Ch 4 elements_compounds_and_mixtures

Elements, Compounds and Mixtures

ELEMENTS

I: Elements

• Element: a pure substance that cannot be separated into simpler substances by physical or chemical means. They contain only one type of particle

• Pure substance: contains only one type of particle

• Meet the Giants

Every Element has a Unique Set of Properties

• Each element can be identified by it’s characteristic properties

Review

• Turn to your partner and tell them what a characteristic property is and give one example of a characteristic property

Characteristic Properties are:

• Properties that do not change regardless of the sample size

• Some examples are boiling point, melting point and density (physical properties) and reactivity with an acid (chemical property).

Elements Are Classified by Their Properties

• Elements are grouped into categories according to their shared properties

• Examples: iron, nickel and cobaltAll three are shiny, all conduct thermal

energy and electric current so all three are grouped together in a single group called metals

If You Know the Category, You Know the Properties!

• If the category is metals, what are the properties?

• Hint: we just talked about them

If You Know the Category, You Know the Properties!

• Elements are classified into three categories: metals, nonmetals, and metalloids

Metals Metals account for about two

thirds of all the elements and about 24% of

the mass of the planet. Metals

have useful properties including strength,

ductility, high melting points,

thermal and electrical

conductivity, and toughness.

Non-metals

Oxygen atomSulfur

Non-metals tend to be brittle and non-conductive of heat or

electricity

Metalloids

(Si) Silicon is a metalloid

Metalloids are also semiconductors

Summarize: which have the following properties?

• Which are good conductors of electric current?

• Brittle and nonmalleable?• Shiny?• Poor conductors of thermal energy?

• Which are good conductors of electric current?

Metals and some metalloids• Brittle and nonmalleable?

nonmetals• Shiny?

metals and some metalloids• Poor conductors of thermal energy?

nonmetals and some metalloids

II: Compounds

• Few elements found in nature are found alone; most elements are found combined with other elements as compounds

• A compound is a pure substance composed of two or more elements that are chemically combined

• In an compound, a particle is formed when atoms from two or more elements join

• In order for atoms to join, they must react or undergo a chemical change with one another

• The compound is a new pure substance that is different from the elements that reacted to form it

Some common compounds:

• Insert table p 86

Elements Combine in a Definite Ratio to Form a Compound

• Compounds form in a specific ratio according to their mass

• For example, in water, the mass ratio of hydrogen to oxygen is always 1:8

Every sample of water has this mass ratio so if the sample has a different mass ratio it cannot possibly be water.

Every Compound Has a Unique Set of Properties

• Each compound has its own unique set of properties such as boiling point, melting point, density and color.

• Compounds can also be identified by chemical properties

Every Compound Has a Unique Set of Properties

A compound has different properties from the elements that form it.

NaCl

Every Compound Has a Unique Set of Properties

Physical properties of sodiumStandard state: solid at 298 K Color: silvery white

Every Compound Has a Unique Set of Properties

Chlorine is a greenish yellow gas which combines directly with nearly all elements. Chlorine is a respiratory irritant. The gas irritates the mucous membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. It was used as a war gas in 1915. It is not found in a free state in nature, but is found commonly as NaCl (solid or seawater).

Sodium chloride

GeneralSystematic name

Sodium chloride

Other names Common salt,halite,table salt

Chemical formula

NaCl

Molar mass 58.442 g/mol

Appearance white and crystalized

Properties

Density and phase

2.16 g/cm³, solid

Solubility in water

35.9 g/100 ml (25 °C)

Melting point 801 °C (1074 K)

Boiling point 1465 °C (1738 K)

GeneralName, symbol, number

sodium, Na, 11

Chemical series alkali metals

Group, period, block

1, 3, s

Appearance silvery white           

Standard atomic weight

22.98976928(2) g·mol−1

Electrons per shell

2, 8, 1

Physical propertiesPhase solid

Density 0.968 g·cm−3

Liquid density 0.927 g·cm−3

Melting point 370.87 K(97.72 °C, 207.9 °F)

Boiling point 1156 K(883 °C, 1621 °F)

GeneralName, symbol, number

chlorine, Cl, 17

Chemical series

halogens

Group, period, block

17, 3, p

Appearance yellowish green             

Standard atomic weight

35.453(2) g·mol−1

Electrons per shell

2, 8, 7

Physical properties

Phase gas

Density (0 °C, 101.325 kPa)3.2 g/L

Melting point 171.6 K(-101.5 °C, -150.7 °F)

Boiling point 239.11 K(-34.04 °C, -29.27 °F)

Compounds Can Be Broken Down Into Simpler Substances

Some compounds can be broken down into elements through chemical changes

NaCl can be broken back down into their original pure substances by adding an electric current to the compound.

Adding electric current to break compounds and elements is called electrolysis.

Compounds Can Be Broken Down Into Simpler Substances

Carbonic acid (or the “stuff” that gives soda its fizz, can be broken down into Carbon Dioxide and Water

These can be further broken down into carbon, oxygen, and hydrogen through additional chemical change

Compounds Can Be Broken Down Into Simpler Substances

Compounds cannot be broken down by physical change

Compounds in Your World

• In nature: remember the bacteria that live as nodules in the roots of plants that help (last year, nitrogen cycle) the plant absorb the usable nitrogen compounds from the soil (nitrogen fixation) to transfer the nitrogen from the ground to the plant?

Compounds in Your World

The element nitrogen is also combined in industry with hydrogen to make ammonia. Plants can also use this as a source of nitrogen when used in combination with fertilizers

Let’s Check Understanding• How are compounds and elements alike?Different?

Both are pure substances , but elements cannot be broken down into simpler substances; compounds can be broken down into simpler substances

Let’s Check Understanding

• What are two ways to break down a compound?

Heating and electrolysis

Let’s Check Understanding• How do the properties of sodium chloride compare

with the properties of sodium and of chloride?

Sodium Chloride is a white solid that dissolves in water and is safe to eat. Sodium is a soft, silvery white metal that reacts violently with water, and chlorine is a poisonous greenish, yellow gas.

Let’s Check Understanding

• What is a compound?

A compound is a pure substance composed of two or more elements that are chemically combined

Let’s Check Understanding• A jar contains samples of the elements carbon and oxygen.

Does the jar contain a compound? Support your response.

A jar containing samples of carbon and oxygen does not contain a compound because the two elements are not chemically combined.

• When magnesium burns in air the principal product is magnesium oxide, but some magnesium nitride is also formed.

• The equations for the reactions are • 2 Mg(s ) + O2(g ) --> 2 MgO(s ) • 3 Mg(s ) + N2(g ) --> Mg3N2(s )