SPI Chemistry · Atomic Structure and Periodic Classification

Structure of the Atom

An atom is a tiny positive nucleus of protons and neutrons, with electrons moving around it in fixed shells.

All Atomic Structure and Periodic Classification notes

Why this matters

Five questions in the bank, two EASY and three MODERATE: who found the nucleus, which way an electron moves when it gains energy, protons and neutrons from atomic and mass numbers, isobars, and the ion an element forms. Each one rests on a fact you can learn on this page.

Concept 1 of 3: The particles of the atom and the models that found them

The atom was opened up one discovery at a time. First the electron, then the proton, then Rutherford's surprise that the atom is mostly empty with a tiny heavy centre, and then Bohr's idea that electrons sit in fixed shells. Learn who found what, and what each experiment showed.

Definition

Thomson found the electron (negative) in cathode rays and pictured the atom as a positive ball with electrons stuck in it, like a Christmas pudding.
Rutherford fired alpha particles at thin gold foil. Most went straight through, a few were turned aside and very few bounced back. So the atom is mostly empty space with a tiny, heavy, positive nucleus.
Bohr said electrons move only in fixed orbits (shells K, L, M, N from the nucleus outwards) and do not lose energy while they stay in one.
Moving to a shell farther from the nucleus, an electron gains energy. Moving closer, it loses energy.

Discovery or model	Scientist	What it showed
Electron	J.J. Thomson	Cathode rays carry negative particles
Proton	E. Goldstein	Canal rays carry positive particles
Neutron	James Chadwick	A particle with no charge and a mass close to a proton's
Nucleus	Ernest Rutherford	Alpha-particle scattering from gold foil: a tiny, dense, positive centre The scattering experiment found the nucleus, not the proton or the neutron.
Fixed orbits	Niels Bohr	Electrons move in shells K, L, M, N without losing energy

Energy rises with distance from the nucleus: K is the lowest shell.

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Example 1 · Atomic Structure and Periodic Classification · Structure of the Atom

Rutherford's alpha - particle scattering experiment was responsible for the discovery of:

Crediting Rutherford with the proton

Rutherford's gold foil experiment showed the nucleus. The electron was found by Thomson, the proton by Goldstein in canal rays and the neutron by Chadwick.

Getting the energy direction backwards

An electron needs energy to move away from the nucleus and gives energy out when it falls closer. If an electron loses energy going from one orbit to another, the second orbit is the nearer one.

Drill 1 more on the particles of the atom and the models that found them

Concept 2 of 3: Atomic number, mass number, isotopes and isobars

Two numbers describe every atom. The atomic number counts its protons and decides which element it is. The mass number counts protons and neutrons together. Subtract and you have the neutrons.

Definition

Atomic number $Z$ = number of protons = number of electrons in a neutral atom.
Mass number $A$ = protons + neutrons.
Written as $^{A}_{Z}\mathrm{X}$ : the mass number on top, the atomic number below.
Isotopes: same atomic number, different mass numbers ( $^{12}_{6}\mathrm{C}$ , $^{14}_{6}\mathrm{C}$ ). Same electrons, so the same chemical properties.
Isobars: same mass number, different atomic numbers. Different elements, so different chemical properties.

Mass number and neutrons

A = Z + N \qquad N = A - Z

Amass number (protons + neutrons)
Zatomic number (protons)
Nnumber of neutrons

Worked example

A phosphorus atom is written

^{31}_{15}\mathrm{P}

. Find its protons, electrons and neutrons.

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Example 2 · Atomic Structure and Periodic Classification · Structure of the Atom

Which of the following statements are incorrect regarding the elements given below?

^{39}_{19}U

^{127}_{53}V

^{35}_{17}W

^{80}_{35}X

^{40}_{18}Y

i. The correct order of increasing proton number is U < W < X < V < Y. ii. The correct order of increasing mass number is W < U < Y < X < V. iii. There is difference in the orders of proton number and mass number. iv. The number of protons is equal to number of neutrons in all the given elements.

Taking the mass number as the neutron count

The mass number counts protons and neutrons together. The neutrons are the mass number minus the atomic number, so

^{31}_{15}\mathrm{P}

has 16 neutrons, not 31.

Thinking a heavier atom always has the larger atomic number

Order by atomic number and order by mass number can differ. Cobalt (atomic number 27, mass number 59) comes before nickel (atomic number 28, mass number 58).

Giving isobars the same chemistry

Isobars share only the mass number. Their atomic numbers differ, so they are different elements with different chemical properties. It is isotopes that share chemistry.

Drill 1 more on atomic number, mass number, isotopes and isobars

Concept 3 of 3: Electron shells, valence electrons and ions

Electrons fill the shells from the inside out. The electrons in the outermost shell, the valence electrons, decide how an atom reacts: it loses or gains electrons to end up with a full outer shell, like a noble gas.

Definition

Shell $n$ holds at most $2n^2$ electrons: K 2, L 8, M 18. For the first 20 elements the outer shell takes no more than 8.
Fill in order: 2 in K, then 8 in L, then M. Sodium (11) is 2, 8, 1.
Valency: with 1 to 4 valence electrons it is that number; with 5 to 7 it is 8 minus that number. A full shell (2 for helium, 8 for the rest) means valency 0.
Metals lose their valence electrons and form positive ions: 2 lost gives a dipositive ion such as $\mathrm{Ca^{2+}}$ .
Non-metals gain electrons and form negative ions: $\mathrm{O^{2-}}$ , $\mathrm{Cl^{-}}$ .

Shell capacity and valency

\text{electrons in shell } n \le 2n^2 \qquad \text{valency} = 8 - \text{valence electrons (5 to 7 valence electrons)}

Worked example

Aluminium has atomic number 13. Write its electronic configuration and the ion it forms.

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Practice questionEasy

Example 3 · Atomic Structure and Periodic Classification · Structure of the Atom

Which of the following element forms dipositive ions very easily?

Taking valency as the valence electrons every time

Chlorine has 7 valence electrons but valency 1: it is easier to gain 1 electron than to lose 7. With 5 or more valence electrons, valency is 8 minus that number.

Overfilling the M shell

Potassium (19) is 2, 8, 8, 1, not 2, 8, 9. For the first 20 elements the outer shell holds at most 8, and the next electron starts the N shell.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Atomic number, mass number, isotopes and isobars
Mass number and neutrons
$A = Z + N \qquad N = A - Z$
Electron shells, valence electrons and ions
Shell capacity and valency
$\text{electrons in shell } n \le 2n^2 \qquad \text{valency} = 8 - \text{valence electrons (5 to 7 valence electrons)}$

Reference tables (1)

The particles of the atom and the models that found them5 rows

Discovery or model	Scientist	What it showed
Electron	J.J. Thomson	Cathode rays carry negative particles
Proton	E. Goldstein	Canal rays carry positive particles
Neutron	James Chadwick	A particle with no charge and a mass close to a proton's
Nucleus	Ernest Rutherford	Alpha-particle scattering from gold foil: a tiny, dense, positive centre The scattering experiment found the nucleus, not the proton or the neutron.
Fixed orbits	Niels Bohr	Electrons move in shells K, L, M, N without losing energy

Energy rises with distance from the nucleus: K is the lowest shell.

Watch out for (7)

Crediting Rutherford with the proton
→ The particles of the atom and the models that found them
Getting the energy direction backwards
→ The particles of the atom and the models that found them
Taking the mass number as the neutron count
→ Atomic number, mass number, isotopes and isobars
Thinking a heavier atom always has the larger atomic number
→ Atomic number, mass number, isotopes and isobars
Giving isobars the same chemistry
→ Atomic number, mass number, isotopes and isobars
Taking valency as the valence electrons every time
→ Electron shells, valence electrons and ions
Overfilling the M shell
→ Electron shells, valence electrons and ions

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