Atomic Structure

Sodium electron shells

Substances that are made up of one type of atom are elements of which there are about 100 (118 at the time I wrote this). They all have a specific symbol on the periodic table. The numbers on the table tell you about the atoms themselves. The largest number is the total number of particles in the nucleus (protons and neutrons). The small number is the number of protons in the nucleus. Therefore, you can easily work out the number of neutrons by taking the small number from the big number. For example, Aluminium has a proton number of 13 and a relative atomic mass of 27. So 27 - 13 = 14 meaning that it has 14 neutrons.

Isotopes of an element must have the same number of protons (or it would be another element) but have different numbers of neutrons e.g. Carbon-12 and Carbon-14 both have 6 protons, one has 6 neutrons and one has 8 neutrons. The relative atomic mass is the mass number of the element and although there is not a Kg unit by it, it shows the mass “relative” to other elements which allows us to calculate the proportions of each element in a compound. We can see from the periodic table that Magnesium has a mass number of 24 and Carbon has 12. This means that if we have 1g of carbon and 1g of magnesium, there will be twice as many carbon atoms as there are magnesium atoms. We can work out the formula mass of a compound e.g. Carbon dioxide is the mass of one carbon plus the masses of two oxygens or 12+16+16=44.

Unbalanced equation

The number of electrons around an atom is the same as the number of protons in the nucleus. Do not forget that if you are talking about an ion, then these numbers will be different as there will be an imbalance of charges making the ion have a change. Above is a Sodium atom, it shows the 11 electrons in the correct orbits as 2:8:1. If the atom reacted, it would give the outer electron to a non metal e.g. Chlorine leaving Sodium with a charge of +1.

Most substances are made from two or more elements chemically combined, these are called compounds. Examples include water (hydrogen and oxygen) and ammonia (hydrogen and nitrogen). If the elements are not chemically combined, then it is simply a mixture like air (mixture of oxygen, nitrogen, argon and many others) or rock salt (various salts and rock fragments). Compounds are formed when the atoms react and either transfer electrons from one to another or they share them within a bond.

Balanced equation

Bonding: If non metals react with metals, the metal will lose the outer electron or electrons making it a positive ion. The non metal will take in those electrons to fill its outer shell and make it a negative ion. All ions will have a full outer shell. The solid, Sodium Chloride is made up of Sodium +1 ions sitting next to Chlorine -1 ions. The ions in the salt are very stable which is why it has such a high melting temperature. The next part is not as simple and even causes dread in some people and that is the balancing of equations.

Above, I have put a simple equation for the combustion of methane. As we know that atoms do not just vanish or pop into existence, the number of atoms of a specific type must be the same on both sides of an equation. Look at the example and see how I have counted them in this unbalanced equation.
Through practice and experience you will get better at this but for now, just try doubling the quantity of certain molecules and recalculating the number of atoms to see what effect it has. You will soon get the answer as shown on the left.

Conservation of mass. If you add the combined mass of methane and oxygen in the above equation, it will be equal to the combined mass of carbon dioxide and water. The mass in is equal to the mass out. In an exam, they may tell you the combined mass of methane and oxygen then tell you the mass of water. To work out the mass of carbon dioxide, simply take the mass of water away from the combined mass of methane and oxygen. Check that both sides of the arrow add up and you are there!

Now that you are aware of the differences between elements, compounds and mixtures, we need to know ways to separate them. Some of obvious like filtering and using a magnet, but there are also processes like:
Evaporation to remove water from a solution.
Distillation to separate two liquids by their boiling temperature and;
Chromatography which separates pigments by how soluble they are when moving up chromatography paper.

For the final part of this section, you need to be aware of how the periodic table was developed. Many scientists in history had noticed patterns in the elements but the main ones to note were: Dalton, Meyer, Newlands and Mendeleyev.
You need to be aware of the properties and reactions of group 1, group 7 and group 0.

If you are studying seperate sciences, you will also need to know about the transition metals.

Difficult first topic but essential to getting you ready for the rest of the course.

Key words and terms for this topic: proton, neutron, atomic mass, mass number, electron, nucleus, element, compound, shell, electronic structure, ionic bond, colvalent bond, product, reactant.

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