Question

Determine the number of each type of atom in each formula. (a) \(\mathrm{MgCl}_{2}\) (b) \(\mathrm{NaNO}_{3}\) (c) \(\mathrm{Ca}\left(\mathrm{NO}_{2}\right)_{2}\) (d) \(\mathrm{Sr}(\mathrm{OH})_{2}\)

Short answer

For each compound: (a) 1 Mg, 2 Cl (b) 1 Na, 1 N, 3 O (c) 1 Ca, 2 N, 4 O (d) 1 Sr, 2 O, 2 H

Step-by-step solution

Analyzing Magnesium Chloride - MgCl2

Examine the chemical formula \(\mathrm{MgCl}_{2}\). The subscript after the chlorine (Cl) indicates that there are 2 chlorine atoms. There is no subscript after magnesium (Mg), meaning there is only 1 magnesium atom.

Analyzing Sodium Nitrate - NaNO3

Inspect the chemical formula \(\mathrm{NaNO}_{3}\). Sodium (Na) does not have a subscript, which implies one sodium atom. Nitrogen (N) also has no subscript indicating one nitrogen atom. The subscript after the oxygen (O) indicates that there are 3 oxygen atoms.

Analyzing Calcium Nitrite - Ca(NO2)2

Look at the chemical formula \(\mathrm{Ca}\left(\mathrm{NO}_{2}\right)_{2}\). Calcium (Ca) has no subscript, which means there is one calcium atom. The \(\left(\mathrm{NO}_{2}\right)_{2}\) portion shows a polyatomic ion, nitrite (NO2), with a subscript of 2, meaning there are 2 nitrite ions. Each nitrite has 1 nitrogen and 2 oxygens, so in total, there are 2 nitrogens and 4 oxygens.

Analyzing Strontium Hydroxide - Sr(OH)2

Consider the chemical formula \(\mathrm{Sr}(\mathrm{OH})_{2}\). Strontium (Sr) does not have a subscript, indicating the presence of one strontium atom. The \(\mathrm{OH}\) group has a subscript of 2, meaning there are 2 hydroxide ions. Each hydroxide ion consists of 1 oxygen and 1 hydrogen, therefore, in total there are 2 hydrogens and 2 oxygens.

Key concepts

Atom Counting in Compounds

Understanding how to count atoms in chemical compounds is fundamental for students embarking on their chemistry journey. For example, in the compound \(\mathrm{MgCl}_{2}\), we analyze the subscripts to determine atom counts. A subscript, or a small number to the right of an element symbol, indicates the number of atoms of that element present in the molecule. In our example, there are two chlorine atoms indicated by the subscript '2' after \(\mathrm{Cl}\). Magnesium (\(\mathrm{Mg}\)) has no subscript, which typically means one atom is present.

This method can be applied to any chemical formula to reveal the atom count within a molecule. It's like reading a recipe—ingredients (elements) followed by a precise measurement (subscripts) telling you how much of each to include in your chemical concoction.

In exercises like determining the number of each type of atom in a given chemical formula, always look for these subscripts and remember that the lack of a subscript implies the number one.

Chemical Subscripts and Stoichiometry

Chemical subscripts don’t only tell us the number of atoms—they are also integral to the concept of stoichiometry in chemistry. Stoichiometry allows us to make calculations on how reactants convert to products in a chemical reaction based on the law of conservation of mass. For example, the coefficient and subscript in \(\mathrm{Ca}\left(\mathrm{NO}_{2}\right)_{2}\) inform us not only about the number of atoms but also their molar ratios if involved in a reaction.

Why is this important?

Imagine baking a cake: The recipe calls for 2 cups of flour to 1 cup of sugar. If you doubled the sugar without adjusting the flour, the cake wouldn’t turn out right. In chemistry, understanding the ratio of elements allows us to predict the outcome of reactions and create balanced equations.

Using stoichiometry, you could deduce how many grams of \(\mathrm{Ca}\left(\mathrm{NO}_{2}\right)_{2}\) you'd need to react with another substance or determine the amount of product formed from given reactants.

Molecular Composition Interpretation

Looking at molecular composition is akin to evaluating a building's blueprint—it tells us the structure and positioning of the component atoms. Taking the compound \(\mathrm{Sr}\left(\mathrm{OH}\right)_{2}\) as an example, we can interpret its molecular composition to see that strontium (Sr) acts as the central atom bonded to two hydroxide (OH) groups.

Understanding Molecular Geometry

This goes beyond counting atoms; it involves picturing how these atoms are arranged in space. For instance, knowing that there are two hydroxide ions gives clues about the geometry of the molecule, which affects properties such as polarity and reactivity.

Through practice, chemists can predict molecular composition based on formulas and subsequently infer the physical and chemical characteristics of the substance. This interpretation is pivotal in designing compounds with desired properties in fields such as pharmaceuticals and materials science.

Polyatomic Ions Identification

Polyatomic ions are clusters of atoms bonded together with an overall charge. Identifying and working with these ions is crucial in understanding chemical reactions and properties. For instance, \(\mathrm{NaNO}_{3}\) contains the polyatomic ion nitrate (\(\mathrm{NO}_{3}^{-}\)).

Recognizing Common Polyatomic Ions

Chemistry often deals with common polyatomic ions, such as sulfate (\(\mathrm{SO}_{4}^{2-}\)) and ammonium (\(\mathrm{NH}_{4}^{+}\)). Learning to recognize these ions in compounds is essential because their presence affects various factors like the chemical formula's total charge, reactivity, and the rules for naming compounds.

Many students find memorizing these ions challenging, yet recognizing them in compounds, as with nitrate in sodium nitrate or nitrite in calcium nitrite (\(\mathrm{Ca}\left(\mathrm{NO}_{2}\right)_{2}\)), is a fundamental skill in chemistry that aids in predicting compound behavior and outcomes in reactions.