How are electrons arranged in atoms?
Each group number will tell us how many electrons are in the outermost shell of the atom. A shell is also known as an energy level. Each electron in an atom is in an energy level. The period an element is in will tell us how many energy levels are in that atom. For example, take oxygen. Oxygen is in group 6, and so therefore has 6 electrons in its outer shell, also known as its highest energy level. Oxygen is in period 2 and so has 2 energy levels/ shells. Electrons begin arrangement in the lowest energy level closest to the nucleus, thus they occupy the lowest possible energy level. The lowest energy level can only hold a maximum of 2 electrons. The following energy levels can hold up to 8 electrons.
Atoms in group 8 (also called group 0) have 8 electrons in their highest energy level, and these are called noble gases. Noble gases are unreactive and highly stable. Other atoms aim to achieve noble gas configuration, and they do so by reacting with other atoms. For example, sodium, a group 1 element, can react with a group 7 element, like chlorine to achieve 8 electrons in the outer shell. Sodium chloride is made, this is salt. Sodium is a metal and chlorine is a non-metal. When a metal reacts with a non-metal, ions are formed. Ions are charged atoms. The non-metal atom gains an electron from the metal, so the metal loses an electron. Both sodium and chlorine now have 8 electrons in their outer shell, but as sodium lost an electron, it has a positive charge (shown with a + sign) and chlorine now has a negative charge (-). Oppositely charged ions attract each other to form ionic bonds which are very strong intermolecular bonds. Ionic bonds can only form between a non-metal and a metal; not two metals or two non-metals. Look at the periodic table - can you work out some more ionic bonds?
Here’s some more examples:
How are each formed?
Potassium (group 1) reacts with bromine (group 7)
Sodium (group 1) reacts with fluorine (group 7)
Magnesium (group 2) reacts with oxygen (group 6)
So how do we know which are metals and which are non-metals?
A lot of periodic tables will have a divide to indicate metals from non-metals, the divide begins on the right side of the table. Group 1 and 2 elements are alkali metals. If you wish to impress your teacher, they are called s block elements (this is A level content; you do not need to know about s block elements). The elements between group 2 and 3 which do not belong to a group are the transition metals. There are a few metals in group 3,4,5 and 6, less as you ascend in groups. All group 7 and group 0 elements are non-metals. But as you progress in the course, it is likely you will start to remember the important elements needed for your course and so will most likely remember roughly where they are on the periodic table!