Dobereiner and Newlands
In a library, books are arranged by subject so you can find them easily. Over 100 elements also needed arranging so that scientists could study their patterns. The first attempts looked for simple patterns in atomic masses.
Classification (वर्गीकरण) of elements means grouping those with similar properties.
Rule 1: Dobereiner's triads — Dobereiner (1817) grouped elements in threes called triads (त्रयी): the atomic mass of the middle element is the mean of the other two. Example: Li 6.9, Na 23.0, K 39.1; the mean of 6.9 and 39.1 is 23.0.
In the figure : In the figure the middle element is the mean of the other two.
- Easy : Easy: Cl 35.5, Br 79.9, I 126.9: the mean of Cl and I is (35.5 + 126.9)/2 = 81.2, close to Br at 79.9.
- Medium : Medium: Ca 40.1, Sr 87.6, Ba 137.3: (40.1 + 137.3)/2 = 88.7, close to Sr; so these three form a triad.
- Exam-type : Exam: a limitation of triads – only a few such groups of three could be found.
Rule 2: Newlands' octaves — Newlands (1866) arranged elements in increasing atomic mass and noticed that every 8th element resembled the first, like the notes of a music octave. This worked only up to calcium.
- Easy : Easy: starting at Li, the 8th element is Na, and both are soft metals reacting with water.
- Medium : Medium: the octave idea failed after calcium, since newly discovered elements did not fit the pattern.
- Exam-type : Exam: compare triads and octaves – triads: groups of 3, middle is the mean; octaves: 8th element repeats properties.
Dobereiner: triads (middle = mean). Newlands: octaves (8th resembles 1st; up to Ca).
Thinking triads need the middle element to be exactly the mean. It is only close to it.
Memory clue: "Dobereiner — Three; Newlands — Eight".
Spot the difference
| Scientist | Idea |
|---|---|
| Dobereiner (1817) | Triads |
| Newlands (1866) | Law of octaves |
Practice questions
- 1Who proposed triads?
- 2What is a triad?
- 3Name a triad of alkali metals.
- 4Who proposed the law of octaves?
- 5Up to which element did the law of octaves hold?
- 6Check the triad Li 6.9, Na 23.0, K 39.1.
- 7Find the mean of S 32.1 and Te 127.6 and compare with Se 79.0.
- 8Why was Newlands' octave rule not accepted for all elements?
Show answers (try them yourself first)
- 1Dobereiner.
- 2A group of three elements where the middle has the mean mass of the others.
- 3Li, Na, K.
- 4Newlands.
- 5Calcium.
- 6(6.9 + 39.1)/2 = 23.0, which equals Na.
- 7(32.1 + 127.6)/2 = 79.85, close to 79.0.
- 8It worked only up to calcium; later elements did not follow the pattern.
Mendeleev's periodic table
When a puzzle has a missing piece, you can still guess its shape from the pieces around it. Mendeleev did exactly this with elements, and later found the missing pieces.
Mendeleev's periodic law: the properties of elements are a periodic function of their atomic masses.
Rule 1: Mendeleev's table — Mendeleev arranged elements by increasing atomic mass in vertical groups and horizontal periods. He left gaps for undiscovered elements and predicted their properties: eka-boron (later scandium), eka-aluminium (gallium) and eka-silicon (germanium).
In the figure : In the figure Mendeleev left gaps and predicted new elements.
- Easy : Easy: eka-aluminium was predicted by Mendeleev and was later discovered as gallium.
- Medium : Medium: since the predicted properties matched the real element, scientists trusted his table.
- Exam-type : Exam: eka means 'one beyond'; eka-silicon is the element placed one step beyond silicon, now called germanium.
Rule 2: Limits of Mendeleev's table — Noble gases, discovered later, were added as the zero group without disturbing the table. Limits: the position of hydrogen was unclear; isotopes (same element, different masses) had no place; and some elements were out of mass order (cobalt 58.9 placed before nickel 58.7).
- Easy : Easy: cobalt (58.9) is placed before nickel (58.7), which breaks the rule of increasing mass.
- Medium : Medium: isotopes of an element have different masses but the same properties, so they cannot be given separate places by mass.
- Exam-type : Exam: name two limits of Mendeleev's table: hydrogen's position, isotopes.
Mendeleev: atomic mass, groups and periods, predicted Sc, Ga, Ge. Limits: H, isotopes, Co/Ni.
Thinking Mendeleev's table used atomic number. It used atomic mass.
Memory clue: "Boron, Aluminium, Silicon → Sc, Ga, Ge".
Spot the difference
| Predicted | Discovered as |
|---|---|
| Eka-boron | Scandium |
| Eka-aluminium | Gallium |
| Eka-silicon | Germanium |
Practice questions
- 1On what basis did Mendeleev arrange the elements?
- 2Which element was predicted as eka-silicon?
- 3Which group was added for noble gases?
- 4Name one limitation of Mendeleev's table.
- 5What are isotopes?
- 6What does 'eka' mean?
- 7Why did the discovery of gallium strengthen Mendeleev's table?
- 8Why was it a problem that cobalt comes before nickel?
Show answers (try them yourself first)
- 1Increasing atomic mass.
- 2Germanium.
- 3Zero group.
- 4Position of hydrogen was unclear.
- 5Atoms of the same element with different masses.
- 6One beyond (the next after).
- 7Its properties matched his prediction for eka-aluminium.
- 8Cobalt has a higher mass, which breaks the order of increasing mass.
Corrections, noble gases and Moseley
A shop that arranged goods by weight would be confusing if two items of the same weight did not match. Moseley found a better number than weight to arrange elements.
Atomic number (अणुअंक) is the number of protons in the nucleus of an atom.
Rule 1: Moseley's modern periodic law — Moseley (1913) showed that the properties of elements are a periodic function of their atomic number, not atomic mass. This removed the problem of cobalt and nickel and of isotopes, which have the same atomic number.
- Easy : Easy: carbon has 6 protons, so its atomic number is 6; sodium has 11 protons, so its atomic number is 11.
- Medium : Medium: Co has atomic number 27 and Ni has 28, so Co comes first by atomic number, which settles the old problem.
- Exam-type : Exam: a neutral atom has equal protons and electrons, so atomic number also equals the number of electrons.
Moseley, 1913: properties depend on atomic number (number of protons).
Using mass number as atomic number. Atomic number = protons only.
Memory clue: "Moseley — Number of protons".
Spot the difference
| Scientist | Basis |
|---|---|
| Mendeleev | Atomic mass |
| Moseley | Atomic number |
Practice questions
- 1Define atomic number.
- 2Who gave the modern periodic law?
- 3State the modern periodic law.
- 4Which problem of Mendeleev's table did atomic number solve?
- 5How many electrons does a neutral atom of atomic number 15 have?
- 6Why do isotopes have the same place in the periodic table?
- 7An element has 17 protons. Give its atomic number and the number of electrons.
- 8Explain how atomic number is a better basis than atomic mass.
Show answers (try them yourself first)
- 1The number of protons in the nucleus of an atom.
- 2Moseley.
- 3Properties of elements are a periodic function of their atomic numbers.
- 4Cobalt–nickel order and isotopes.
- 515.
- 6They have the same atomic number.
- 7Atomic number 17; electrons 17.
- 8It is unique to each element and fixes the order without exceptions.
Structure of the modern table
Seats in a cinema hall are numbered by row and seat. In the same way, every element has an address in the periodic table: its period (row) and its group (column).
A group (गट) is a vertical column and a period (आवर्त) is a horizontal row of the periodic table.
Rule 1: Structure of the modern table — The modern table has 18 groups, 7 periods and 118 elements. Elements in the same group have the same number of valence electrons. Elements in the same period have the same number of shells. Valency of oxygen (2, 6) is 2. Silicon is the metalloid in period 3.
In the figure : In the figure the modern periodic table.
Structure of the modern table
In the app you can move this figure with your finger.
Open in the app- Easy : Easy: Na has 2, 8, 1; it has 3 shells and 1 valence electron, so it is in period 3, group 1.
- Medium : Medium: Cl has 2, 8, 7; period 3 (3 shells) and group 17 (7 valence electrons), with valency 1 (it needs one more electron).
- Exam-type : Exam: an element with atomic number 12: 2, 8, 2; three shells so period 3; two valence electrons so group 2; valency 2; it is magnesium.
18 groups, 7 periods, 118 elements. Group: valence electrons. Period: shells.
Mixing up groups and periods. Groups are columns; periods are rows.
Memory clue: "Group — Guards the Ground (column); Period — flat (row)".
Spot the difference
| Feature | Same group | Same period |
|---|---|---|
| Direction | Vertical | Horizontal |
| Same in all | Valence electrons | Number of shells |
| Total | 18 | 7 |
Practice questions
- 1How many groups are there in the modern table?
- 2How many periods are there?
- 3How many elements are known in the modern table?
- 4What is the same for elements in a group?
- 5What is the same for elements in a period?
- 6Name the metalloid in period 3.
- 7Find the period, group and valency of the element with atomic number 17.
- 8An element has 2 shells and 7 valence electrons. Name its period, group and the element.
Show answers (try them yourself first)
- 118.
- 27.
- 3118.
- 4Number of valence electrons.
- 5Number of shells.
- 6Silicon.
- 7Configuration 2, 8, 7; period 3; group 17; valency 1.
- 8Period 2, group 17, fluorine (atomic number 9).
Valency and periodic trends
Walk from the left end to the right end of a row in the periodic table: the atoms get smaller and more eager to take electrons. Walk down a column: atoms get bigger. These are periodic trends.
Atomic radius (अणुत्रिज्या) is the distance from the nucleus to the outermost electron shell of an atom.
Rule 1: Atomic radius and valency — Across a period, the atomic radius decreases (Li 152 pm to O 66 pm), because the nuclear charge increases and pulls the electrons in. Down a group it increases, because new shells are added. Valency is the number of electrons an atom gains, loses or shares to complete its octet.
In the figure : In the figure trends in the periodic table.
- Easy : Easy: Na (period 3) is bigger than Li (period 2) as it has one more shell.
- Medium : Medium: in period 2, Li to O radius falls from 152 pm to 66 pm, so oxygen is much smaller than lithium.
- Exam-type : Exam: arrange Na, Mg, Al in order of atomic radius (period 3): Na > Mg > Al, because nuclear charge increases.
Rule 2: Metallic character — Metallic character decreases across a period and increases down a group. So the most metallic elements are at the bottom left and the least metallic at the top right.
- Easy : Easy: sodium is more metallic than chlorine, since it is on the left of period 3.
- Medium : Medium: potassium is more reactive than sodium, since metallic character grows down group 1.
- Exam-type : Exam: explain why atomic radius increases down a group: each period adds a new electron shell.
Across period: radius down, metallic character down. Down group: radius up, metallic character up.
Thinking radius increases across a period. It decreases.
Memory clue: "Right is Small, Down is Big".
Spot the difference
| Trend | Across a period | Down a group |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Metallic character | Decreases | Increases |
Practice questions
- 1What is atomic radius?
- 2How does atomic radius change across a period?
- 3How does atomic radius change down a group?
- 4Which is larger, Li or O?
- 5How does metallic character change down a group?
- 6What is valency?
- 7Arrange K, Na, Li in increasing order of atomic radius.
- 8Why does atomic radius decrease across a period?
Show answers (try them yourself first)
- 1Distance from the nucleus to the outermost shell.
- 2It decreases.
- 3It increases.
- 4Li.
- 5It increases.
- 6Number of electrons an atom gains, loses or shares to complete its octet.
- 7Li < Na < K.
- 8The nuclear charge increases and pulls the electrons closer while the shells stay the same.
Quick revision
- Dobereiner (triads), Newlands (octaves), Mendeleev (mass), Moseley (atomic number).
- 18 groups, 7 periods; radius decreases across a period.