atoms and molecules

atoms

• an atom is an entity that is electrically neutral and is made up of a positively charged nucleus (protons and neutrons) with negatively charged electrons moving around them nucleus.
• atoms have a mass that is mainly contributed by the mass of the nucleus as protons and neutrons are much heavier than electrons
• basic unit of an element, a form of matter which may not be further broken down into any other simpler substances using any chemical means

sub-atomic particles

• proton: has a relative electric charge of +1, and its relative mass is 1
• neutron: has a relative electric charge of 0, and its relative mass is 1
• electron: has a relative electric charge of -1, and its relative mass is 1/1840

in the periodic table

• proton number/atomic number is the number of protons found in an atom
• nucleon number/mass number is the total number of protons and neutrons found in the nucleus of an atom

molecules

• a group of two or more atoms that are chemically combined

most non-metal elements and some compounds are made up of molecules

arrangement of electrons in an atom

• electrons move around the nucleus in regions known as electron shells
• first shell (closest to the nucleus) holds a maximum of 2 electrons and is always filled first when drawing a diagram
• second shell can hold up to 8 electrons
• third shell can usually hold up to 8 electrons and is filled up after the 2nd shell
• electronic configuration/structure shows the arrangement of electrons in an atom (eg 2.8.8)
• valence shell is the shell that is furthest away from the nucleus of the atom

group number

• elements with the same number of valence electrons belong to the same group in the periodic table
• number of valence electrons = group number

cations and anions

• cations—when the valence electrons are removed
• anions—when a valence electron is added