SQA Higher Chemistry

## The periodic table The elements are arranged in order of increasing **atomic number**, which is the number of protons in the nucleus. Because the arrangement follows the electron arrangement of the atoms, an element's position lets a chemist predict its physical properties and its chemical behaviour, even for an element that has never been handled. A **group** is a vertical column. The elements in a group have similar chemical properties because their atoms have the same number of electrons in the outer shell. The alkali metals (group 1) all have one outer electron, and all form a $1+$ ion by losing it; the halogens (group 7) all have seven outer electrons, and all tend to gain one more. A **period** is a horizontal row. Moving from left to right across a period the atomic number rises, so the number of outer electrons rises by one at each step, and the elements change from metallic to non-metallic. Sodium to argon, for example, runs from a reactive metal with one outer electron, through a semi-metal (silicon), to reactive non-metals (phosphorus, sulfur, chlorine) and finally an unreactive monatomic gas with a full outer shell. ## Bonding and structure in the first 20 elements Each of the first 20 elements falls into one of four categories, and the category decides its physical properties. | Category | Elements | Particles and bonding | |---|---|---| | Metallic | Li, Be, Na, Mg, Al, K, Ca | a lattice of positive ions in a sea of delocalised electrons | | Covalent…