Grade 9 ¥ Physical Science (Elements, Compounds, and Reactions)

 

Describe how elements are characterized by the nature of their particles

 

Elements are defined as substances that consist of one type of atom, for example Carbon atoms make up diamond, and also graphite. Pure (24K) gold is composed of only one type of atom. Atoms are the smallest particle into which an element can be divided.

The proton is located in the center (or nucleus) of an atom, each atom has at least one proton. Protons have a charge of positive one, and a mass of approximately 1 atomic mass unit (amu). Elements differ from each other in the number of protons they have, e.g. Hydrogen has 1 proton; Helium has 2.

The neutron also is located in the atomic nucleus (except in Hydrogen). The neutron has no charge, and a mass of slightly over 1 amu.

The electron is a very small particle located outside the nucleus. Because they move at speeds near the speed of light the precise location of electrons is hard to pin down. Electrons occupy orbitals, or areas where they have a high statistical probability of occurring. (An orbital is also an area of space in which an electron will be found 90% of the time).The charge on an electron is -1. Its mass is negligible (approximately 1800 electrons are needed to equal the mass of one proton).

 

Inert Elements - Have complete valence shells and are stable

 

Reactive Elements - Valence shells are not full and are unstable. Tend to gain, lose, or share electrons. Allows for bond formation, which produces stable valence

 

Things to Remember - It is important to remember that a model is a simplified representation of an object. Some of the models on the previous page are more accurate than others, but none of them are completely correct. Here are a couple of the things we have ignored:

 

The Size of the Nucleus - In the drawings above, the nucleus is too large. Or, put another way, if the nucleus is going to be that large, the electrons are too close. Real atoms are mostly empty space. If we wanted our drawings to be accurate, we would have to place the electrons about a mile away. Clearly, it would be difficult to bring a drawing that large to class.

Electrons do not Orbit the Nucleus - In the drawings above, we have drawn nice circles showing where the electrons go around the atom. In reality, scientists cannot tell exactly where an electron is at a given moment or where it is going. They can calculate the probability that an electron will be found in a given volume of space, but that isn't the same as knowing where that electron is. This behavior is described in the Quantum Model of the atom. Although it is the most accurate description that scientists currently have of the atom, it is much more difficult to understand.

 

Using the information in your Periodic Table to draw the Bohr ModelÕs for:

H, He, Li, Be, B, C, N. O. F. Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K & Ca.

 

Predict the properties of elements based on their position in the periodic table

 

The Periodic Table of the Elements provides a great deal of information about various elements. During the nineteenth century chemists arranged the then-known elements according to chemical bonding.

 

The two main groups are the metals and non-metals.

 

Metals typically have the following properties: They are shiny, They are ductile (they can be stretched into wires), They are malleable (they can be beaten into sheets), They are good conductors (they are able to conduct/transfer electricity and heat)

 

While non-metals typically do not have the above properties.

 

What are the other more specific categories that metals and non-metals can belong to?

 

Write formulae and names for simple compounds

 

A chemical compound is a substance made of two or more elements combined in a definite formula. Every compound has its own chemical formula, which shows the proportion of elements present in that compound.

 

Ionic Bonds - Form when electrons are completely transferred from one atom to another

 

Ionic bonds are formed when atoms become ions by gaining or losing electrons.

 

Ionic bonds form between a metal and a non-metal.

 

Chlorine is in a group of elements having seven electrons in their outer shells. Members of this group tend to gain one electron, acquiring a charge of negative 1.

 

Sodium is in another group with elements having one electron in their outer shells. Members of this group tend to lose that outer electron, acquiring a charge of  positive1.

 

Oppositely charged ions are attracted to each other, thus Cl- and Na+ form an ionic bond, becoming the molecule sodium chloride. Ionic bonds generally form between elements in Group I (having one electron in their outer shell) and Group VIIa (having seven electrons in their outer shell). Such bonds are relatively weak, and tend to disassociate in water, producing solutions that have both Na and Cl ions.

 

In the formation of a crystal of sodium chloride, each positively charged sodium ion is surrounded by six negatively charged chloride ions; likewise each negatively charged chloride ion is surrounded by six positively charged sodium ions. The overall effect is electrical neutrality.

 

The metal is always named first and the root of the nonmetal is followed by the suffix ÒideÓ Ex. BaCl2 is Barium Chloride.

 

Conversely, the formula can be determined from the name. Ex. For Aluminum Oxide we know that the two atoms involved are Aluminum and Oxygen. To determine the necessary subscripts look at the charges on each atom. From the periodic table Al has a charge of +3 and O has a charge of -2. The smallest number of each atom that can balance the charges is therefore Al2O3

 

Covalent Bonds - Atoms become stable through shared electrons

 

Covalent bonds form between a non-metal and a non-metal (i.e. two non-metals).

 

Covalent bonds form when atoms share electrons. Remember that electrons move very fast and thus can be shared, effectively filling or emptying the outer shells of the atoms involved in the bond. Such bonds are referred to as electron-sharing bonds.

An analogy can be made to child custody: the children are like electrons, and tend to spend some time with one parent and the rest of their time with their other parent.

 

Carbon (C) is in Group IVa, meaning it has 4 electrons in its outer shell. Thus to become a "happy atom", Carbon can either gain or lose four electrons. By sharing the electrons with other atoms, Carbon can become a happy atom,. alternately filling and emptying its outer shell. The molecule methane (chemical formula CH4) has four covalent bonds, one between Carbon and each of the four Hydrogens. Carbon contributes an electron, and Hydrogen contributes an electron.

 

The sharing of a single electron pair is termed a single bond. When two pairs of electrons are shared, a double bond results, as in carbon dioxide.

 

Naming: Since there is no metal the second non-metal will have its ending modified by the ending ÒideÓ.  A greek prefix is added to tell us the number of atoms present.

1 atom = mono, 2 atoms = di, 3 atoms = tri, 4 atoms = tetra, 5 atoms = penta,

6 atoms = hexa, 7 atoms = hepta, 8 atoms = octa, 9 atoms = nona, 10 atoms = deca

 

For Example: The prefix ÒmonoÓ is used to distinguish Carbon Monoxide and Carbon Dioxide.

 

SO3 is named Sulfur Trioxide.       The formula for Dinitrogen Pentoxide is N2O5

 

Read page 49 – 51 and answer questions 2, 3 on page 51

 

Complete 3, 4 and 5 on page 58

 

Complete C3. And C5. On page 59

 

Compare and contrast physical and chemical changes

 

Science probe 9 pg. 23 to 27

 

Infer the Law of Conservation of Mass through experimentation

 

Democritus (460-370 BC) and somewhat later John Dalton (1766-1844) were the first to consider matter at its most microscopic form. They both came up with the concept of the "atom" as being the smallest unit of matter and thus being undivisible*. This observation has an important and fundamental consequence: mass is neither created nor destroyed during the course of a chemical reaction. How do we come to this conclusion? We know that chemical reactions take place at the atomic/molecular level. That is molecules and atoms interact with one another during a chemical reaction. If atoms are indivisible then they cannot be destroyed during a chemical reaction. If atoms cannot be destroyed then the mass of reactants must equal the mass of the products in a chemical reaction. e.g.,

 

 Reactants -------> Products

 

Mass of Reactants = Mass of Products

 

This can be visualized by considering the formation of water from oxygen and hydrogen molecules:

 

Note that the hydrogen and oxygen atoms simply rearrange themselves but are not destroyed. Therefore mass is conserved.

 

Mass conservation can be used in chemical calculations. For example iron rusts by combining with oxygen to form rust (iron oxide). Suppose 100 g of iron metal rusts. We weigh the rust and find that the rust has a mass of 143 g. What mass of oxygen reacted with the iron?

 

 Iron + Oxygen -----> Rust

 

100 g + ?g ------> 143g

 

mass reactants = mass products

 

mass products = 143g = mass reactants

 

= 100 + mass of oxygen

 

mass oxygen = 43 g

 

The law of conservation of mass/matter, also known as law of mass/matter conservation (or the Lomonosov-Lavoisier law), states that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. An equivalent statement is that matter changes form, but cannot be created nor destroyed. This implies that for any chemical process in a closed system, the mass of the reactants must equal the mass of the products. In chemistry, so long as no nuclear reactions take place, a special form the conservation of mass also holds in regard to the conservation of the mass (and number of atoms) of each chemical element. In most basic chemical reactions and equations, no atoms of any element may be created or destroyed. They must only come out exactly as found in the reactant side of an equation, with a different location in regard to their new chemical formula, as may be found on the product side of an equation.

 

The notion that mass, or matter, can be neither created nor destroyed. According to conservation of mass, reactions and interactions which change the properties of substances leave unchanged their total mass; for instance, when charcoal burns, the mass of all of the products of combustion, such as ashes, soot, and gases, equals the original mass of charcoal and the oxygen with which it reacted.

 

Identify the effects of various factors on the rate of chemical reactions

 

Reaction rates are greatly influenced by a number of factors:

CONCENTRATION: the greater the concentration of a reactant, the greater the rate of the reaction. A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate

 

PRESSURE: increases the rates of reactions which take place with the reactants in the gaseous phase.

 

TEMPERATURE: The higher the temperature, the higher the reaction rate. Temperature is a measure of the kinetic energy of a system, so higher temperature implies higher average kinetic energy of molecules and more collisions per unit time. A general rule of thumb for most (not all) chemical reactions is that the rate at which the reaction proceeds will approximately double for each 10&degC increase in temperature.

 

CATALYSTS are substances which speed up reaction rates without undergoing a permanent change in the process. Catalysts (e.g., enzymes) lower the activation energy of a chemical reaction and increase the rate of a chemical reaction without being consumed in the process. Catalysts work by increasing the frequency of collisions between reactants, altering the orientation of reactants so that more collisions are effective, reducing intramolecular bonding within reactant molecules, or donating electron density to the reactants.

 

BOND TYPES: Reactions between ionic compounds are usually very much faster than those involving compounds where covalent bonds have to be made or broken.

 

 

STATE OF SUBDIVISION: This is very important when solids are involved. The more finely divided the solid is, the faster the reaction will take place. In fact, for solids, it is the surface area, and not the concentration, which affects the rate.

 

In general, a factor that increases the number of collisions between particles will increase the reaction rate and a factor that decreases the number of collisions between particles will decrease the chemical reaction rate.