Atomic Structure and Periodic Table: Models and Trends
Notes on atomic structure and the periodic table for exams: discovery of the particles, Thomson, Rutherford and Bohr models, shells, isotopes and periodic trends.
By GK24 Editorial Team· Published · 5 min read
Everything in chemistry begins with the atom, and every competitive paper that sets a chemistry question begins with the same two areas: how the atom is built, and how the elements are arranged. Both are settled science, so the facts here do not change: a discoverer, a year, a number of electrons, a group number. This note takes the atom first and the periodic table second, because the second grew out of the first.
Who found what inside the atom
| Particle | Discoverer | Charge | Where it sits |
|---|---|---|---|
| Electron | J. J. Thomson, 1897, from cathode ray experiments | Negative | In shells around the nucleus |
| Proton | E. Goldstein found canal rays; Rutherford identified the proton | Positive | In the nucleus |
| Neutron | James Chadwick, 1932 | Neutral | In the nucleus |
The word atom itself is much older. It comes from the Greek atomos, meaning that which cannot be cut, and the Greek philosopher Democritus is credited with coining it. John Dalton turned the idea into a scientific theory in 1808, proposing that matter is made of indivisible atoms, that all atoms of an element are identical, and that atoms combine in fixed whole-number ratios.
The three models of the atom
- Thomson's model: the atom is a sphere of positive charge with electrons embedded in it, like seeds in a watermelon or plums in a pudding. It explained electrical neutrality but not the nucleus.
- Rutherford's model: from the alpha-particle scattering experiment on a thin gold foil, he concluded that almost all the mass and all the positive charge sit in a tiny central nucleus, that most of the atom is empty space, and that electrons move around the nucleus. Its weakness was that an electron circling a nucleus should lose energy and fall into it.
- Bohr's model: electrons revolve only in certain permitted orbits or shells and do not radiate energy while in them, which made the atom stable. The shells are named K, L, M and N outward from the nucleus.
Shells, electrons and the numbers that follow
The largest number of electrons a shell can hold is given by the formula 2n squared, where n is the shell number. So K holds 2, L holds 8, M holds 18 and N holds 32. Two further rules of the Bohr-Bury scheme are asked: the outermost shell of an atom cannot hold more than eight electrons, and a shell begins to fill only after the one before it is complete.
The atomic number, written Z, is the number of protons in the nucleus, and it is the identity of the element. The mass number, written A, is the number of protons and neutrons together. Atoms of the same element with the same atomic number but different mass numbers are isotopes: protium, deuterium and tritium are the isotopes of hydrogen, and chlorine occurs as chlorine-35 and chlorine-37. Atoms of different elements with the same mass number are isobars, such as argon-40 and calcium-40. Valency is the combining capacity of an atom and is decided by the electrons in its outermost shell.
How the periodic table was built
- Dobereiner's triads, 1817: groups of three elements in which the atomic mass of the middle one was about the average of the other two. Only a few triads could be found.
- Newlands' Law of Octaves, 1866: elements arranged by increasing atomic mass showed every eighth element resembling the first. He arranged 56 elements, beginning with hydrogen and ending with thorium, and the law broke down once more elements were discovered.
- Mendeleev's periodic law, 1869: the properties of elements are a periodic function of their atomic masses. He left gaps for undiscovered elements and predicted the properties of eka-boron, eka-aluminium and eka-silicon, which were later found as scandium, gallium and germanium. His table could not place hydrogen properly, had no room for isotopes and needed some pairs, such as cobalt and nickel, to be put out of strict mass order.
- Moseley's work, 1913: showed that the atomic number, not the atomic mass, is the fundamental property of an element. This gave the modern periodic law, that the properties of elements are a periodic function of their atomic numbers.
The modern periodic table
The modern table has 7 horizontal rows called periods and 18 vertical columns called groups. The first period holds 2 elements, the second and third hold 8 each, the fourth and fifth hold 18 each and the sixth holds 32. Elements in the same group have the same number of outermost electrons and therefore similar chemical behaviour, which is why groups carry family names: group 1 is the alkali metals, group 2 the alkaline earth metals, group 17 the halogens and group 18 the noble gases. The lanthanides and actinides are placed in two separate rows at the bottom. By the sub-shell being filled, the table divides into the s-block on the left, the p-block on the right, the d-block of transition elements in the middle and the f-block of the inner transition elements below.
Periodic trends worth memorising
Moving left to right across a period, the atomic radius decreases because the growing nuclear charge pulls the same shell inward, while electronegativity and electron affinity increase and metallic character decreases. Moving down a group, a new shell is added at each step, so the atomic radius increases, electronegativity decreases and metallic character increases. Fluorine is the most electronegative element, which is why it sits at the top of the halogens, and the noble gases are chemically inert because their outermost shells are already complete.
Exam Point of View
Questions here are almost always single facts. Expect 'who discovered the neutron', 'who coined the word atom', 'maximum electrons in the M shell', 'which element is most electronegative' and 'which group are the alkali metals'. Railway and SSC papers also ask which scientist gave which model and what the alpha-particle scattering experiment proved. Matching questions pair the particle with its discoverer and the group with its family name. The traps are these: Thomson discovered the electron but Rutherford named the proton, and Goldstein only found canal rays; Mendeleev's law used atomic mass while the modern law uses atomic number, and papers often swap the two; Newlands ended his table at thorium, not at uranium; and the 2n squared rule gives the capacity of a shell, but the outermost shell still cannot hold more than eight electrons, which is why calcium has 20 electrons arranged 2, 8, 8, 2 and not 2, 8, 10.
Important Facts
| Origin of the word atom | Greek atomos, meaning indivisible; credited to Democritus |
|---|---|
| Dalton's atomic theory | 1808; atoms indivisible and combining in fixed whole-number ratios |
| Electron | Discovered by J. J. Thomson in 1897; negative charge |
| Proton | Canal rays found by E. Goldstein; proton identified by Rutherford |
| Neutron | Discovered by James Chadwick in 1932; no charge |
| Thomson's model | Plum pudding or watermelon model; electrons in a sphere of positive charge |
| Rutherford's experiment | Alpha-particle scattering by thin gold foil; discovery of the nucleus |
| Bohr's model | Electrons in fixed permitted shells K, L, M, N; explains stability |
| Electrons in a shell | 2n squared: K 2, L 8, M 18, N 32 |
| Outermost shell limit | Not more than 8 electrons |
| Atomic number (Z) | Number of protons in the nucleus |
| Mass number (A) | Protons plus neutrons |
| Isotopes | Same atomic number, different mass number: protium, deuterium, tritium |
| Isobars | Same mass number, different atomic number: argon-40 and calcium-40 |
| Dobereiner | Triads, 1817; the middle mass is about the average of the other two |
| Newlands | Law of Octaves, 1866; 56 elements from hydrogen to thorium |
| Mendeleev | Periodic law of 1869 based on atomic mass; predicted three eka elements |
| Moseley | 1913; atomic number is the fundamental property of an element |
| Modern periodic table | 7 periods and 18 groups; s, p, d and f blocks |
| Most electronegative element | Fluorine |
Practice MCQs on this topic
Who coined the word 'atom'?
- A.Democritus
- B.Thomson
- C.E Rutherford
- D.John Dalton
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The Greek philosopher Democritus, working in the fifth century before the common era, argued that matter could be divided only up to a limit, and he called that smallest indivisible particle atomos, from which the word atom comes.
Read the full explanation →Who among the following discovered the neutrons?
- A.J. Chadwick
- B.E. Goldstein
- C.Neils Bohr
- D.Ernest Rutherford
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James Chadwick discovered the neutron in 1932 by bombarding beryllium with alpha particles and showing that the radiation produced consisted of neutral particles of nearly the same mass as the proton.
Read the full explanation →Rutherford's alpha-particle scattering experiment on a thin gold foil led to which conclusion?
- A.Electrons are embedded in a sphere of positive charge
- B.The atom is indivisible
- C.Electrons revolve in fixed permitted orbits
- D.Almost all the mass and the positive charge lie in a tiny nucleus
Show answer
Most alpha particles passed straight through the foil, a few were deflected through small angles and about one in twenty thousand bounced back.
Read the full explanation →The maximum number of electrons that can be accommodated in the M shell is:
- A.8
- B.2
- C.18
- D.32
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The capacity of a shell is given by the formula 2n squared, where n is the number of the shell counted outward from the nucleus.
Read the full explanation →Alkali metals are assigned which group in the Modern Periodic Table?
- A.Second group
- B.Eighteenth group
- C.Third group
- D.First group
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The alkali metals are lithium, sodium, potassium, rubidium, caesium and francium. Each has a single electron in its outermost shell, loses it readily to form a positive ion, and therefore stands in group 1 of the modern periodic table.
Read the full explanation →Which of the following elements has the highest electronegativity?
- A.Fluorine
- B.Bromine
- C.Iodine
- D.Chlorine
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Electronegativity is the tendency of an atom to attract a shared pair of electrons towards itself. It increases from left to right across a period, as nuclear charge grows, and decreases down a group, as each new shell puts the outer electrons further from the nucleus.
Read the full explanation →The modern periodic law states that the properties of elements are a periodic function of their:
- A.Atomic mass
- B.Atomic number
- C.Density
- D.Valency
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Henry Moseley showed in 1913, from the frequencies of the X-rays that elements emit, that the atomic number, the number of protons in the nucleus, is the more fundamental property of an element.
Read the full explanation →Which of the following is the last element in the Newlands Law of Octaves classification?
- A.Radium
- B.Thorium
- C.Iron
- D.Rhodium
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John Newlands arranged the fifty-six elements known in 1866 in order of increasing atomic mass, beginning with hydrogen, the lightest, and ending with thorium, the fifty-sixth.
Read the full explanation →How many periods and groups are there in the modern periodic table?
- A.8 periods and 7 groups
- B.18 periods and 7 groups
- C.7 periods and 18 groups
- D.7 periods and 8 groups
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The modern periodic table has seven horizontal rows, called periods, and eighteen vertical columns, called groups.
Read the full explanation →Isotopes of an element have the same:
- A.Atomic number but different mass number
- B.Mass number but different atomic number
- C.Number of neutrons but different number of protons
- D.Number of protons and the same number of neutrons
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Isotopes are atoms of the same element, so they have the same number of protons and therefore the same atomic number, but they carry different numbers of neutrons, which makes their mass numbers differ.
Read the full explanation →The plum pudding model of the atom, in which electrons are embedded in a sphere of positive charge, was proposed by:
- A.John Dalton
- B.J. J. Thomson
- C.Ernest Rutherford
- D.Niels Bohr
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After discovering the electron in 1897, J. J. Thomson pictured the atom as a sphere of positive charge with negatively charged electrons stuck in it, a picture often compared to plums in a pudding or seeds in a watermelon.
Read the full explanation →Mendeleev left gaps in his periodic table and predicted the properties of eka-boron, eka-aluminium and eka-silicon. These elements were later discovered as:
- A.Sodium, potassium and calcium
- B.Cobalt, nickel and copper
- C.Scandium, gallium and germanium
- D.Radium, radon and rhenium
Show answer
Mendeleev arranged the elements by increasing atomic mass in 1869 and left blank spaces where his pattern demanded an element that was not yet known.
Read the full explanation →Frequently Asked Questions
Who discovered the electron, the proton and the neutron?
J. J. Thomson discovered the electron in 1897 from cathode ray experiments. The proton was identified by Ernest Rutherford, building on the canal rays found by E. Goldstein. James Chadwick discovered the neutron in 1932.
What did Rutherford's alpha-particle scattering experiment prove?
That an atom has a very small, dense, positively charged nucleus holding almost all its mass, that most of the atom is empty space, since most alpha particles passed straight through the gold foil, and that electrons move around this nucleus.
How many electrons can each shell hold?
The capacity of a shell is 2n squared, where n is the shell number, so the K shell holds 2 electrons, L holds 8, M holds 18 and N holds 32. Separately, the outermost shell of an atom can never hold more than 8 electrons.
What is the difference between isotopes and isobars?
Isotopes are atoms of the same element with the same atomic number but different mass numbers, such as chlorine-35 and chlorine-37. Isobars are atoms of different elements that happen to have the same mass number, such as argon-40 and calcium-40.
What is the difference between Mendeleev's periodic law and the modern periodic law?
Mendeleev's law of 1869 said the properties of elements are a periodic function of their atomic masses. After Moseley's work in 1913 the modern law states that they are a periodic function of atomic numbers, which fixed the position of isotopes and of pairs such as cobalt and nickel.
How many periods and groups are there in the modern periodic table?
There are 7 periods, the horizontal rows, and 18 groups, the vertical columns. The first period has 2 elements, the second and third have 8 each, the fourth and fifth have 18 each and the sixth has 32.
Which element is the most electronegative?
Fluorine. Electronegativity increases across a period and decreases down a group, so fluorine, at the top of group 17, attracts a shared electron pair more strongly than any other element.
