Pure Substances

pure substance

 a sample of matter, either a single element or a single compound, that has definite chemical and physical properties. 

elements

pure substances that only contain one kind of matter.

Elements cannot be

 

separated or broken down into simpler substances by physical or chemical means. 

Each element has its

own unique set of physical and chemical properties. 

An atom is the smallest unit of an element that

t maintains the properties of that element

Matter exists in many different forms but there are only

118 types of atoms. 

On Earth, more than ___ elements occur naturally.

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There are also several elements that have been developed by

scientists. 

Atoms are joined together to make up all the

 

 different kinds of matter. 

Each element is represented by a

 

distinct name and chemical symbol. 

chemical symbols are made up of

1 or 2 letters

if there are two letters the

1^st one is always capital, 2^nd one always is a lowercase).

The periodic table organizes the elements into a grid of

periods and groups/families

periods

horizontal rows

groups or families. 

vertical columns

Elements in the same group have similar

chemical and physical properties. 

The table is called periodic because the

pattern of similar properties repeats from period to period. 

diatomic elements

 

elements that naturally exist as molecules of two atoms of the same element bonded together, rather than as single atoms.

Diatomic elements exist as

 

two atoms of the same element joined together.  

Allotropes are

different forms of the same element, in the same state. The properties of allotropes vary widely.  

The properties of allotropes

vary widely.  

example of allotropes

 Oxygen (O2): colorless and odorless  

Ozone (O3): pale blue, toxic 

 

compounds

Pure substances that are not elements are compounds.  

 

Compounds are composed of

more than one kind of atom combined in a definite ratio.

 

example of a compounds

carbon dioxide (CO2). 

There may be easier ways of preparing them, but compounds can be made from

their elements, and can be broken down into their elements,

compounds can be made or broken by using 

external energy such as heat or electricity. (ex. Electrolysis) 

Because every compound is made up of the same kinds of atoms arranged the same way, a compound has

characteristic properties and composition.  

 

Most matter in the universe exists as  

 

compounds

Molecules are composed of

two or more atoms,

unlike covalent compounds, molecules can be of the

same kind, the substance is then classified as “element”. 

Compounds can be represented by

 

an abbreviation or formula that has subscripts

the subscripts written with the compounds formula represents

 the ratio of different atoms in the compound. 

A structural formula shows how the

atoms are connected.  

A ball-and-stick model shows the distances between

atoms and the angles between them in three dimensions. 

A space-filling model attempts to represent the

actual sizes of the atoms and not just their relative positions.  

The properties of a compound are different from its

 component elements. 

Eg: When water is broken down, its  components—hydrogen and oxygen—are  dramatically .  

 

different than the liquid they form when combined

The law of definite proportions states

that a compound is always composed of the same elements in the same proportion by mass, no matter how large or small the sample.  

the key idea for the law of definite proportions

A compound has a fixed composition 

 

 

Example: 

Water (H₂O) always has hydrogen and oxygen in a mass ratio of 1:8 

2 g of H combines with 16 g of O  

 → ratio = 1:8 

the ratio of the two compounds when divdided

has to be the same for each element

No matter if water comes from a river, lab, or rain

this ratio never changes 

The law of multiple proportions states that

when different compounds are formed by a combination of the same elements, different masses of one element combine with the same fixed mass of the other element in a ratio of small whole numbers. 

Key idea for law of multiple proportions

When the same elements form multiple compounds, their combining masses are in simple ratios 

example Carbon (C) and Oxygen (O) form two compounds: 

CO (Carbon monoxide) → 12 g of C combines with 16 g of O. 

CO₂ (Carbon dioxide) → 12 g of C combines with 32 g of O. 

  

Now compare oxygen masses for the same 12 g of carbon: 

Ratio of O in CO: CO₂ = 16: 32 = 1 : 2 (a simple whole number ratio). 

 

the main aspect for the law of multiple proportions is that

one element aspect is equilavent in the ratio

• Each compound contains the same elements. 

• The mass of hydrogen is the same in both compounds. 

• The mass of oxygen in hydrogen peroxide compared to water gives a 2:1 ratio.