Simple molecules & covalent bonding – core flashcards1:14 know what is meant by the terms atom and molecule
Atom: An atom is the smallest part of an element.
Molecule: A molecule is made of a fixed number of atoms covalently bonded together.
1:15 know the structure of an atom in terms of the positions, relative masses and relative charges of sub-atomic particles
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.
| Mass | Charge | |
|---|---|---|
| Proton | 1 | +1 |
| Neutron | 1 | 0 |
| Electron | almost 0 | -1 |
1:16a know what is meant by the terms atomic number, mass number and relative atomic mass (Aᵣ)
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.
1:18 understand how elements are arranged in the Periodic Table: in order of atomic number, in groups and periods
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.
1:19 understand how to deduce the electronic configurations of the first 20 elements from their positions in the Periodic Table
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):
- 1st shell can hold a maximum of 2
- 2nd can hold a maximum of 8
- 3rd can also hold 8
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:
1:21 identify an element as a metal or a non-metal according to its position in the Periodic Table
Metals on the left of the Periodic Table.
Non-Metals on the top-right, plus Hydrogen.
1:22 understand how the electronic configuration of a main group element is related to its position in the Periodic Table
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.
1:23 Understand why elements in the same group of the Periodic Table have similar chemical 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.
1:24 understand why the noble gases (Group 0) do not readily react
The noble gases are inert (unreactive) because they have a full outer shell of electrons.
Dogs teach ionic versus covalent bonding – video
This should give you a ruff idea of what the difference is between ionic and covalent bonding.
1:44 know that a covalent bond is formed between atoms by the sharing of a pair of electrons
A covalent bond is formed between two non-metal atoms by sharing a pair of electrons in order to fill the outer shell.
1:45 understand covalent bonds in terms of electrostatic attractions
Covalent bonding: a strong attraction between a shared pair of electrons and two nuclei.
Covalent bonding – videos
These videos are a good introduction to covalent bonding:
Diatomic elements – Tyler de Witt video
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.
1:47 explain why substances with a simple molecular structures are gases or liquids, or solids with low melting and boiling points. The term intermolecular forces of attraction can be used to represent all forces between molecules
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.
1:48 explain why the melting and boiling points of substances with simple molecular structures increase, in general, with increasing relative molecular mass
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.
2:44 describe tests for these gases: hydrogen, oxygen, carbon dioxide, ammonia, chlorine
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 moist litmus paper | Turns moist litmus paper white (bleaches) |
2:49 describe a test for the presence of water using anhydrous copper(II) sulfate
Add anhydrous copper (II) sulfate (CuSO4) to a sample.
If water is present the anhydrous copper (II) sulfate will change from white to blue.
