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Cheatography

Atoms and Molecules Cheat Sheet (DRAFT) by

for students of grade 9

This is a draft cheat sheet. It is a work in progress and is not finished yet.

LAWS OF CHEMICAL COMBIN­ATION

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

atoms are smaller than anything we can imagine. atomic radius is measured in nm.
1/109m = 1nm; 1m = 109nm
IUPAC: Intern­ational Union of Pure and Applied Chemistry

TERMS: atoms

atomic mass unit is a mass unit equal to exactly 1/12th 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 1/12th the mass of a carbon-12 atom.

ATOMIC MASSES OF ELEMENTS

H
1
C
12
N
14
O
16
Na
23
Mg
24
S
32
Cl
35.5
Ca
40
 

TERMS: molecules

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

ATOMICITY OF ELEMENTS [non-m­etals]

Ar
monoatomic
He
monoatomic
O
diatomic
H
diatomic
N
diatomic
Cl
diatomic
PO
4
tetra-­atomic
S
poly-a­tomic

MOLECULES OF COMPOUNDS

COMPOUND
COMBINING ELEMENTS
RATIO BY MASS
water
H, O
1:8
ammonia
N, H
14:3
carbon dioxide
C, O
3:8
quick lime - calcium oxide - Ca, O
baking powder - sodium hydrogen carbonate - Na, H, C, O
potassium sulphate - K, S, O

IONS

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.
 

IONIC COMPOUNDS

IONIC COMPOUND
CONSTI­TUTING ELEMENTS
RATIO BY MASS
calcium oxide
Ca, O
5:2
magnesium sulphide
Mg, S
3:4
sodium chloride
Na, Cl
23:35.5

VALENCY

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.

MOLECULAR MASS

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

FORMULA UNIT MASS

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