Shared pairs of electrons, drawing simple molecules, why substances made of small molecules melt at low temperatures, and the difference between bonds and intermolecular forces.
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1.Between which types of element does covalent bonding occur?
Between non-metal atoms only.
2.Define a covalent bond.
A shared pair of electrons between two atoms, held together by the strong attraction between the shared electrons and the nuclei of both atoms.
3.Are covalent bonds weak or strong?
Very strong. The bonds themselves are strong even in substances that melt easily.
4.How many covalent bonds does hydrogen form?
One, because it needs one more electron to fill its only shell.
5.How many covalent bonds does oxygen form?
Two, because it has six outer electrons and needs two more.
6.How many covalent bonds does carbon form?
Four, because it has four outer electrons and needs four more.
7.How many covalent bonds does nitrogen form?
Three.
8.Describe the bonding in a molecule of hydrogen, H2.
The two hydrogen atoms share one pair of electrons, giving each a full first shell.
9.Describe the bonding in a molecule of water, H2O.
The oxygen atom shares one pair of electrons with each of the two hydrogen atoms, giving oxygen a full outer shell of eight.
10.What is a double bond?
Two shared pairs of electrons between the same two atoms, as in oxygen, O2, and carbon dioxide.
11.What are the three categories of covalently bonded substance?
Small molecules, very large molecules such as polymers, and giant covalent structures.
12.Why do substances made of small molecules have low melting and boiling points?
The forces between the molecules, the intermolecular forces, are weak and need little energy to overcome. The covalent bonds themselves are not broken.
13.What is broken when a simple molecular substance melts?
Only the weak intermolecular forces between molecules. The strong covalent bonds inside each molecule stay intact.
14.What happens to intermolecular forces as molecules get larger?
They get stronger, so melting and boiling points increase.
15.Are most simple molecular substances solid, liquid or gas at room temperature?
Most are gases or liquids, because the intermolecular forces are so weak.
16.Do simple molecular substances conduct electricity?
No. The molecules have no overall electric charge, so there are no free charged particles to carry current.
17.Give a limitation of using a two dimensional diagram for a molecule.
It does not show the true shape of the molecule or the relative sizes of the atoms.
18.What is a displayed formula?
A diagram showing each atom and each covalent bond drawn as a line, for example H-O-H for water.
19.What does a line between two atoms represent?
One shared pair of electrons, that is one covalent bond.
20.Why does a covalent bond hold atoms together strongly?
The shared pair of electrons is attracted to the positive nuclei of both atoms at the same time.
21.What is a polymer?
A very large molecule made of many small repeating units joined by covalent bonds.
22.Why are polymers solid at room temperature?
The molecules are so large that the intermolecular forces between them are relatively strong, so more energy is needed to separate them.
23.How is a polymer represented?
By its repeating unit written in brackets with the letter n outside, showing that the unit repeats many times.
24.Compare the melting point of a polymer with a small molecule.
The polymer is much higher, because although both are covalent, the far larger molecules have much stronger intermolecular forces.
25.What kind of forces are broken when you boil water?
The intermolecular forces between water molecules. The O-H covalent bonds inside each molecule are not broken.
26.Why is the boiling point of water higher than you might expect for a small molecule?
The intermolecular forces between water molecules are unusually strong for a molecule of that size.
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