Download Lesson 7.8 Basic Properties of the Main Group Elements Suggested

Lesson 7.8 Basic Properties of the Main Group Elements
Suggested Reading

Zumdahl Chapter 7 Section 7.13
Essential Question

What are the basic properties of the main group elements?
Learning Objectives


Describe the basic characteristics of the main-group elements.
Discuss the similarities and differences in the chemical properties of
elements in the same group.
Introduction
In this lesson we will look at the periodicity of chemical properties of the
main-group elements (s & p block elements). However, before we can do
this we must review the metallic character of the elements and the
properties of oxides.
Metallic Character
When looking at the basic characteristics of the main-group elements, it is
important to recall how metallic/nonmetallic character of the elements
changes going through the periodic table. Metallic elements lie to the left of
the "staircase" (amphoteric line), nonmetals lie to the right and the
metalloids (with intermediate characteristics) lie along the staircase. Thus
the elements tend to increase in metallic character in going from right to left
in any period.
The elements also tend to increase in metallic character going down any
column of elements. Some groups show this tendency more clearly than
others. For example, Group IVA starts with carbon (nonmetal), and goes to
silicon and germanium (metalloids), and concludes with tin and lead
(metal).
The trends in metallic/nonmetallic character should roughly follow the
trends in ionization energies. The situation with electron affinities is less
straightforward, but you can say that the reactive nonmetals have large
negative electron affinities. Recall from the previous lesson that large
negative electron affinities indicate a very stable negative ion is formed.
These substances are the elements in Groups VIA and VIIA.
Although the elements in a given group are expected to be similar, the
degree of similarity varies. The alkali metal elements show marked
similarities to one another, as do the halogens. On the other hand, the
Group IVA elements range from a nonmetal at the top of the column to a
metal at the bottom. The periodic table helps you to understand these
systematic changes.
Most main-group elements from compounds with oxygen. Because the
basic/acidic behavior of the oxides is a good indicator of the
metallic/nonmetallic character of the elements, we need to look at the
general behavior of the oxides.
Basic and Acidic Oxides
Oxides can be divided into two main types, basic oxides and acidic oxides,
depending on their reaction with acids and bases. A basic oxide (basic
anhydride) is an oxide that reacts with acids. Most metal oxides are basic
oxides. The soluble ionic oxides give basic solutions with water to give the
OH- ion and therefore a basic solution (According to the solubility rules, all
oxides are insoluble EXCEPT those of calcium, barium and Alkali metal
(Group I) cations.).
CaO(s) + H2O(l) → Ca2+(aq) + 2OH-(aq)
Many metal oxides are not soluble in water but do react with acids; they are
therefore basic oxides. Nickel(II) oxide, NiO, is an example.
NiO(s) + 2H+(aq) → Ni2+(aq) + 2H2O(aq)
These equations provide an excellent example of our different views of
bases. Note how the definition of an Arrhenius base applies to the the first
reaction while the Bronsted-Lowry definition applies to the second
equation.
An acidic oxide (or acidic anhydride) is an oxide that reacts with bases.
Most nonmetal oxides are acidic oxides. The soluble acidic oxides dissolve
in water to produce acidic solutions. Carbon dioxide, for example, dissolves
in water to give unstable carbonic acid, (H2CO3).
CO2(g) + H2O(l) ⇌ H2CO3(aq)
Silicon dioxide (silica, glass) is an inert solid in water, but when fused
(melted) with a base or basic oxide, it gives a salt, and so it acts like an
acidic oxide. For example,
SiO2(s) + CaO(s) + ∆ → CaSiO3(l) (calcium silicate)
Now that we have reviewed metallic character and oxides, we can discuss
the main-group elements. We will look at some of the major characteristic
of these elements, paying particular attention to the oxides. In the case of
the nonmetals, we will also examine the compounds with hydrogen. You
may want to make flash cards to help you remember the major
characteristics of the elements we are about to discuss. You will be tested
on these characteristics on in class, AP, and IB exams.
Hydrogen (1s1)
Although he electron configuration of hydrogen is like that of the alkali
metals, its physical and chemical properties are quite different. The
element is a colorless, odorless gas composed of H2 molecules. It is not
reactive with water but does burn in oxygen.
2H2(g) + O2(g) → 2H2O(g)
At very high pressures, however, hydrogen is believed to have metallic
properties.
Group IA Elements, the Alkali Metals (ns1)
YouTube Video
The alkali metals are soft and reactive. All react with water. For example,
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
The reactivity increases going down the group of elements. When a small
piece of lithium is added to water, it moves about the surface, evolving
hydrogen. Sodium reacts similarly, though hydrogen is evolved more
rapidly. The release of heat is also noticeable; it may ignite the hydrogen!
In the case of potassium, the reaction is quite vigorous, and the heat of
reaction ignites the hydrogen to produce a flame. All alkali metals can form
basic oxides with the general formula R2O.
Watch this You Tube video
https://www.youtube.com/watch?v=l9z5-mJ8NZk
Group IIA Elements, the Alkaline Earth Metals (ns2)
The alkaline earth metals are also reactive, but much less so than the alkali
metals. Reactivity increases going down a group. Beryllium shows no
reaction with water while magnesium reacts very slowly. Calcium,
strontium, and barium react readily with water.
Ca(s) + 2H2O(l) → 2Ca(OH)2(aq) + H2(g)
All alkaline earth elements form basic oxides with the general formula RO.
Group IIIA Elements (ns2np1)
The Group IA and Group IIA elements are all relatively reactive metals, and
there is only a slight variation in metallic character. Within the group IIIA
elements, we see a significant increase in metallic character going down
the column of elements. The first Group IIIA element, boron, is a metalloid
having several allotropes, or different forms of the element. In one form,
boron is a black, crystalline substance with a metallic luster. Boric acid,
(B(OH)3 or H3BO3), is a common boron compound used as a mild
antiseptic. Heating boric acid yields boric oxide, B2O3, an acidic oxide used
to make heat-resistant glass (Pyrex).
Other elements in this group are metals and form oxides with the general
formula R2O3. Aluminum is a common structural material. Aluminum oxide,
Al2O3, is an amphoteric oxide. An amphoteric substance is a substance
that has both acidic and basic properties. Aluminum oxide dissolves in
acids to produce aluminum ion, as expected for a basic oxide.
Al2O3(s) + 6H+(aq) → 2Al3+(aq) + 3H2O(l)
The oxide also dissolves in strong bases, as expected for an acidic oxide.
Al2O3(s) + 3H2O(l) + 2OH-(aq) → 2Al(OH)4-(aq)
The metal underneath aluminum is gallium, which metals when held in the
palm of your hand. Gallium forms the amphoteric oxide, Ga2O3. Indium and
thallium are metals with basic oxides, In2O3 and Tl2O3. Note the trend in the
Group IIIA elements from acidic to amphoteric to basic oxides, which
follows the trend of greater metallic character as we go down a column.
Group IVA Elements (ns2np2)
YouTube Video
We have already mentioned the distinct trend from nonmetal to metal going
down the Group IVA elements. Carbon Exists in two well-known allotropic
forms: graphite, a soft, black substance used in pencil leads and diamond,
a very hard, clear, crystalline substance. Another allotrope a carbon is a
molecule buckministerfullerene,C60, which has a cage-like fused ring
structures that form buckyballs and nanotubes.
Both tin and lead have been known and used by humans since ancient
times. Tin is alloyed, or mixed with copper to make bronze, which contains
about 90% copper and 10% tin. Some believe that lead acetate, which was
used as a sweetener by ancient Romans, brought down the entire empire!
Here is a link to in article about this (optional, but interesting, reading).
All of the elements in this group form oxides with the general formula, RO2.
Carbon dioxide is a gas and an acidic oxide. Carbon also forms a stable
monoxide, CO. Silicon dioxide, SiO2, an acidic oxide, exists as quartz and
with sand (particles of quartz). Amphoteric tin dioxide, SnO2, is found in
the mineral cassiterite, the principle ore of tin. Lead forms, PbO2, and the
monoxide PbO, which are both amphoteric oxides. Lead monoxide is more
stable.
Watch this You Tube video
https://www.youtube.com/watch?v=Kl083LAYnoU
Group VA Elements (ns2np3)
Group VA elements also show the transition from nonmetal (nitrogen, N,
and phosphorus, P), to metalloid (arsenic, As, and antimony, Sb) to metal
(bismuth, Bi). Nitrogen occurs as a colorless, odorless gas with N2
molecules. It is a relatively unreactive element. White phosphorus is a
white, waxy solid with P4 molecules. It is quite reactive and is normally
stored under water. In air it bursts into flame reacting with O2 to give the
acidic oxides P4O6 and P4O10. Other allotropes, red and black phosphorus
are much less reactive. Grey arsenic, the common form of the element, is a
brittle solid with metallic luster. Yellow arsenic is an unstable, crystalline
solid composed of As4 molecules. Antimony is a brittle solid with a silvery,
metallic luster. Bismuth is a hard lustrous metal with a pinkish tinge.
Group VA elements form oxides with empirical formulas R2O3 and R2O5. In
some cases the molecular formulas are twice these formulas, as in the
case of phosphorus. Nitrogen has the acidic oxides N2O3 and N2O5,
although it has other better known oxides. Nitric oxide, NO, is a gas
produced as a byproduct of combustion when nitrogen reacts with
oxygen. Many people take supplements of this oxide, because is also an
important cellular signaling molecule is mammals. Nitric oxide combines
readily with more oxygen to give nitrogen dioxide, NO2, a brown gas.
N2(g) + O2(g) → 2NO(g)
2NO(g) + O2(g) → 2NO2(g)
Group VIA Elements (ns2np4)
These elements show transition from nonmetal (oxygen, O, sulfur, S, and
selenium, Se) to metalloid (tellurium, Te) to metal (polonium, Po). Oxygen
occurs as a colorless, odorless gas with O2 molecules. It also has an
allotrope, ozone, of molecular formula O3. Sulfur is a brittle, yellow solid
with molecular formula, S8. It burns in air to produce the gas sulfur dioxide,
SO2. and traces of sulfur trioxide, SO3. Both oxides are acidic.
Selenium exists in two allotropic forms, red and gray. Red selenium is a
solid composed of Se8 molecules. Gray selenium is a brittle solid that
becomes electrically conductive with light shines on it (It has been used to
construct devices for measuring light intensity). Selenium forms the acidic
oxides SeO2 and SeO3. Selenium dioxide is a white solid that forms when
selenium is burned in air. Tellurium is a white, metallic looking substance; it
is brittle and a weak electrical conductor. Polonium is a radioactive metal.
Group VIA elements form compound with hydrogen of the form H2R; the
best know is water, H2O. Hydrogen sulfide or hydrosulfuric acid, H2S, is a
poisonous gas with the odor of rotten eggs.
Group VIIA Elements, the Halogens (ns2np5)
The halogens are reactive nonmetals with the general molecular formula
X2, where X symbolizes a halogen. Fluorine, F2, is a pale yellow gas;
chlorine, Cl2, a pale greenish-yellow gas; bromine, Br2 , a reddish-brown
liquid; and iodine, I2, a bluish-black solid that gives off violet vapor. Little is
known about astatine, At, because it is a synthetic elements and is
radioactive. It is expected be more metallic than iodine and is perhaps a
metalloid. Each halogen forms several compounds with oxygen, but these
are unstable. The halogens form compounds with hydrogen with the
general formula HX. Hydrogen fluoride is a liquid (B.P. 20∘C); the other
binary compounds with hydrogen, called hydrogen hallides, are gases. All
are corrosive and dissolve in water to give solutions known generally as
hydrohalic acids. For example, hydrogen chloride (HCl) dissolves in water
to give hydrochloric acid.
Group VIIIA Elements, the Noble Gases (ns2np6)
Group VIIIA elements exist as gases consisting of uncombined atoms such
as helium, He. For a long time these elements were thought to be
chemically inert, because no compounds were known. In the early 1960's
several compounds of xenon were formed. Now compounds are also
known for krypton and radon. However, these elements re knows as noble
gases because the their relative unreactivity. They are also called rare
gases or inert gases.
Homework:
Book questions pg. 324 questions 85, 87, ,89, 90, 92, 95, 97
Practice exercises 7.1-7.7