The atom is the basic building block of all matter, but it is itself made of still smaller charged particles arranged in a definite way. These notes move from the atom and its three sub-atomic particles, through the historical atomic models, to atomic number and mass number, isotopes and their cousins, and finally the way electrons fill up shells to decide an element's valency.
The atom is the smallest unit of an element that takes part in a chemical reaction, yet it can be broken further into smaller charged particles.
The word 'atom' comes from the Greek 'atomos', meaning:
Who first used the term 'atom' in modern science?
An atom as a whole is electrically because protons equal electrons.
The radius of an atom is of the order of 10⁻¹⁰ metre, i.e. one .
An atom is built from three fundamental particles — a negatively charged electron, a positively charged proton, and a neutral neutron.
| Particle | Symbol | Charge | Relative mass | Discoverer |
|---|---|---|---|---|
| Electron | e⁻ | -1 | 1/1837 (≈ negligible) | J.J. Thomson |
| Proton | p⁺ | +1 | 1 (1.6726 × 10⁻²⁷ kg) | Goldstein / Rutherford |
| Neutron | n⁰ | 0 | 1 (1.6749 × 10⁻²⁷ kg) | James Chadwick |
Which sub-atomic particle carries no charge?
Protons and neutrons together are called:
The electron was discovered by .
The neutron was discovered by James .
The electron is the lightest of the three particles and carries the unit negative charge.
The mass of an electron is about what fraction of a proton's mass?
Electrons are found:
The charge on an electron is -1.6 × 10⁻¹⁹ .
The electron was discovered through his -ray (discharge tube) experiments.
The proton carries the unit positive charge and sits inside the nucleus.
The number of protons in an atom equals its:
Who gave the name 'proton'?
The proton was discovered by in his canal-ray experiment.
The charge on a proton is +1.6 × 10⁻¹⁹ .
The neutron is electrically neutral and was the last of the three particles to be discovered.
Which is the only atom that has no neutron?
The neutron is:
The neutron carries a charge of .
Neutrons are present inside the along with protons.
John Dalton gave the first scientific theory of the atom, treating it as a tiny, solid, indivisible ball.
According to Dalton's theory, atoms in a chemical reaction:
The main drawback of Dalton's theory was that it held the atom to be:
According to Dalton, atoms combine in small -number ratios to form compounds.
Dalton's atomic theory was proposed in the year .
After discovering the electron, J.J. Thomson pictured the atom as a positive sphere with electrons stuck in it.
Thomson's model of the atom is popularly called the:
In Thomson's model, the atom is a sphere of:
A drawback of Thomson's model was that it could not explain the .
In Thomson's model the positive and negative charges are , so the atom is neutral.
Rutherford fired alpha particles at a thin gold foil and from the way they scattered concluded that the atom is mostly empty space with a tiny dense nucleus.
Rutherford's gold-foil experiment showed that the atom is:
The tiny, dense, positively charged centre of the atom is the:
Rutherford fired particles at a thin gold foil.
A drawback was that a revolving electron should lose energy and into the nucleus.
Niels Bohr fixed the flaw in Rutherford's model by allowing electrons only in certain fixed, stable orbits.
In Bohr's model, electrons revolve only in:
According to Bohr, energy is absorbed when an electron:
Bohr's shells are named K, L, M, from the nucleus outward.
Bohr's model explained the of the atom that Rutherford's could not.
Two numbers fix the make-up of an atom's nucleus — the atomic number counts the protons, the mass number counts protons plus neutrons.
The mass number (A) of an atom equals:
For sodium ₁₁Na²³, the number of neutrons is:
The number of neutrons N = A − .
In a neutral atom the number of protons equals the number of .
Atoms can match each other in protons, in mass number, or in neutrons — giving three different "iso-" families.
| Type | What is same | What differs | Example |
|---|---|---|---|
| Isotopes | Atomic number Z (protons) | Mass number A (neutrons) | ₁H¹, ₁H², ₁H³ (Protium, Deuterium, Tritium); ₆C¹², ₆C¹⁴ |
| Isobars | Mass number A | Atomic number Z | ₁₈Ar⁴⁰ and ₂₀Ca⁴⁰ |
| Isotones | Number of neutrons N | Z and A | ₆C¹⁴ and ₈O¹⁶ (each has 8 neutrons) |
Atoms with the same atomic number but different mass numbers are called:
₁₈Ar⁴⁰ and ₂₀Ca⁴⁰ are examples of:
Isotones have the same number of but different Z and A.
Carbon-14 (₆C¹⁴) is a radioactive isotope used in carbon .
Electrons are arranged in shells around the nucleus, and each shell can hold only a fixed maximum number of electrons.
| Shell | n | Maximum electrons (2n²) |
|---|---|---|
| K | 1 | 2 × 1² = 2 |
| L | 2 | 2 × 2² = 8 |
| M | 3 | 2 × 3² = 18 |
| N | 4 | 2 × 4² = 32 |
The maximum number of electrons in a shell is given by:
The maximum number of electrons the M shell can hold is:
The outermost shell can hold a maximum of electrons.
The electronic configuration of sodium (Z = 11) is 2, 8, .
The outermost shell decides how an atom bonds, through the number of electrons it has and how many it needs to become stable.
The electrons in the outermost shell of an atom are called:
The valency of chlorine (2, 8, 7) is:
Valency is the capacity of an atom.
Noble gases have a complete outer shell, so their valency is .
Although an atom contains charged particles, it shows no overall charge because the positive and negative charges exactly cancel.
An atom is electrically neutral because:
An atom that loses electrons becomes a:
The has no charge, so it does not affect the charge balance of the atom.
An atom that gains electrons becomes a negative ion called a/an .
Take 5 questions at a time — tap an option to check. After each round, revise the notes above and take the retest for 5 fresh questions, until you've mastered the whole chapter.