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The Easiest Way to Understand Chemistry

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Chemistry Concepts, Problems and Solutions

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My grandma used to bake cakes.

To bake a cake for five people you have to mix:

1 cup of flour

3 eggs

1/2 cup of sugar

100 grams of butter

There was no such thing as a fat free cakes at that time. That is why they tasted so good.:)

What if you need to bake a cake for 20 people? She had the common sense to calculate proportions.

20 is 4 times greater than 5.

So, you have to use 4 times the number of cups of flour, 4 times as many number eggs and so on. As a result, you will have a new recipe.

1 * 4=4 cups of flour

3 * 4=12 eggs

1/2 * 4 = 2 cups of sugar

100 * 4 = 400 grams of butter.

In the same way you can solve a problem about a chemical reaction.

All chemical reactions occur in equivalent proportions.

If 10 grams of Na2CO3 react with CaCL 2 how many grams of CaCO3 is produced?

10g? g

1. Na2CO3 + CaCL2 = CaCO3 +2NaCL

All compounds react with each other in certain proportions In a given reaction one mole of Na2CO3 produces one mole of CaCO3.

Mole is molecular mass (MW) in grams.

Atomic mass of Na = 23

Atomic mass of C = 12

Atomic mass of O = 16

For Na2CO3 the MW is 23 *2 +12 +16*3 = 106 g. — 1 mole

For CaCO3 the MW is 40 +12 +48 = 100g. 1 mole.

106 g Na2CO3 produces 100g CaCO3

10 g Na2CO3 produces X g CaCO3

To calculate X, multiply the matched up values on the opposite ends of the diagonal and divide the product by the unmatched value as shown in the figure below.

X=10 * 100 / 106 = 9.4 g of CaCO3

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 element´s 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 element´s 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 electron´s orbital has spherical shape that is called an S orbital

When l=1 the electron´s orbital has a dumbbell shape and is called a p orbital.

When l=2 the electron´s orbital is called a d orbital.

When l=3 the electron´s 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 and 3 electrons. Its electronic configuration is:

1s 2 2s 1

Lithium: 1s 2 2s 1

The next element, Beryllium (Be) has the order number of 4 and it has 2 electrons on the 1S orbital and 2 electrons on the 2S orbital. Two S orbitals are completely filled for Beryllium. You may wonder why Beryllium is not a noble element if it has completely filled its S orbitals? The answer is that on the second shell the completed number of electrons is 8 (2 S electrons and 6 P electrons). For Beryllium, the quantum number n=2 and the quantum number l=1. As a result, an additional p orbital appears for n=2. This orbital is not filled for Beryllium. The Beryllium electronic configuration is:

1s 2 2s 2

Beryllium: 1s 2 2s 2

The next element is Boron (B). It has 5 electrons: 2 electrons on the 1S orbital, 2 electrons on the 2S orbital and 1 electron on the 2P orbital. Filling of the P orbital starts from Boron. Boron electronic configuration is:

1s 2 2s 2 2p 1

Boron: 1s 2 2s 2 2p 1

Next 5 elements have the outmost electrons on the P orbital and the number of P electrons is incremented by one for each consequential element.

1s 2 2s 2 2p 2

Carbon: 1s 2 2s 2 2p 2

Nitrogen: 1s 2 2s 2 2p 3

Oxigen: 1s 2 2s 2 2p 4

Fluorine: 1s 2 2s 2 2p 5

Neon: 1s 2 2s 2 2p 6

Neon (Ne) has 6 P electrons. The P orbital is completely filled for Neon. As a result, Neon is a noble gas.

The next element is Sodium (Na). Na order number is 11. It has the same electronic configuration as Neon plus additionally, it has 1 electron on the 3S orbital. The Sodium electronic configuration can be written in short form as [Ne] 3s1 or in long form:

1s 2 2s 2 2p 63s 1

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