Class 9 Science: Structure of Atoms

This set of vocabulary flashcards covers essential concepts from the Class 9 Science lecture on the Structure of Atoms, including atomic models, sub-atomic particles, electronic configuration, and periodicity.

Canal rays

Positively charged rays discovered by E. Goldstein.

Sub-atomic particles

The three constituent particles of an atom: Proton, Neutron, and Electron.

Atomic Number ($Z$)

The number of protons in an atom, which defines the element and remains constant during chemical reactions.

Mass Number ($A$)

The sum of the number of protons and neutrons present in the nucleus of an atom.

Charge of a proton

The positive charge value of a proton, which is exactly $+1.6 \times 10^{-19}\,C$.

Isotopes

Atoms of the same element having identical atomic numbers but different mass numbers due to a different number of neutrons.

Isobars

Atoms of different atomic species that have different atomic numbers but share the same mass number.

Valence electrons

The electrons present in the outermost shell of an atom which determine its chemical properties and valency.

Valency

The combining capacity of an atom, determined by the number of electrons it needs to gain, lose, or share to complete its octet.

Thomson’s Model

Proposed that an atom consists of a positively charged sphere with negatively charged electrons embedded in it, making the atom neutral as a whole.

Rutherford’s Model

Identified the presence of a central nucleus containing protons, with electrons revolving around it.

Bohr’s Model

States that electrons revolve around the nucleus in discrete orbits or shells ($K, L, M, N...$) with fixed energy levels.

$2n^2$ Rule

The formula used to find the maximum number of electrons that can exist in a shell, where $n$ is the orbit number.

Mendeleev’s Periodic Law

The principle stating that the properties of elements are a periodic function of their atomic mass.

Ionization Energy

The quantity of energy that must be supplied to one mole of a gaseous element to produce one mole of gaseous cations.

Electron Affinity

The energy change associated with the formation of one mole of gaseous anions from one mole of gaseous atoms.

Atomicity

The number of atoms that constitute a single molecule of an element, such as 2 for fluorine ($F_2$).

Cathode Rays

Negatively charged rays that exhibit deflection by magnetic fields in the direction of the anode.

Beta ($\beta^-$) Particle Emission

A nuclear process where the atomic number of an atom increases by 1 while its mass number remains unchanged.

Gamma ($\gamma$) Ray Emission

A process where a nucleus releases energy as a photon, resulting in no change to mass or charge.

Electropositive

The nature of metals to give away electrons from their outermost shell to achieve stability and form positive charges.

Electronegative

The nature of non-metals to gain electrons to complete their octet and form negative charges.