Atomic Structure, Bonding and the Periodic Table
Atomic structure
An atom has protons and neutrons in the nucleus and electrons in shells. Atomic number = number of protons; mass number = protons + neutrons. Electrons fill shells 2, 8, 8. Isotopes have the same number of protons but different numbers of neutrons, so they have the same chemical properties but different masses. Relative atomic mass is the weighted average of isotope masses.
Bonding
Ionic bonding occurs between metals and non-metals: electrons are transferred, forming oppositely charged ions in a giant lattice. Ionic compounds have high melting points and conduct electricity when molten or dissolved. Covalent bonding occurs between non-metals: electron pairs are shared, forming simple molecules or giant covalent structures such as diamond. Metallic bonding has positive ions in a sea of delocalised electrons, which explains conduction and malleability.
The periodic table
Groups are vertical columns and show the number of outer electrons; periods are horizontal rows and show the number of shells. Group 1 contains reactive alkali metals, Group 7 the halogens, and Group 0 the unreactive noble gases. Reactivity increases down Group 1 and decreases down Group 7.
Worked example
Sodium has atomic number 11, so its electron arrangement is 2, 8, 1. It loses one outer electron to form Na+, with arrangement 2, 8. Chlorine has 17 electrons (2, 8, 7) and gains one to form Cl− with arrangement 2, 8, 8. The oppositely charged ions attract in sodium chloride.
Traps that cost marks
Mass number is not the same as relative atomic mass. Group number gives outer electrons; period number gives shells. Ionic substances do not conduct when solid because the ions are not free to move.