The sources define the mass number as the sum of the number of protons \( ( Z ) \) and the number of neutrons \( ( n ) \).

J.J. Thomson proposed that an atom is a sphere of positive charge with electrons embedded in it, similar to seeds in a watermelon.

Chromium has an exceptional configuration of \( [ Ar ] 3d^{ 5 } 4s^{ 1 } \) because half-filled subshells have extra stability.

The Balmer series is the only series of lines in the hydrogen spectrum that appears in the visible region of the electromagnetic spectrum.

Isobars are atoms with the same mass number but different atomic numbers, such as \( ^{ 14 }{ 6 } C \) and \( ^{ 14 }{ 7 } N \).

Calculations by Rutherford showed that the radius of the atom is about \( 10^{ -10 } m \), while that of the nucleus is only \( 10^{ -15 } m \).

The azimuthal quantum number \( ( l ) \), also known as the orbital angular momentum quantum number, defines the shape.

The sources note that while Millikan found it to be \( -1.6 \times 10^{ -19 } C \), the presently accepted value is \( -1.602176 \times 10^{ -19 } C \).

Planck suggested that energy is emitted or absorbed in discrete amounts called quanta, where \( E = h\nu \).

The radius of the first orbit \( ( n = 1 ) \) is \( a_{ 0 } = 52.9 pm \).

Stability is due to symmetrical distribution of electrons and maximum exchange energy.

J.J. Thomson measured the ratio of electrical charge to the mass of an electron in 1897 by applying perpendicular electrical and magnetic fields.

Regardless of wavelength, all electromagnetic radiations travel at the speed of light, \( 3.0 \times 10^{ 8 } m s^{ -1 } \) in a vacuum.

The sources confirm that for a given value of \( n \), the number of allowed orbitals is \( n^{ 2 } \).

Bohr’s fourth postulate states that angular momentum \( m_{ e } vr = n h / 2\pi \) where \( n \) is an integer.

According to the sources, Chadwick discovered neutrons in 1932. These are electrically neutral particles with a mass slightly greater than that of protons.

If two orbitals have the same \( ( n + l ) \) value, the one with the lower value of \( n \) has the lower energy.

The sources state that the number of electrons ejected is proportional to the intensity or brightness of light, while their kinetic energy depends on frequency.

According to Maxwell’s theory, accelerated charged particles (revolving electrons) should emit radiation and spiral into the nucleus, making the model unstable.

The principle states that the exact position and exact momentum (or velocity) of an electron cannot be determined simultaneously.

The negative sign means the energy of the electron in the atom is lower than the energy of a free electron at rest (which is assigned zero energy).

The sources state that in the absence of an electrical or magnetic field, cathode rays travel in straight lines.

de Broglie proposed that matter exhibits dual behavior and gave the relation \( \lambda = h / p = h / mv \).

The Pauli Exclusion Principle restricts the number of electrons in an orbital to two, and they must have opposite spins.