Question 1

A WHITE CRYSTALLINE SOLID MELTS AT 801°C. IT CONDUCTS ELECTRICITY WHEN MOLTEN OR DISSOLVED IN WATER, BUT NOT AS A SOLID. WHAT IS ITS STRUCTURE?

Accepted Answer

A GIANT IONIC LATTICE. A high melting point plus conduction only when molten or dissolved is the ionic pattern: the ions are fixed in the solid lattice and free to carry charge once it melts or dissolves. A metal would conduct as a solid too.

Question 2

MAGNESIUM IS IN GROUP 2 AND OXYGEN IS IN GROUP 6. WHAT CHARGES DO THEIR IONS CARRY IN MAGNESIUM OXIDE?

Accepted Answer

2+ AND 2−. A Group 2 metal atom loses its two outer electrons to form a 2+ ion, and a Group 6 non-metal atom gains two electrons to form a 2− ion. Both ions then have a noble gas electronic structure.

Question 3

THE COVALENT BONDS BETWEEN OXYGEN AND HYDROGEN ATOMS IN WATER ARE STRONG, YET WATER BOILS AT ONLY 100°C. WHY?

Accepted Answer

BOILING OVERCOMES THE WEAK FORCES BETWEEN MOLECULES, NOT THE COVALENT BONDS. In a substance made of small molecules, boiling only separates the molecules from each other. The weak intermolecular forces are overcome; the strong covalent bonds inside each molecule stay intact.

Question 4

METHANE (CH4) BOILS AT ABOUT −162°C AND PENTANE (C5H12) BOILS AT ABOUT 36°C. WHAT BEST EXPLAINS THE DIFFERENCE?

Accepted Answer

PENTANE MOLECULES ARE LARGER, SO THE FORCES BETWEEN THEM ARE STRONGER. Intermolecular forces increase with the size of the molecules, so the larger pentane molecules need more energy to separate. No covalent bonds are broken when either substance boils.

Question 5

SOLID SODIUM CHLORIDE DOES NOT CONDUCT ELECTRICITY, BUT MOLTEN SODIUM CHLORIDE DOES. WHAT CHANGES WHEN IT MELTS?

Accepted Answer

THE IONS BECOME FREE TO MOVE. In the solid, the ions are held in fixed positions in the lattice. Melting lets them move, and moving charged particles carry the current. The charge carriers are ions, not electrons.

Question 6

DIAMOND AND GRAPHITE ARE BOTH PURE CARBON. WHY IS GRAPHITE SOFT AND SLIPPERY WHILE DIAMOND IS VERY HARD?

Accepted Answer

GRAPHITE HAS NO COVALENT BONDS BETWEEN ITS LAYERS, SO THE LAYERS CAN SLIDE. Each carbon in graphite bonds to three others in flat hexagonal layers with no covalent bonds between the layers, so they slide over each other. In diamond every carbon bonds to four others in a rigid 3D network.

Question 7

BRASS, AN ALLOY OF COPPER AND ZINC, IS HARDER THAN PURE COPPER. WHY?

Accepted Answer

ATOMS OF A DIFFERENT SIZE DISTORT THE LAYERS, SO THEY SLIDE LESS EASILY. In a pure metal the atoms sit in regular layers that can slide when a force is applied. Zinc atoms are a different size, so they distort the layers and make it harder for them to slide over each other.

Question 8

A WHITE SOLID HAS A VERY HIGH MELTING POINT AND DOES NOT CONDUCT ELECTRICITY AS A SOLID OR WHEN MOLTEN. WHICH SUBSTANCE COULD IT BE?

Accepted Answer

SILICON DIOXIDE. Only a giant covalent structure fits: a very high melting point because many strong covalent bonds must break, and no conduction because it has no ions or delocalised electrons. Molten sodium chloride would conduct, and methane melts far below room temperature.

Question 9

THE SIDE OF A CUBE-SHAPED PARTICLE SHRINKS FROM 100 NM TO 10 NM. WHAT HAPPENS TO ITS SURFACE AREA TO VOLUME RATIO?

Accepted Answer

IT INCREASES BY A FACTOR OF 10. For a cube the ratio is 6 ÷ side length. Dividing the side by 10 multiplies the ratio by 10: from 0.06 per nm to 0.6 per nm. That higher ratio is why nanoparticles can behave differently from the bulk material.

Question 10

ETHENE IS A GAS AT ROOM TEMPERATURE, BUT POLY(ETHENE) IS A SOLID. WHY?

Accepted Answer

POLYMER MOLECULES ARE VERY LARGE, SO THE FORCES BETWEEN THEM ARE RELATIVELY STRONG. A polymer molecule is a long chain of atoms joined by strong covalent bonds. Because the molecules are so large, the intermolecular forces between them are relatively strong, which keeps the polymer solid at room temperature.

Question 11

IN A DOT AND CROSS DIAGRAM OF A NITROGEN MOLECULE (N2), HOW MANY PAIRS OF ELECTRONS ARE SHARED BETWEEN THE TWO ATOMS?

Accepted Answer

THREE. Nitrogen is in Group 5, so each atom has five outer electrons and needs three more for a full shell. Sharing three pairs gives each atom eight outer electrons: a triple bond.

Question 12

A SUBSTANCE MELTS AT −7°C AND BOILS AT 59°C. WHAT STATE IS IT IN AT 25°C?

Accepted Answer

LIQUID. 25°C is above the melting point, so it is no longer solid, and below the boiling point, so it has not become a gas. Between the two points a substance is a liquid. These are the values for bromine.

Question 13

CALCIUM IS IN GROUP 2 AND CHLORINE IS IN GROUP 7. IN CALCIUM CHLORIDE, EACH CALCIUM ION IS BALANCED BY ___ CHLORIDE IONS.

Accepted Answer

TWO. Calcium forms a 2+ ion and chlorine forms a 1− ion, so two chloride ions balance the charge on one calcium ion. That gives the formula CaCl2.

Question 14

HYDROGEN CHLORIDE IS A GAS AT ROOM TEMPERATURE AND DOES NOT CONDUCT AS A LIQUID, BECAUSE IT IS MADE OF SMALL ___.

Accepted Answer

MOLECULES. Hydrogen chloride is a compound of two non-metals, so its atoms share electrons in covalent bonds. Small molecules have weak forces between them, giving a low boiling point, and no overall charge, so no conduction.

Question 15

A MATERIAL USED IN ELECTRONICS IS ONE ATOM THICK, IS MADE OF CARBON IN HEXAGONAL RINGS, AND CONDUCTS ELECTRICITY. IT IS ___.

Accepted Answer

GRAPHENE. A single layer of graphite is graphene. It keeps graphite's delocalised electrons, so it conducts, and its strong covalent network makes it useful in electronics and composites.

Bonding, Structure and Properties