Classification of Elements and Periodicity in Properties · Lesson 2 of 10
The Modern Periodic Table and Element Names
“Read the modern table using atomic number, periods, groups, and systematic element names.”
• State the modern periodic law and explain why atomic number is its basis. • Identify periods, groups, and the detached rows in the long-form table. • Construct temporary element names and symbols from atomic-number digits. • Distinguish temporary names from official names and recognise the named elements through 118.
Atomic number gives a consistent ordering
Mendeleev’s arrangement successfully connected chemical properties, but atomic-weight ordering produced awkward cases such as tellurium and iodine. The discovery of the atom’s internal structure supplied a more fundamental property. Each element has a particular number of protons, and that number determines its identity.
The number of protons in an atom’s nucleus, represented by . In a neutral atom it also equals the number of electrons. Gaining or losing electrons changes charge but does not change the element.
In 1913 Henry Moseley investigated characteristic X-rays emitted by elements. The square root of their frequency varied regularly with atomic number. A straight-line relationship with , rather than atomic mass, showed that nuclear charge supplied the consistent ordering. Tellurium has and iodine , so their chemically appropriate order now follows the fundamental ordering too.
The physical and chemical properties of elements are periodic functions of their atomic numbers. Similar outer electron arrangements recur as atomic number increases, producing recurring chemical behaviour.
The law does not say that every property changes smoothly or repeats with a single fixed interval. It identifies atomic number as the organising variable; electron configurations explain the repeating families.
Problem
Two neutral atoms have 17 protons but different numbers of neutrons. Should they occupy different positions?
- 1.The atomic number of each atom is 17 because each has 17 protons.
- 2.Their different neutron counts change their masses, so they are isotopes.
- 3.Both neutral atoms have 17 electrons and essentially the same chemical electron arrangement. They therefore occupy the same chlorine position.
- 4.This illustrates why mass is not the defining variable of the modern table.
Reading the long form
The table is a map with two useful directions. A horizontal row is a period, and a vertical column is a group or family. Across a period the atomic numbers are consecutive; down a group they are not consecutive, because a new outer electron arrangement must be reached before the family pattern returns.
There are seven periods and eighteen numbered groups. The older letter-and-Roman-numeral group labels can be ambiguous, so the numbers 1–18 are used here. Elements in the same group usually have related valence configurations and similar chemistry; the next lesson explains why.
| Period | Elements in the completed period | Opening and closing elements |
|---|---|---|
| 1 | 2 | H to He |
| 2 | 8 | Li to Ne |
| 3 | 8 | Na to Ar |
| 4 | 18 | K to Kr |
| 5 | 18 | Rb to Xe |
| 6 | 32 | Cs to Rn |
| 7 | 32 | Fr to Og |
The two detached rows shorten the printed table. They are not an eighth and ninth period. The first fits within period 6 and the second within period 7. Seaborg’s work on heavy elements helped establish the placement of actinoids below lanthanoids. The arrangement groups elements with closely related behaviour while keeping the main table manageable.
Problem
Locate chlorine and flerovium using the table.
- 1.Chlorine, Cl, has atomic number 17. It lies in the third horizontal row and the seventeenth column.
- 2.Its position is period 3, group 17.
- 3.Flerovium, Fl, has atomic number 114 and lies in period 7, group 14.
- 4.A position includes both a row and a column: the atomic number alone is not a period or group label.
Temporary names from atomic-number digits
Producing a very heavy element may yield only a few atoms, so establishing a discovery requires strong evidence. Historically, competing claims also led to competing names. A systematic temporary name identifies an element unambiguously by atomic number while its discovery and permanent name are considered.
| Digit | Root | Symbol letter |
|---|---|---|
| 0 | nil | n |
| 1 | un | u |
| 2 | bi | b |
| 3 | tri | t |
| 4 | quad | q |
| 5 | pent | p |
| 6 | hex | h |
| 7 | sept | s |
| 8 | oct | o |
| 9 | enn | e |
Read the digits in order, join their roots, and add the ending “ium”. The temporary symbol uses the root letters in the same order, with only the first capitalised. Minor spelling adjustments avoid awkward repeated letters: the final i of bi or tri is dropped before ium, and the final n of enn is dropped before nil. A temporary symbol has three letters; an established official symbol normally has one or two.
Problem
Construct the systematic temporary name and symbol for atomic number 120.
- 1.Read the digits separately: 1, 2, 0.
- 2.Their roots are un, bi, and nil. Joining them and adding ium gives unbinilium.
- 3.The symbol letters are u, b, n, so the temporary symbol is Ubn.
- 4.This names a hypothetical atomic-number position. It does not itself announce a discovery or an official permanent name.
Problem
An old table labels element 118 Uuo. What do the name and symbol mean?
- 1.The digits 1, 1, 8 give un–un–oct and the systematic name ununoctium.
- 2.The corresponding temporary symbol is Uuo.
- 3.The official name is oganesson, with symbol Og. Use Og when referring to the officially named element.
- 4.A change from a temporary to a permanent name does not change the atomic number.
| Atomic number | Temporary name and symbol | Official name and symbol |
|---|---|---|
| 101 | Unnilunium — Unu | Mendelevium — Md |
| 102 | Unnilbium — Unb | Nobelium — No |
| 103 | Unniltrium — Unt | Lawrencium — Lr |
| 104 | Unnilquadium — Unq | Rutherfordium — Rf |
| 105 | Unnilpentium — Unp | Dubnium — Db |
| 106 | Unnilhexium — Unh | Seaborgium — Sg |
| 107 | Unnilseptium — Uns | Bohrium — Bh |
| 108 | Unniloctium — Uno | Hassium — Hs |
| 109 | Unnilennium — Une | Meitnerium — Mt |
| 110 | Ununnillium — Uun | Darmstadtium — Ds |
| 111 | Unununnium — Uuu | Roentgenium — Rg |
| 112 | Ununbium — Uub | Copernicium — Cn |
| 113 | Ununtrium — Uut | Nihonium — Nh |
| 114 | Ununquadium — Uuq | Flerovium — Fl |
| 115 | Ununpentium — Uup | Moscovium — Mc |
| 116 | Ununhexium — Uuh | Livermorium — Lv |
| 117 | Ununseptium — Uus | Tennessine — Ts |
| 118 | Ununoctium — Uuo | Oganesson — Og |
Permanent names may honour a scientist, a place, or another accepted source of a name. Lawrencium honours Ernest Lawrence, and seaborgium honours Glenn Seaborg. IUPAC’s recognition and naming process replaces a temporary label with an agreed name and symbol. Elements through 118 are officially named, so the seventh period is complete through oganesson; older descriptions of it as incomplete are historical.
Quiz
Which property fixes an element’s position in the modern ordering?
Which statement correctly describes the two detached rows?
What is the temporary symbol for atomic number 120?
Which official name and symbol belong to element 117?
What did Moseley’s X-ray observations help establish?
Practice Problems
- State the modern periodic law and explain its connection to electron arrangement.
- Why do isotopes occupy one position despite differing masses?
- Locate the elements with atomic numbers 17 and 114 by period and group.
- Construct the temporary name and symbol for 119 using the digit-root table.
- Explain why Uuq and Fl refer to the same atomic-number position but are not equally suitable as current names.
- Identify the scientist honoured by each of Lr and Sg.
- Explain why the detached rows do not increase the number of periods from seven to nine.
Key Takeaways
Atomic number is proton number and supplies the modern ordering of elements. Periods are rows and groups are columns; the detached rows are parts of periods 6 and 7. Temporary systematic names encode atomic-number digits and are distinct from official names. Elements through atomic number 118 have official names; Ts and Og occupy the final two positions of period 7.