Atomic Structure and the Periodic Table

Atoms have a radius of about 0.1 nanometres in the middle of the atom containing protons and nuetrons with a positive charge. This is where almost the whole mass of the atom is concentrated. The electrons move around in shells around the nucleus and cover alot of space even though they are tiny. The volume of their orbits determnes the size of the atom.

Atoms are nuetral as they have an equal number of protons and neutrons. The atomic number tells you how many protons there are and the atomic mass tells you the mass of the nucleus (protons + neutrons).

The number of protons determines what type of atom it is. A substance with all the same atoms is called an element. There are just over 100 elements in the periodic table.

Element symbols is a type of shorthand, saving time and making the elements universal.

Isotopes are different forms of the same element with the same number of protons but a different number of neutrons. Therefore, they have the same atomic number but different mass numbers. Relative atomic mass is used to show an average mass taking acount the different masses and abundance of all the isotpes that make up the element. This is found using the formula:

Ar = sum of (isotope abundance x isotope mass number) / sum of abundance of all the isotopes.

Compounds are substances formed by two or more elements, the atoms of each are fixed in proportions throughout the compound and they are held together by chemical bonds. This involves atoms sharing, taking or giving electrons. It is often difficult to seperate the original elemts out of a compund and a chemical reaction is needed.

Ionic bonding - metal and non-metal. The metal loses electrons to become positive and the non-metal gains electrons to become negative. The opposite charges means they're strongly attracted to each other. For example, sodium chloride, magnesium oxide and calcium oxide.

Covalent bonding - non-metals. Each atoms shares an electron and becomes a molecule. For example, carbon monoxide and water.

The properties of a compound are usually very different to the element, for example iron sulfide. Some common formulas include:

Carbon dioxide = CO2   Ammonia = NH3   Water = H2O   Sodium chloride = NaCl   Sulfuric acid = H2SO4   Sodium carbonate = Na2CO3   Calcium chlorideCaCl2   Carbon monoxide = CO Nitric acid = HNO3 Hydrochloric acid = HCl

John Dalton - Described atoms as solid spheres made up of different elements.

JJ Thomson - Measurements of the charge and mass showed that an atom must contain smaller negatively charged particals - electrons. Proposed the "plum pudding model", showing the atom as a ball of positive charge with electrons in it.

Ernest Rutherford - Conducted the alpha partical scattering experiment where they fired alpha particals at a thin sheet of gold. They were expecting the particals to pass straight through or be slightly defected but some were largely deflected and some were deflected backwards. Therefore he proposed the nuclear model - a tiny, positively charged nucleus is in the centre where most of the mass is concentrated and a "cloud" of negativly charged electrons surround it, therfore most of the atom is empty space.

Niel Bohr - Realised that a "cloud" of electrons would be attracted to the nucleus and collapse. Therfore proposed that electrons orbit the nucleus in fixed shellswhich are a fixed distance from the nucleus.

Further experiments by Rutherford and others showed that the nucleus could be divided into smaller particals which have the same charge as a hydrogen nucleaus - protons.

James Chadwick - Conducted an experiment which proved there were neutral particals in the nucleus - nuetrons.

Before the knowledge of protons, neutrons and electrons the known elements were organised in order of atomic mass. When this was done, a periodic pattern was noticed and the first periodic table was formed. However, early periodic tales were not compleate and some elements were placed in the wrong group as they were placed in order of mass, not properties.

Dmitri Mendeleev took the 50 known elements and arranged them in a table in order of mass, however he switched the order if the properties meant it should be changed and left gaps to ensure elements with similar properties stayed in the same group. Therfore, when newly discovered elements fitted the pattern and properties he predicted, it helped confirm his ideas. The discovery of isotopes in the early 20th century confirmed that Mendeleev was correct not to place elements in a strict order but to take account of their properties. Isotopes of the same element have different atomic masses but have the same chemicl properties so occupy the same position on the periodic table.