Patterns in the table

To determine the charges, examine their groups: Beryllium is in Group 2, losing two electrons to form a $B{e}^{2+}$ ion. Fluorine is in Group 17, gaining one electron to form an $F^{-}$ ion.

The compound must be neutral, so the total positive and negative charges must balance. Let $x$ be beryllium ions and $y$ be fluoride ions, then: $x(+2)+y(-1)=0$

Solve the equation for the simplest integers: $2x-y=02x=y\frac{x}{y}=\frac{1}{2}$ So, one beryllium ion requires two fluoride ions.

Reviewing options: 2:3 results in $+1$, 1:1 results in $+1$, and 2:1 results in $+3$. None are neutral.

The 1:2 ratio matches the need for charge neutrality in the compound.

Using the periodic table, beryllium in Group 2 has 2 valence electrons, and fluorine in Group 17 has 7.

Beryllium sheds its two valence electrons to form a $B{e}^{2+}$ ion: $Be\to B{e}^{2+}+2e^{-}$

Fluorine acquires one electron, forming an $F^{-}$ ion: $F+e^{-}\to F^{-}$

Beryllium's 2 lost electrons need 2 fluorine atoms to accept, as each fluorine accepts only 1.

Options of 1:1 and 2:1 don't balance the charges. Option 2:3 leaves a net charge of $+1$.

The electron transfer shows a need for 1 beryllium for every 2 fluoride ions, confirming the 1:2 ratio.

The task is to determine which element from Period 4 has four electrons in its outermost shell. This can be figured out by examining the group number of these elements.

For elements in the main groups, the count of valence electrons is indicated by the last digit of their group number.

• Groups 1–2: Valence electrons are equal to the group number.
• Groups 13–18: Subtract 10 from the group number to find the count of valence electrons.

Consider the first two choices:

• Potassium ($\text{K}$) belongs to Group 1, thus it has 1 valence electron.
• Calcium ($\text{Ca}$) is in Group 2, so it possesses 2 valence electrons.

Neither fits the criteria.

Now, evaluate Gallium ($\text{Ga}$), which is in Group 13.

Applying the group rule: $\text{Valence electrons}=13-10=3$

This option does not satisfy the requirement of having 4 electrons.

Germanium ($\text{Ge}$) resides in Group 14. Using our group analysis:

$\text{Valence electrons}=14-10=4$

Thus, Germanium has four valence electrons, matching the requirement.

To utilize the electron configuration method, remember that valence electrons are those in the highest principal energy level ($n$) of an atom.

For Period 4 elements, the highest energy level is $n=4$, so we search for an element with exactly 4 electrons in the $4s$ and $4p$ subshells combined.

Potassium, with an atomic number of 19, has the electronic configuration:

$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$

Here, the highest energy level $n=4$ contains only 1 electron in the $4s$ subshell. Hence, Potassium does not meet the criterion of having 4 valence electrons.

Calcium, atomic number 20, has this configuration:

$1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^2$

The $n=4$ level holds 2 electrons in the $4s$ subshell. This count is insufficient for our needs, so it's not the correct choice.

Gallium, atomic number 31, has the electron configuration expressed as:

$[Ar]3d^{10}4s^{2}4p^1$

The highest level $n=4$ comprises the $4s$ and $4p$ subshells. Total electrons:

$2+1=3\text{ valence electrons}$

The $3d$ electrons are not counted as they belong to $n=3$. This option falls short of the 4-electron requirement.

Germanium, with an atomic number of 32, has its configuration as:

$[Ar]3d^{10}4s^{2}4p^2$

Counting electrons in the $n=4$ level:

• $4s$: 2 electrons
• $4p$: 2 electrons

$\text{Total valence electrons}=2+2=4$

Thus, Germanium indeed possesses the required four valence electrons.

Begin by recognizing the principal quantum number, $n$, mentioned. This value indicates the main energy shell being considered. Here, the focus is on the third shell.

The angular quantum number, $l$, defines subshell shape. For $l$, values range from $0$ to $n-1$.

Match each $l$ number to its corresponding subshell: $l=0$ for s, $l=1$ for p, and $l=2$ for d.

Review options: A lists only $s$; B lacks $d$; D includes $f$, which appears in $n=4$.

The third energy level includes the subshells $s,p,extandd$, confirming option C.

The query requires identifying subshells within the third shell, corresponding to $n=3$.

The rule states that the number of subshell types equals the principal quantum number $n$.

Subshells are sequentially listed as $s$, $p$, $d$, $f$. For $n=3$, include the first three: $s$, $p$, $d$.

Option A includes only $s$; B has $s,p$; D includes $f$, suitable for $n=4$.

Option C, listing $s,p,extandd$, is correct for $n=3$.

The question concerns subshells in the third shell, or $n=3$, a core part of understanding electronic configurations.

Using the Aufbau principle, subshells fill in a set order: $1s,2s,2p,3s,3p,4s,3d$.

Option A only lists $s$; however, $3p$ and $3d$ also belong to this level.

Option B lists $s$ and $p$, missing $d$, which is integral to $n=3$.

Option D incorrectly includes $f$, which is not in $n=3$.

Option C is accurate, listing $3s$, $3p$, and $3d$ for the third energy level.

The question aims to define valency. This is a key term in understanding how atoms bond, which is covered in the Matter section under Electronic configuration and valency.

The phrase "amount of protons an element has" describes the atomic number ($Z$). Although it determines the element's identity, it's not what valency means.

"Weight compared to carbon-12" refers to relative atomic mass ($A_r$). This concerns an atom's weight based on isotopes rather than its bonding nature.

While the "sum of electrons in an atom" matches the number of protons in a neutral atom, valency specifically deals with the electrons used in bonding, not the total count.

In chemistry, valency is described as an atom's combining capacity or "ability to form bonds". It indicates how many chemical bonds an atom can make, based on how many valence electrons it needs to lose, gain, or share to become stable.

The task is to find the best definition of 'valency'. We'll use process of elimination to discard options that describe other chemical properties until we find the correct one.

"Amount of protons an element has" is the definition of the atomic number ($Z$). Although it identifies the element, it does not define valency.

"Weight compared to carbon-12" defines relative atomic mass ($A_r$). As explained in Topic: The mole concept and chemical calculations, it is calculated as: $A_r=\frac{\text{weighted average mass of one atom of an element}}{\frac{1}{12}\times \text{mass of one atom of carbon-12}}$ This measure concerns mass, not bonding ability.

The "sum of electrons in an atom" in a neutral atom equals the number of protons. Although electrons are crucial for reactions, valency focuses on the outermost electrons' behavior, not the total number.

Valency describes the combining power of an atom. It is determined by the number of electrons in the outer shell (valence electrons) that an atom must lose, gain, or share to achieve a stable electronic configuration. This confirms the definition in Topic: Electronic configuration and valency.