Video Transcript
How is the modern periodic table
arranged? (A) By date of discovery, (B) by
alphabetical order, (C) by atomic weight, (D) by atomic number, (E) by melting
point.
First of all, the periodic table is
a chart that arranges the 118 known elements. But how exactly are the elements
arranged? In ancient times, useful metals
like iron, copper, silver, and gold were already known. Much later, in the 18th century,
scientists discovered gases like hydrogen, nitrogen, and oxygen and studied their
properties. The noble gases helium and neon
were not discovered until almost the 20th century. We can tell from the location of
all of these elements on the periodic table that the elements must not be arranged
by the date of discovery. So, answer choice (A) is
incorrect.
The elements labeled here in pink
are gases at room temperature because they have very low melting and boiling
points. The only elements that are liquids
at room temperature are mercury and bromine. All of the remaining elements,
except for those that scientists know little about, are solids at room
temperature. These elements, which are shaded in
orange, have relatively high melting and boiling points. Now, we can see that the elements
are definitely not arranged by increasing melting point. So, answer choice (E) is not
correct.
Each box on the periodic table
represents one element. The box usually includes the
element symbol and, sometimes, the element name. Although it’s not true for all
elements, many element symbols are the first one or two letters of the English
name. For example, the element symbol of
nitrogen is N, which is the first letter of the English name. Now, let’s look at the names and
symbols of the first 10 elements. Even though this is a small set of
elements, we can clearly see that the elements are not arranged by alphabetical
order. So, answer choice (B) is
incorrect.
Let’s look at the element box of
nitrogen again. Each element box on the periodic
table may include two numbers. The whole number is the atomic
number of the element, and it is the smaller of the two numbers. The atomic number is the number of
protons in the nucleus of an atom of a specific element. The atomic number of nitrogen is
seven, which means that there are seven protons in the nucleus of a nitrogen
atom.
Let’s look at the atomic numbers of
the first 10 elements. We can see that each time we move
to the next element, the atomic number increases by one. Now, we know that the elements on
the periodic table are arranged by atomic number.
The other number in an element box
is the atomic mass or atomic weight. This is usually a decimal
number. The atomic mass does generally
increase as we move from element to element across the periodic table, but not
always. For example, the atomic number of
cobalt is 27, and the atomic number of nickel is 28. However, nickel’s atomic weight is
actually smaller than cobalt’s atomic weight. There are other examples of
elements that have a greater atomic number but a smaller atomic weight. So, the modern periodic table is
not arranged by increasing atomic weight.
In conclusion, on the modern
periodic table, the elements are arranged by atomic number. So, the correct answer is choice
(D).
