The first consideration is molecule size. Because large molecules include more electrons and nuclei, which generate van der Waals attractive forces, their compounds often have higher boiling temperatures than identical compounds composed of smaller molecules. For example, carbon dioxide has a much lower boiling point (100°C or 212°F) than water (100°C or 212°F), even though both are gaseous at standard pressure. The reason is that the larger molecules experience greater intermolecular forces as they try to arrange themselves into a liquid state.
Second, shape affects the boiling point. Spherical molecules tend to be less stable than flat ones of equal size because there are more pairs of surface atoms than there are triplets of opposite charges. For example, sodium melts at 133°C but lithium melts at 35°C. Lithium's low melting point can be used to construct all-solid-state batteries, while sodium-ion batteries require a liquid electrolyte.
Third, electrical charge affects the boiling point. Positively charged particles attract each other, so compounds containing them will boil at higher temperatures than those without any net charge. For example, hydrogen gas is only stable under high pressure, while helium is always found in nature in its gaseous form.
The boiling point is related to the van der Waals forces that exist between atoms and molecules. The higher the temperature necessary to overcome this binding force, and hence the higher the boiling point, the stronger the force of attraction. Helium has a boiling point about 500 degrees below zero Fahrenheit (240 degrees below zero Celsius), which is one reason it is used as a refrigerant.
Atomic size differences between elements can have a significant effect on their boiling points. For example, the radius of helium atoms is only 4% that of carbon atoms, but because of the greater distance between the electrons in helium they experience more repulsion than carbon at equal temperatures, so helium boils at lower temperatures than carbon. The radius of oxygen atoms is much larger than those of carbon or helium; therefore, at any given temperature, its boiling point is higher than either carbon or helium.
Helium also has the lowest boiling point of all the elements. Because of its small size, each helium atom is able to interact with many other atoms, which results in a strong attractive force between them. This makes helium very reactive - even at low temperatures - which is why it is found in nature mostly in compounds. It is the most common element in the universe after hydrogen and oxygen, which are both more abundant.
Carbon's boiling point is about 1000 degrees Fahrenheit (540 degrees Celsius).
The chemical connections between the atoms in all hydrocarbon molecules are extremely strong. Intermolecular forces are greater in longer hydrocarbon molecules. More energy is required to separate them so that their boiling points are greater. At least four units of carbon are needed for liquid water to appear in the gaseous state.
Hexane has a boiling point of 78 degrees Fahrenheit and can be used as an indicator of temperature because it changes color from clear to yellow when it reaches this point. If you were to heat up a sample of hexane, it would be very difficult to tell if the oil had reached its boiling point or not. It might seem like it was close but just below the boiling point.
Heptane has a boiling point of 98 degrees Fahrenheit and octane has one of 108 degrees. These numbers show that there is more intermolecular force holding these molecules together than hexane has. Therefore, they require more energy to break away from each other and become gas molecules.
Nonane has three units of carbon and therefore it should have a boiling point of about 110 degrees F. But even after energy is added to it, nonane will never reach this point because it is too heavy to exist as a gas at such low temperatures.
I think that as the density of the pure material grows, molecules with comparable sorts of chemical bonding (e.g., covalent bonds) will have greater boiling points. That is why substances that are very stable at low temperatures (such as ice) tend to melt or decompose at higher temperatures.
The example you gave about hydrogen and helium is right, because they are both gasses but they have different boiling points because they have different molecular weights. As the molecular weight increases, so does the boiling point.
Density tends to increase as the temperature increases because atoms move faster when there is more heat energy available, so they interact with each other more frequently which causes them to be less likely to stay together. This means that there are more empty space between particles and thus less mass per unit volume.
At lower temperatures, molecules have more time to rearrange themselves into different configurations so they can match up with other molecules in order to form crystals. Since there is more room for movement, molecules can avoid each other more easily and thus have a lower probability of being paired up correctly. This means that there is more mass per unit volume at these temperatures since there are more molecules present in the volume searched by physicists when measuring density.
Because the van der Waals force rises with atom size, the boiling and melting points increase from He to Rn. Helium has a boiling point of 269 degrees Celsius. Argon is heavier than helium and has greater dispersion forces. As a result, its boiling point (-186 C) is greater than He's. Although radon is a gaseous element like helium, it has a much higher boiling point (4803 K).