Simple Molecules & Covalent Bonding quiz
Atom: An atom is the smallest part of an element.
Molecule: A molecule is made of a fixed number of atoms covalently bonded together.
An atom consists of a central nucleus, composed of protons and neutrons.
This is surrounded by electrons, orbiting in shells (energy levels).
Atoms are neutral because the numbers of electrons and protons are equal.
Atomic number: The number of protons in an atom.
Mass number: The number of protons and neutrons in an atom.
Relative atomic mass (Ar): The average mass of an atom compared to 1/12th the mass of carbon-12.
The elements in the Periodic Table are arranged in order of increasing atomic number.
Columns are called Groups. They indicate the number of electrons in the outer shell of an atom.
Rows are called Periods. They indicate the number of shells (energy levels) in an atom.
Electrons are found in a series of shells (or energy levels) around the nucleus of an atom.
Each energy level can only hold a certain number of electrons. Low energy levels are always filled up first.
Rules for working out the arrangement (configuration) of electrons:
Example – chlorine (Cl)
1) Use the periodic table to look up the atomic number. Chlorine’s atomic number (number of protons) is 17.
2) Remember the number of protons = number of electrons. Therefore chlorine has 17 electrons.
3) Arrange the electrons in levels (shells):
Therefore the electron arrangement for chlorine (17 electrons in total) will be written as 2,8,7
4) Check to make sure that the electrons add up to the right number
The electron arrangement can also be draw in a diagram.
Electron arrangement for the first 20 elements:
Metals on the left of the Periodic Table.
Non-Metals on the top-right, plus Hydrogen.
Elements in the same group have the same number of electrons in their outer shell.
This is why elements from the same group have similar properties.
Elements in the same group of the periodic table have the same number of electrons in their outer shells, which means they have similar chemical properties.
The noble gases are inert (unreactive) because they have a full outer shell of electrons.
This should give you a ruff idea of what the difference is between ionic and covalent bonding.
A covalent bond is formed between two non-metal atoms by sharing a pair of electrons in order to fill the outer shell.
Covalent bonding: a strong attraction between a shared pair of electrons and two nuclei.
These videos are a good introduction to covalent bonding:
This excellent Tyler de Witt video addresses a common mistake around diatomic elements (H2, N2, O2, F2, Cl2, Br2, I2).
For example, if chlorine is in its elemental form (i.e. not bonded in a compound) the formula is Cl2, but when chlorine is bonded to sodium the formula of sodium chloride is NaCl.
Carbon dioxide (CO2) has a simple molecular structure. This just means that it is made up of molecules.
Within each molecule are atoms bonded to each other covalently. These covalent bonds inside the molecules are strong.
However, between the molecules are weak forces of attraction that require little energy to break. These forces are not covalent bonds. This is why simple molecular substances such as carbon dioxide have a low boiling point.
So when carbon dioxide changes from a solid to a gas, for example, that process can be represented as:
CO₂ (s) → CO₂ (g)
Notice that even though there has been a dramatic change of state from solid to gas, the substance before and after the change is always made up of carbon dioxide molecules. During the change of the state the covalent bonds within each molecule remain unbroken. Instead it is the weak forces of attraction between the molecules which have been overcome.
Larger molecules tend to have higher boiling points.
This is because larger molecules (molecules with more mass) have more forces of attraction between them. These forces, although weak, must be overcome if the substance is to boil, and larger molecules have more attractions which must be overcome.
Tests for gases
|Gas||Test||Result if gas present|
|hydrogen (H2)||Use a lit splint||Gas pops|
|oxygen (O2)||Use a glowing splint||Glowing splint relights|
|carbon dioxide (CO2)||Bubble the gas through limewater||Limewater turns cloudy|
|ammonia (NH3)||Use red litmus paper||Turns damp red litmus paper blue|
|chlorine (Cl2)||Use damp litmus paper||Turns damp litmus paper white (bleaches)|
Add anhydrous copper (II) sulfate (CuSO4) to a sample.
If water is present the anhydrous copper (II) sulfate will change from white to blue.