It means that Hydrogen has only one electron on the first S orbital.

      The electronic configuration of the Helium (He) atom is:

      1s ²

      Helium: 1s ²

      It means that He has two electrons on the first S orbital.

      For an He atom, n quantum number =1, l (small L) quantum number =0. There is no p orbital for l=0. S orbital is completely filled. Helium cannot have more than 2 electrons on the 1S orbital. As a result, It is a noble gas. Helium cannot form bound with any other elements.

      The next element in the periodic table is Lithium (Li). Li starts the second period and has the order number of 3. It means that it has 3 protons

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