Three elements: What do the numbers mean? Quiz

Answer: Atomic number

Each element has its own unique atomic number. The atomic number is determined by the number of protons of one atom of that element.

Answer: Melting point in degrees Celsius

Tungsten is at the extreme high end but, on the other end, elements that are gases at room temperature have a negative melting and boiling point in degrees Celsius. For example, hydrogen has a melting point of -259°C and a boiling point of -252°C. (-273.15°C is absolute zero.)

It is interesting that gallium has a melting point of 30°C. This means that if it is hot outside, or if gallium is in a hot room, gallium can turn from solid to liquid just because of the temperature change.

Answer: Number of stable isotopes

The number of stable isotopes for each element is different. Elements with even atomic numbers will usually have more stable isotopes than those with odd atomic numbers. Elements with no stable isotopes are always radioactive.

Answer: Density (g/cm³)

Osmium has the highest density, at 22 g/cm³. Even though lead is thought to be heavy (and it is), its density is 11 g/cm³, which is much less than gold. Since mercury (liquid at room temperature) has a density of 13 g/cm³, lead will float in mercury, but gold will sink. (Of course, they will both sink quickly in water, since water has a density of 1 g/cm³).

Gases have a much lower density. For example, oxygen has a density of 0.0014 g/cm³. Gases often have their density written in terms of grams per liter, giving oxygen a density of 1.4 g/L.

Answer: Percentage of matter in the Earth's crust

It may be surprising that silicon makes up 27% of the Earth's crust. However, silicon dioxide (quartz) is a common part of Earth's crust, causing both silicon and oxygen to make up a large percentage of the Earth's crust.

Answer: Period number in the Periodic Table

Germanium, francium, and americium are all named after countries, but that's not important right now. The period number in the Periodic Table is determined by the row that it is in. Some sections of the Periodic Table don't exist in low-numbered periods (for example, the first transition metals are in period 4).

Comparing atomic numbers to period numbers:
1, 2: Period 1
3-10: Period 2
11-18: Period 3
19-36: Period 4
37-54: Period 5
55-86: Period 6
87-118: Period 7

Answer: Most common number of neutrons

The most common number of neutrons in one atom of lead is 126. Because of its atomic number (82), lead always has 82 protons and 82 electrons (if not charged). 82+126=208, so the most common form of lead is called ²⁰⁸Pb (Pb being the symbol for lead, and 208 being the number of protons plus the number of neutrons). Some lead atoms have more or fewer neutrons, but those are much less common.

Elements with low atomic numbers (usually 14 or less) often have the same number of protons as neutrons. This is why carbon and oxygen usually have 6 and 8 neutrons, respectively. However, with higher atomic numbers, the number of neutrons exceeds the number of protons.

Answer: Half-life of the most stable isotope in seconds

Elements with atomic numbers of 84 or higher are always radioactive. Usually, higher atomic numbers mean shorter half-lives. Radioactive elements with odd atomic numbers also often have shorter half-lives than the even-numbered elements with one more proton or one fewer proton, although this is not always true.

140 quadrillion seconds is 4.4 billion years, while 2.5 quadrillion seconds is 80 million years. 10 seconds is... ten seconds. With half-lives this short, the atoms have to be made artificially with just a few atoms at a time.

Answer: Percentage of volume in the Earth's atmosphere

The Earth's atmosphere is mostly nitrogen, even though oxygen is needed in order to be able to breathe. Atmospheric nitrogen and oxygen are in molecules (N₂ and O₂) because neither nitrogen atoms nor oxygen atoms can exist alone.

78+21+0.93=99.93. (The total is closer to 99.96 because the 78 and the 21 were rounded.) Since the total must be 100 percent, these three elements make up almost all of the air in the atmosphere.

Answer: Most common charge of an ion of that atom

The charge of an ion is important for making ionic compounds. Anything in the leftmost column of the Periodic Table will have a charge of +1, so Cs₂S is a valid compound (each of the two cesium/caesium atoms is +1, for a total of +2, while sulfur/sulphur is -2, balancing to 0). However, transition metals can sometimes have more than one possible charge (iron can be +2 or +3).