- The Russian Mendeleev created the periodic table in the 19th century, using "atomic weights" (now called atomic masses), arranging them in order of increasing atomic weight but sometimes breaking the rules so that elements with different properties were lined up.
- Mendeleev also left gaps where he thought elements should exist which had not yet been discovered, and predicted their properties. For example, he predicted that "eka-aluminium" existed with an atominc mass of about 68; when gallum, with its mass of 70, was discovered, it turned out to be very similar to his prediction.
C2.2: Structure of the atom:
- The atom consists of positively-charged protons with a mass of 1 and neutrons with no charge and a mass of 1 in the nucleus, and electrons with negliable mass and negative charge surrounding the nucleus in shells. All atoms contain the same number of protons and electrons and thus have no charge overall, and all atoms of an element contain the same number of protons.
C2.3: The modern periodic table:
- The modern periodic table consists of columns called groups and rows called periods. The elements in each group have the same number of electrons in their outermost shell; the elements in each period have the same number of electron shells. An element's atomic number is its number of protons (and thus also its number of electrons); its mass number is its number of protons and neutrons.
- To measure the mass of an atom we use relative atomic mass (Ar): this is the mass of an atom relative to the mass of carbon-12, which is exactly 12.
- Isotopes are forms of an element with different numbers of neutrons. The relative atomic mass can be calculated by comparing the relative abundance of isotopes. For example, chlorine-35 has a relative abundance of 75% whilst that of chlorine-37 is 25%: the relative atomic mass of chlorine is thus ((35 * 75) + (37 * 25)) / 100 = 35.5.
C2.4: Electron shells:
- Electrons are arranged in shells around atoms; the first shell can hold two electrons, subsequent shells can hold eight. The way in which an atom's electrons are organised is called its electronic configuration.
C2.5: Ionic bonding:
- An ion is an atom or group of atoms which has charge. Metal atoms lose electrons and become postive cations; non-metals gain electrons and become anions.
- Ionic bonding occurs because atoms with incomplete outermost shells gain or lose electrons in reactions. Take, for example, magnesium chloride. Magnesium (Mg) is in group 2 and thus loses 2 electrons, thus gaining a charge of 2+. Chlorine (which changes its name as it is the non-metal) is in group 6 and thus loses one electron, gaining a charge of -. In order for the magnesium ions and chloride ions to form magnesium chloride, the charges of the ions must sum to 0, and so there must be two chloride ions for each magnesium ion. Thus the formula of mangesium chloride is MgCl2.
C2.6: Ionic compounds:
- As mentioned about, ionic…