Atoms and Molecules Cheat Sheet (DRAFT) by Ayesha Talib

law of conser­vation of mass: mass can neither be created nor be destroyed in a chemical reaction. [Antoine Lavois­ier]; atoms are indivi­sible particles, which can neither be created nor destroyed in a chemical reaction.
law of constant proportion: in a chemical substance the elements are always present in definite propor­tions by mass. [Joseph Proust]; the relative number and kinds of atoms are constant in a given compound.

atoms are smaller than anything we can imagine. atomic radius is measured in nm.
1/10⁹m = 1nm; 1m = 10⁹nm

atomic mass unit is a mass unit equal to exactly 1/12^th the mass of one carbon-12 atom.
atomic mass of an element is the relative mass of its atom as compared with the mass of a carbon-12 atom.
relative atomic mass is the ratio of the average mass of atoms of an element to 1/12^th the mass of a carbon-12 atom.

molecules are the smallest particle of an elemen­t/c­ompound which can exist indepe­ndently and show all the properties of that substance. molecules constitute same type of atoms. it may be monoat­omic, diatomic, polyat­omic. they join together in definite propor­tions.
number of atoms consti­tuting a molecule is known as its atomicity. ex. Ar: monoatomic

compounds composed of metals and non-metals contain charged species. the charged species are called ions. anion: -vely charged; cation: +vely charged. ex. Na⁺ Cl^-.
a group of atoms carrying a charge is known as a polyatomic ion. ex. Ammonium ion - NH

4

; Carbonate ion: CO

3

².

combining capacity of an element. can be used to find out how the atoms of an element will combine with the atom[s] of a compound.

sum of atomic masses of all the atoms in a molecule of the substance. expressed in [u].

sum of atomic masses of all atoms in a formula unit of a compound. expressed in [u].