Question

Write the name and number of atoms of each element in each of the following compounds: (a) \(\mathrm{Fe}_{2} \mathrm{O}_{3}\) (b) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) (c) \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\) (e) \(\mathrm{K}_{2} \mathrm{CO}_{3}\) (f) \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) (g) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (h) \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\)

Short answer

(a) 2 Fe, 3 O; (b) 1 Ca, 2 N, 6 O; (c) 1 Co, 4 C, 6 H, 4 O; (d) 3 C, 6 H, 1 O; (e) 2 K, 1 C, 3 O; (f) 3 Cu, 2 P, 8 O; (g) 2 C, 6 H, 1 O; (h) 2 Na, 2 Cr, 7 O.

Step-by-step solution

- Identify atoms and their counts in \(\mathrm{Fe}_{2} \mathrm{O}_{3}\)

For \(\mathrm{Fe}_{2} \mathrm{O}_{3}\), count the atoms of Iron (Fe) and Oxygen (O). There are 2 atoms of Iron and 3 atoms of Oxygen.

- Identify atoms and their counts in \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

For \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\), count the atoms of Calcium, Nitrogen, and Oxygen. There is 1 atom of Calcium, 2 atoms of Nitrogen (since \(\mathrm{NO}_{3}\) is repeated twice), and 6 atoms of Oxygen (since there are 3 atoms in each \(\mathrm{NO}_{3}\) and it is repeated twice).

- Identify atoms and their counts in \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}\)

For \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}\), count the atoms of Cobalt, Carbon, Hydrogen, and Oxygen. There is 1 atom of Cobalt, 4 atoms of Carbon (since \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) is repeated twice), 6 atoms of Hydrogen, and 4 atoms of Oxygen.

- Identify atoms and their counts in \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\)

For \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), count the atoms of Carbon, Hydrogen, and Oxygen. There are 3 atoms of Carbon, 6 atoms of Hydrogen, and 1 atom of Oxygen.

- Identify atoms and their counts in \(\mathrm{K}_{2} \mathrm{CO}_{3}\)

For \(\mathrm{K}_{2} \mathrm{CO}_{3}\), count the atoms of Potassium, Carbon, and Oxygen. There are 2 atoms of Potassium, 1 atom of Carbon, and 3 atoms of Oxygen.

- Identify atoms and their counts in \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

For \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\), count the atoms of Copper, Phosphorus, and Oxygen. There are 3 atoms of Copper, 2 atoms of Phosphorus, and 8 atoms of Oxygen.

- Identify atoms and their counts in \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\)

For \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\), count the atoms of Carbon, Hydrogen, and Oxygen. There are 2 atoms of Carbon, 6 atoms of Hydrogen, and 1 atom of Oxygen.

- Identify atoms and their counts in \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\)

For \(\mathrm{Na}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\), count the atoms of Sodium, Chromium, and Oxygen. There are 2 atoms of Sodium, 2 atoms of Chromium, and 7 atoms of Oxygen.

Key concepts

element identification

Identifying elements in a chemical compound is the first and crucial step in chemical analysis. A chemical formula represents a compound using symbols for the elements. Each element symbol tells us which elements are in the compound.
For example, in the formula \(\text{Fe}_2\text{O}_3\), \(\text{Fe}\) stands for iron and \(\text{O}\) stands for oxygen.
Understanding these symbols is essential for identifying which elements are present in the compound. Similarly, in \(\text{Ca}(\text{NO}_3)_2\), \(\text{Ca}\) represents calcium, \(\text{N}\) represents nitrogen, and \(\text{O}\) represents oxygen.
Each element in the periodic table has a unique one- or two-letter symbol, which is used to represent the element in chemical formulas.

Elements are identified by their symbols, and the number following the symbol (if any) indicates the count of atoms of that element within one molecule of the compound.

counting atoms

Once you have identified the elements in a compound, the next step is to count the number of atoms of each element. This involves reading the subscript numbers in the chemical formula.
The subscript number following an element symbol indicates how many atoms of that element are present. For example, in \(\text{Fe}_2\text{O}_3\), the subscript 2 means there are 2 iron atoms and the subscript 3 means there are 3 oxygen atoms.
If an element symbol does not have a subscript, it implies there is only one atom of that element in the compound.

Sometimes, subscripts are found within parentheses in more complex compounds, like in \(\text{Ca}(\text{NO}_3)_2\). Here, the subscript outside the parentheses multiplies the count of each element inside the parentheses.
For \(\text{Ca}(\text{NO}_3)_2\), nitrogen and oxygen atoms within the parentheses must be multiplied by 2, giving 2 nitrogen and 6 oxygen atoms.
Paying attention to subscripts is crucial to accurately count the number of atoms of each element in a compound.

chemical formulas

Chemical formulas are a concise way of representing the elements in a compound and the ratio of their atoms. The chemical formula is made up of the element symbols and subscripts to show the number of each type of atom present.
For example, \(\text{CH}_3\text{COCH}_3\) is the formula for acetone. The symbols (C, H, O) show which elements are in the compound, and the subscripts show the number of atoms for each element.
In a chemical formula, the elements are usually listed in a specific order often determined by conventional rules.

Understanding chemical formulas is key to grasping molecular composition and performing chemical calculations. For instance, knowing acetone has 3 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom, helps us predict its chemical behavior and properties.
Additionally, being familiar with chemical formulas enables us to balance chemical equations, understand molecular shapes, and calculate molar masses.

molecular composition

Molecular composition refers to the particular arrangement and ratio of atoms within a molecule. Knowing the molecular composition offers insight into a compound’s properties, reactions, and function.
For instance, the molecular composition of \(\text{C}_2\text{H}_5\text{OH}\) reveals it contains 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom, which are arranged to form ethanol.
Each type of atom and its count contribute to the unique makeup of the molecule, affecting attributes such as polarity, boiling point, and reactivity.

By understanding molecular composition, one can also predict how a compound interacts with others. In \(\text{Cu}_3(\text{PO}_4)_2\), the arrangement offers two phosphorus and eight oxygen atoms bonded to copper atoms, forming copper(II) phosphate.
They make it easier to determine the overall structure and bond types, leading to better comprehension of chemical reactions and synthesis.
Consequently, mastering molecular composition fundamentals plays a vital role in fields like chemistry, biology, and materials science.