Question

How many atoms of each kind are represented in the following formulas? For each, name the elements present. (a) \(\mathrm{Ni}\left(\mathrm{ClO}_{4}\right)_{2},\) (b) \(\mathrm{COCl}_{2}\), (c) \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) (d) \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H},\) (e) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{HPO}_{4}\)

Short answer

For each formula: (a) 1 Ni, 2 Cl, 8 O, (b) 1 C, 1 O, 2 Cl, (c) 2 K, 2 Cr, 7 O, (d) 2 C, 4 H, 2 O, (e) 2 N, 8 H, 1 P, 4 O. Elements present are (a) Nickel, Chlorine, Oxygen, (b) Carbon, Oxygen, Chlorine, (c) Potassium, Chromium, Oxygen, (d) Carbon, Hydrogen, Oxygen, (e) Nitrogen, Hydrogen, Phosphorus, Oxygen.

Step-by-step solution

Identify Elements in (a)

Examine the chemical formula \(\mathrm{Ni}(\mathrm{ClO}_{4})_{2}\) to identify the elements present. There is Nickel (Ni), Chlorine (Cl), and Oxygen (O).

Count Atoms in (a)

Count the number of each atom in the chemical formula. There is 1 Nickel atom, as indicated by the absence of a subscript next to Ni. For the perchlorate ion \((\mathrm{ClO}_{4})\), there is 1 Chlorine atom and 4 Oxygen atoms in each ion. Since we have 2 such ions (indicated by the subscript 2), in total there are 2 Chlorine atoms and 8 Oxygen atoms.

Identify Elements in (b)

Look at the chemical formula \(\mathrm{COCl}_{2}\) to identify elements present. There is Carbon (C), Oxygen (O) and Chlorine (Cl).

Count Atoms in (b)

The number of each atom is given by the subscripts. There is 1 Carbon atom, 1 Oxygen atom, and 2 Chlorine atoms as shown by the subscripts.

Identify Elements in (c)

Examine \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) for elements. The elements are Potassium (K), Chromium (Cr), and Oxygen (O).

Count Atoms in (c)

Determine the atom count: 2 Potassium atoms (indicated by subscript 2), 2 Chromium atoms (indicated by subscript 2), and 7 Oxygen atoms (indicated by subscript 7).

Identify Elements in (d)

Inspect \(\mathrm{CH}_{3}\mathrm{CO}_{2}\mathrm{H}\) for elements. The elements are Carbon (C), Hydrogen (H), and Oxygen (O).

Count Atoms in (d)

There are a total of 2 Carbon atoms (1 in \(\mathrm{CH}_{3}\) and 1 in \(\mathrm{CO}_{2}\)), 4 Hydrogen atoms (3 in \(\mathrm{CH}_{3}\) and 1 in \(\mathrm{H}\)), and 2 Oxygen atoms (both in \(\mathrm{CO}_{2}\)).

Identify Elements in (e)

Analyze \((\mathrm{NH}_{4})_{2} \mathrm{HPO}_{4}\) for constituent elements. The elements are Nitrogen (N), Hydrogen (H), Phosphorus (P), and Oxygen (O).

Count Atoms in (e)

In the chemical formula, there are 2 Nitrogen atoms (each ammonium ion \(\mathrm{NH}_{4}\) has 1 Nitrogen, and there are 2 ions), 8 Hydrogen atoms (each ammonium ion has 4 Hydrogen atoms), 1 Phosphorus atom, and 4 Oxygen atoms.

Key concepts

Counting Atoms in Compounds

To grasp how many atoms are in a chemical compound, we look closely at its chemical formula. A chemical formula provides a wealth of information, revealing not only which elements are present but also how many atoms of each element can be found within the molecule. For instance, in water (H_2O), the subscript '2' tells us there are two hydrogen atoms for every one oxygen atom. No subscript typically means there is only one atom of that element, like in carbon dioxide (CO_2), which contains one carbon atom.

Let's consider a more complex example, such as C_6H_{12}O_6 (glucose). The subscripts indicate that there are 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms in one molecule of glucose. This methodical counting using subscripts is essential for understanding the composition of compounds, which is fundamental to chemical reactions and stoichiometry in chemistry.

Chemical Elements Identification

Identifying chemical elements in a compound is the first step toward understanding its properties. Each element in the periodic table has a unique symbol, often derived from its English or Latin name. For example, Hydrogen is represented by 'H', Oxygen by 'O', and Carbon by 'C'. By analyzing the chemical formula, one can identify the component elements. In the example of NaCl, it's clear that the compound consists of Sodium (Na) and Chlorine (Cl).

It is also crucial to recognize that some elements exist as diatomic molecules naturally, such as H_2, N_2, and O_2, which are not compounds but elemental molecules made up of two atoms. Understanding elements and their symbols is a prerequisite for studying chemical reactions, as it enables students to comprehend and predict how different substances will interact.

Molecular Composition

Molecular composition refers to the types and numbers of atoms that constitute a molecule. By examining the molecular formula, you can determine the molecular composition of the substance. This tells us not only the proportions of elements within the compound but also the ratio in which the atoms combine to form the molecule.

For example, ethanol (C_2H_5OH) consists of two carbons (C), six hydrogens (H), and one oxygen (O). The molecular composition gives insight into the molecule's structure and, by extension, its chemical properties and behavior. Knowing this is essential for students when they're predicting the outcome of chemical reactions or when they're balancing chemical equations, which requires an accurate count of atoms on both sides of the reaction.

Subscripts in Chemical Formulas

Subscripts in chemical formulas are numbers written just below the line to indicate the number of atoms of the element that immediately precedes them in the molecule. They are the backbone of understanding a compound's composition. For instance, in the compound H_2O, the subscript '2' identifies that there are two atoms of hydrogen for every one of oxygen. If a chemical element does not have a subscript, it implies that there is only one atom of that element in the molecule.

Moreover, parentheses combined with subscripts show a group of atoms that is repeated in the molecule. For example, in the compound Fe(OH)_3, the '(OH)' is a group with one oxygen and one hydrogen. The subscript '3' outside the parenthesis informs us that there are three 'OH' groups attached to the iron (Fe) atom. Students must pay careful attention to subscripts to accurately determine the molecular composition and avoid mistakes in stoichiometric calculations or when describing chemical reactions.