elements combine to form many substances, each with its
TRANSCRIPT
Elements combine to form many substances, each with its own set of properties.
General Outcome #2 – Explain, using the periodic table, how elements combine to form compounds, follow IUPAC guidelines for naming ionic compounds and simple molecular compounds
Explain the importance of and need for the IUPAC system of naming compounds, in terms of the work that scientists do and the need to communicate clearly and precisely
Explain, using the periodic table, how and why elements combine to form compounds in specific ratios
Predict formulas and write names for ionic and molecular compounds and common acids
Classify ionic and molecular compounds, acids and bases on the basis of their properties
Predict whether an ionic compound is relatively soluble in water, using a solubility chart
Relate the molecular structure of simple substances to their properties: conductivity, pH, solubility and state
Outline the issues related to personal and societal use of potentially toxic or hazardous compounds
1.3: Electrons and the Formation of Compounds
Did you know?? ◦ The blue color in blue jeans comes from the
pigment copper phthalocyanine, a synthetic pigment made from the elements copper, carbon, hydrogen and nitrogen
The component of all materials can be separated into about 115 basic building blocks called elements (substances that cannot be broken down into other substances)
All elements, based on their properties, can be divided into three classes: metals, non-metals and metalloids.
Most elements are metals and are silver or grey and shiny.
They are all good conductors of electricity and heat
They are also malleable (can be beaten or rolled into sheets without crumbling) and ductile (they can be stretched into long wires)
Most metals are solids at room temperature (25oC); mercury is the exception and melts at -39oC
Another variable characteristic is how strongly they react with other substances; some metals are highly reactive while others are inert (unreactive)
Consists of 17 elements Grouped together because of their lack of
resemblance to metals 11 are gases at 25oC, 5 are solids and 1,
bromine, is a red-brown liquid Tremendous variation in color and some exist in
different forms (eg. phosphorus has a red form and a white form)
Some are highly reactive (fluorine can etch glass) and some noble gases are unreactive
About half non-metals exist at 25oC are called molecules (group of non-metallic atoms bound together by covalent bonds eg. O2); others, however exist only as individual atoms
They have properties that are intermediate between metals and non-metals
eg. some metalloids conduct electricity but not very well
Organizes the elements according to their chemical properties
Metals are located on the left hand side and center and non-metals are on the far right (one exception is hydrogen because it behaves like a metal in chemical reactions)
Each horizontal line or row is a period (numbered 1 – 7)
Each vertical line or column is a group or family (numbered 1 – 18)
Chemical families are groups of elements that have similar chemical and physical properties
Alkali metals – group 1, far left;soft, shiny and silver in color and are reactive with water; their compounds tend to be white solids and are soluble in water
Alkaline-earth metals – group 2; shiny and silver but not as soft as alkali metals; compounds are white but are less soluble than compounds formed by alkali metals
Halogens – group 17; these elements are poisonous and react readily with the alkali metals to form salts (such as sodium chloride or table salt; salts are compounds produced in neutralization reactions between acids and bases)
Noble gases – group 18; very unreactive
Elements are made up of particles called atoms - are made up of three kinds of subatomic particles: electrons, protons, and neutrons
Recall that protons have a positive charge and neutrons have a neutral charge
The nucleus, made up of the protons and neutrons, make up 99.9% of the total mass of the atom however electrons (negative charge) occupy 99.9% of the volume of an atom
Energy level – region of space near the atom’s nucleus that may be empty or may contain electrons; electrons in energy levels (or shells) nearest the nucleus have the lowest energy but are the most tightly held in the atom because they are closest to the positive charged nucleus (opposite charges attract)
Each electron in an atom is in a particular energy level (or shell) and the electrons must occupy the lowest available energy level (or shell) available nearest the nucleus.
When the level is full, the next electron goes into the next highest level (shell) available.
There are rules about the maximum number of electrons allowed in each shell and you have to be able to work out the arrangements for the first 20 elements (see the Periodic Table diagrams further down). ◦ The 1st shell has a maximum of 2 electrons ◦ The 2nd shell has a maximum of 8 electrons ◦ The 3rd shell has a maximum of 8 electrons
This is only true up to atomic number 20, after that it is 18 for the 4th shell and is studied in advanced level chemistry
◦ The 19th and 20th electrons go into the 4th shell.
Particle Symbol Charge Mass Location
proton p+ 1+ 1.7×10-24g nucleus
neutron no 0 1.7×10-24g nucleus
electron e- 1- 9.1×10-28g surrounds the nucleus
The atomic number is the number of protons in the nucleus and is also known as the proton number of the particular element.
It is the proton/atomic number that determines the number of electrons an element has, its specific electron structure and therefore the specific identity of a particular element in terms of its physical and chemical properties. It cannot be overemphasised that it is the electronic structure that determines the chemical character of an element, hence the proton/atomic number determines everything about a particular element
To find the number of neutrons, follow neutrons = mass number - proton/atomic number
Isotope – atoms of the same element that contain different numbers of neutrons
Mass Number – an integer equal to the total number of protons and neutrons in the nucleus of an atom
Even though elements and isotopes have the same number of protons (proton/atomic number) isotopes of a particular element have the same electronic structure and identical chemistry.
The atomic symbol for an element is sometimes shown with both the mass number and atomic number
Let’s take hydrogen for example, hydrogen has
an atomic number of 1 therefore it only has 1 proton
, and are the three isotopes of hydrogen with mass numbers of 1, 2 and 3, with 0, 1 and 2 neutrons respectively. Hydrogen-1 is the most common, there is a trace of hydrogen-2 naturally but hydrogen-3 is very unstable and is used in atomic fusion weapons.
What would the symbol for the three naturally occurring oxygen isotopes with mass numbers 16, 17 and 18? Also, give the number of protons and neutrons.
- oxygen 16 isotope (most common) has 8 protons and 8 neutrons
- oxygen 17 isotope has 8 protons and 9 neutrons
- oxygen 18 isotope has 8 protons and 10 neutrons
Atomic Molar Mass – is related to the mass number
The atomic molar mass is an average mass of the elements isotopes; they do not have exactly the same mass because of the different number of neutrons
In general, atoms are neutral, so the number of electrons in an atom equals the number of protons
Practice 5-8 pg. 23
Ionization is the process of gaining or losing an electron
Ions form when an atom gains or loses one or more of the electrons in their outer energy level.
Positively charged ions are called cations, they form when an atom loses an electron.
Negatively charged ions are called anions, they form when an atom gains an electron.
Metals tend to lose electrons (cations) while non-metals tend to gain them (anions).
Atoms gain or lose electrons so that they have to same number of electrons as the nearest noble gas, which makes them more stable.
Recall that noble gases are more stable, therefore become close to a noble gas makes them less reactive
Practice pg. 28
Certain elements combine to form compounds. Elements that combine to form these compounds are those that are reactive. Reactivity of an element is determined by the number of electrons the element has in its outer energy level.
When the outer energy level is filled, the element is more stable. When the outer energy level is not filled, the element is more reactive.
Electrons that are in the outer energy level are called valence electrons. Valence is the capacity of an atom to gain or lose electrons.
Valence number – the number of electrons an element can gain or lose
Elements in the same family have the same number of valence electrons; this results in similar chemical properties
also called “the rule of eight”, states that atoms bond in such a way as to have eight electrons in their valence energy levels
1. If fluorine has an atomic number of 9, how many valence electrons does it have and will it lose or gain electrons? Fluorine has 7 valence electrons and will gain 1.
2. Neon is in group 18, how does that effect the valence electrons? Neon is a noble gas therefore will not gain or lose any
electrons.
3. Magnesium has 2 electrons in it’s outer level, according to the rule of eight, what is most likely going to happen to them? The two electrons will join other elements.
Recall:
protons = proton/atomic number
neutrons = mass number - proton/atomic number
electrons = protons - electric charge
Name Symbol Mass #
Atomic #
Protons Neutrons
Electrons
Electric Charge
silicon atom
Si 28 14 14 14 14 0
phosphide ion
P3- 31 15 15 16 18 3-
carbon isotope
14 6C 14 6 6 8 6 0
barium ion Ba+ 137 56 56 81 55 1+
Practice 9-12 page 27
2.1: Naming Ionic and Molecular Compounds
General Outcome #2 – Explain, using the periodic table, how elements combine to form compounds, follow IUPAC guidelines for naming ionic compounds and simple molecular compounds
Explain the importance of and need for the IUPAC system of naming compounds, in terms of the work that scientists do and the need to communicate clearly and precisely
Predict formulas and write names for ionic and molecular compounds and common acids
We throw salt on the roads in winter but you don’t throw road salt on your food! Road salt is calcium chloride and table salt is sodium chloride. The term “salt” is not really the name of the compound, it is the name of a class related to compounds.
IUPAC (International Union of Pure and Applied Chemistry) is the body responsible for naming compounds. It ensures the use of a consistent, practical way of naming compounds that allows scientists to communicate clearly and precisely. What would happen if we didn’t have the IUPAC??
Did you know that sodium chloride (table salt) is one of the most common compounds on Earth! Besides the supper table, where else do you think we find sodium chloride??
Oceans (contain sodium chloride as well as other salts)
Our cells control the amount of water they contain by controlling the concentration of salts. A high salt content in our body triggers thirst!
Salt is also used for electrical messaging in our bodies.
Ionic compounds form when electrons transfer from one atom to another ◦ Lets take another look at table salt
In sodium chloride, the two sodium and chloride ions combine in an organized array called a crystal lattice. There is one sodium ion for each chlorine ion. This combination of one sodium ion and one chlorine ion is called a formula unit (smallest amount of an ionic compound with the composition shown by the chemical formula).
Ionic bonding – type of bond formed when electrons transfer between metals and non-metals
All ionic compounds are solid at room temperature and tend to dissolve in water
Solutions of ionic compounds always conduct electricity
Examine the following table:
the little “s” means solid, (l) liquid, (g) gas or (aq) aqueous (element or compound can dissolve in water)
many elements and compounds have a common name as well as a chemical name, you will not have to memorise all of them!
Common Name Formula Chemical Name Application
baking soda NaHCO3 (s) sodium hydrogencarbonate
rises breads
milk of magnesia Mg(OH)2 (s) magnesium hydroxide antacid
All names of ionic compounds have two parts because all ionic compounds are made up of two parts. Every ionic compound is made up of a cation and an anion. Here are the rules:
1. Name the cation first by using the elements name (it is usually a metal ion)
2. Name the anion second by using the element`s name and changing the last part to `-ide` (it is usually a non-metal ion)
Fill in the table: Formula Cation Anion Name
NaCl (s) Na+ Cl- sodium chloride
BaF2 (s) Ba2+ F- barium fluoride
K3N (s) K+ N3- potassium nitride
In an ionic formula unit, there are equal numbers of positive and negative charges. If the cation has a charge of 1+ and the anion has a charge of 1-, the formula unit would have one ion of each element. This is what happens when Na+ and Cl- combine to form NaCl(s).
If the cation has a charge of 3+ and the anion has a charge of 1-, you need three anion charges to balance the single cation. This occurs when aluminium and chlorine combine to form aluminium chloride. The aluminium ion (Al3+) has a charge of 3+, and the chloride ion (Cl-) has a charge of 1-. Therefore, it takes three chloride ions to balance one aluminium ion, making the formula for aluminium chloride AlCl3(s).
Write the formula for the following ionic compounds.
a. sodium nitride a. Na3N(s)
b. magnesium fluoride b. MgF2(s)
c. calcium nitride c. Ca3N2(s)
Now complete the worksheet...
Practice 5-8 pg. 45 Practice 9-10 pg. 47
Some elements have more than one stable ion. For example, iron has Fe3+ and Fe2+ as stable ions. These elements are called multivalent elements.
If you are given a formula containing a multivalent element, you need to be able to determine which valence number has been used before you can write the name of the compound. It is also necessary to use an upper case Roman numeral to indicate which valence number is used, like iron(III) and lead(IV).
How do you know that FeBr2 must be iron(II) bromide and FeBr3 must be iron(III) bromide?
The subscript in the first one is a 2 and the second is a 3.
Write out the names for the following? a. FeCl2 (s)
a. iron (II) chloride
b. Cr2S3 (s)
b. chromium (III) sulphide
Now complete the worksheet
Polyatomic Ions – ions made up of several non-metallic atoms joined together
The most common two suffixes are ‘-ate’ and ‘-ite’
Some common polyatomic ion names are: ◦ ammonium (NH4
+)
◦ hydroxide (OH-)
◦ carbonate (CO32-)
For naming, you follow the same rules as you did for ionic compounds except this time we have a cation followed by the polyatomic compound. You do not need to change the ending of a polyatomic’s name
Writing formulas for polyatomic ions is similar to ionic compounds. Remember that the subscript of the cations applies to all of the elements inside the brackets
What is the formula for iron (III) sulfate? iron (III) – Fe3+
sulfate – SO42-
therefore the formula is Fe2(SO4)3 (s)
What is the formula of ammonium dihydrogenphosphate?
ammonium – NH4+
dihydrogenphosphate – H2PO4-
therefore the formula is NH4H2PO4 (s)
Now complete the worksheet
Practice 13-16 pg. 52 Practice 17-19 pg. 53
Molecule – forms when two or more non-metallic atoms bond together
Recall that the formula unit of an ionic compound represents a ratio of ions in a crystal lattice. In a solid ionic compound, this lattice extends in all directions. A formula unit is not an independent unit – it is just one part of a crystal lattice. Molecules are independent units made up of fixed numbers of atoms bonded together.
Unlike ionic compounds, molecular compounds can be solid, liquid or gas, many do not dissolve in water and they tend to be poor conductors of electricity
Covalent bonds – bond formed when non-metallic atoms share electrons; atoms in a molecule are bound together by covalent bonds ◦ Recall: Ionic bonding – type of bond formed when
electrons transfer between metals and non-metals
Don`t get confused between them! Think of them like this...
Let`s look at chlorine gas (Cl2 (g)); it has the atomic number 17 which means that there is 7 valence electrons, when two chlorine atoms combine to form one molecule of chlorine gas, one pair of electrons are shared to make a stable outer energy level.
Another example is carbon dioxide gas (CO2 (g)); because carbon has an atomic number of 6 (4 valence electrons) and oxygen has an atomic number of 8 (6 valence electrons), each share two pairs of electrons.
Molecular element – element that forms molecules made up of only its own atoms
Diatomic molecule – molecule composed of two atoms of the same element
For example, nitrogen is a molecular element but its molecules are diatomic!
Some elements are polyatomic (have more than two atoms of the same element) while some are monatomic (their atoms can exist on their own)
Monatomic C(s) noble gases all metals
Diatomic H2(g) N2(g) O2(g) F2(g) Cl2(g) Br2(l) I2(s)
Polyatomic O3(g) (ozone) P4(s) S8(s)
The Chemical Formulas of Molecular Elements
It`s helpful to remember the diatomic formulas with the following: Help Our Needy Class Find Brains Immediately
Naming binary molecular compounds (contains two elements) is similar to naming ionic compounds.
For molecules, prefixes indicate how many atoms of each element are present
Use the following table for naming:
Prefix Number
mono 1
di 2
tri 3
tetra 4
penta 5
Prefix Number
hexa 6
hepta 7
octa 8
enna (nona)
9
deca 10
The format for naming looks like: prefix + first element followed by prefix + second element ending in `ide`
The only ‘trick’ is that mono- is not required on the first atom
Try the following a. SO2 (g)
• sulfur dioxide
b. CS2 (g) • carbon disulfide
c. N2O3 (g) dinitrogen trioxide
d. carbon tetrachloride CCl4 (l)
e. tetraphosphorus decaoxide P4O10 (s)
f. oxygen difluoride OF2 (g)
Molecular compounds that contain hydrogen are very common. Some of these compounds have common names that do not follow any rules. These have to be memorized or found in a table. Water, hydrogen peroxide, ammonia, and sucrose are just a few of the molecular compounds containing hydrogen that do not follow specific naming rules.
Common Compounds Containing Hydrogen
IUPAC Name Formula and State at 25oC
water H2O (l)
hydrogen peroxide
H2O2 (l)
ammonia NH3 (g)
sucrose C12H22O11 (s)
methane CH4 (g)
propane C3H8 (g)
methanol CH3OH (l)
ethanol C2H5OH (l)
hydrogen sulfide
H2S (g)
Practice 1-4 pg. 44
See page 53
