Question

What monatomic ions would you expect potassium \((Z=19)\) and bromine \((Z=35)\) to form?

Short answer

Potassium forms \(\text{K}^+\) and bromine forms \(\text{Br}^-\).

Step-by-step solution

Determine the Electron Configuration

Find the electron configuration of potassium and bromine. For potassium \((Z=19)\), the electron configuration is \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 \]. For bromine \((Z=35)\), the electron configuration is \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5 \].

Determine the Tendency to Gain or Lose Electrons

Potassium, with one electron in its 4s orbital, will readily lose one electron to achieve a stable noble gas configuration (like argon, which has \[1s^2 2s^2 2p^6 3s^2 3p^6 \]). Bromine, with seven electrons in its 4p orbital, will tend to gain one electron to complete its octet, achieving a stable noble gas configuration (like krypton, which has \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 \]).

Identify the Monatomic Ions Formed

When potassium loses one electron, it becomes a cation with a charge of \(+1\), forming \(\text{K}^+\). When bromine gains one electron, it becomes an anion with a charge of \(-1\), forming \(\text{Br}^-\).

Key concepts

electron configuration

Electron configuration describes the distribution of electrons in an atom's orbitals. It's key to understanding how atoms interact and bond. Electrons occupy different energy levels or shells around a nucleus. These shells (s, p, d, f) fill based on increasing energy.
For potassium \((Z=19)\), the electron configuration is \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^1 \]. This sequence shows how 19 electrons are arranged. They fill lower energy orbitals before higher ones.
Bromine \((Z=35)\) has a different sequence: \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5 \]. Here, the electrons fill up to the 4th energy level, and the 4p subshell has 5 electrons.

noble gas configuration

Noble gas configuration is about achieving electron arrangements like those of noble gases. Noble gases (like helium, neon, argon) have very stable and complete electron shells. Atoms often react to reach similar stability.
Potassium (K) loses one electron to mimic argon \[ 1s^2 2s^2 2p^6 3s^2 3p^6 \]. This way, it achieves a stable noble gas configuration.
Bromine (Br), in contrast, gains one electron to have the same electron configuration as krypton \[ 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 \]. It fills its 4p orbital to complete the octet.

cation and anion formation

Cations and anions form when atoms lose or gain electrons, respectively. This process helps atoms reach a more stable electron configuration.
Potassium (\text{K}), being a metal, loses one electron to form a cation: \[ \text{K}^+ \]. This makes it positively charged with a configuration like argon.
Bromine (\text{Br}), being a non-metal, gains one electron to form an anion: \[ \text{Br}^- \]. It becomes negatively charged while achieving a krypton-like configuration.
In summary, potassium forms \[ \text{K}^+ \] by losing an electron, and bromine forms \[ \text{Br}^- \] by gaining one. This gain or loss ensures both reach more stable electron arrangements.