Chemical symbols and formulae are the language of chemistry. They allow chemists to communicate clearly about elements, compounds, and reactions. Understanding how to interpret and use these symbols is essential for success in chemistry.
Elements and Their Symbols #
What is an Element? #
An element is a pure substance that cannot be broken down into simpler substances by chemical methods. Each element has its own unique identity and properties. The periodic table organizes all known elements and gives each a unique symbol.
Chemical Symbols #
- Chemical symbols consist of one or two letters
- The first letter is always capitalized
- If there is a second letter, it is always lowercase
- Examples: H (hydrogen), O (oxygen), Na (sodium), Cl (chlorine)
Common Elements and Their Symbols #
This is small list of some of the most common element and their Symbols.
| Element | Symbol | Location in Periodic Table |
|---|---|---|
| Hydrogen | H | Group 1 |
| Oxygen | O | Group 16 |
| Nitrogen | N | Group 15 |
| Carbon | C | Group 14 |
| Chlorine | Cl | Group 17 |
| Bromine | Br | Group 17 |
| Sodium | Na | Group 1 |
| Potassium | K | Group 1 |
| Sulfur | S | Group 16 |
Molecules and Formulae #
What is a Molecule? #
A molecule is a group of atoms chemically bonded together. The chemical formula shows the types of atoms and numbers of atoms in the molecule. You can think of it like a cooking recipe of how the final element or molecule is made.
Reading Chemical Formulae #
- Numbers written as subscripts (small numbers at the bottom) show how many atoms of each element are present
- Example: H2O means 2 hydrogen atoms and 1 oxygen atom
- When no number is written, it is understood to be 1
- The total number of atoms in a molecule is the sum of all the atoms of each element
Examples of Counting Atoms in Molecules: #
- H2O (water): 3 atoms in total (2 hydrogen + 1 oxygen)
- CO2 (carbon dioxide): 3 atoms in total (1 carbon + 2 oxygen)
- CH4 (methane): 5 atoms in total (1 carbon + 4 hydrogen)
- C6H12O6 (glucose): 24 atoms in total (6 carbon + 12 hydrogen + 6 oxygen)
- H2SO4 (sulfuric acid): 7 atoms in total (2 hydrogen + 1 sulfur + 4 oxygen)
Diatomic Molecules #
Some elements exist naturally as a pair. These are called diatomic molecules (molecules containing two atoms of the same element):
- H2 (hydrogen)
- O2 (oxygen)
- N2 (nitrogen)
- F2 (fluorine)
- Cl2 (chlorine)
- Br2 (bromine)
- I2 (iodine)
The Group 17 (VII) elements (F, Cl, Br, I) are also known as halogens and all exist naturally as diatomic molecules. This is an important characteristic to remember for exams.
Ions and Ionic Compounds #
What is an Ion? #
An ion is an atom or group of atoms that has gained or lost electrons, giving it a positive or negative charge.
Types of Ions #
-
Cations: Positively charged ions (they are formed when atoms lose electrons)
- Examples: Na+, K+, Ca2+
- Group 1 elements form +1 ions (e.g., K+)
- Group 2 elements form +2 ions (e.g., Ca2+)
-
Anions: Negatively charged ions (they are formed when atoms gain electrons)
- Examples: Cl–, O2-, SO42-
- Group 17 (VII) elements form -1 ions (e.g., Cl–)
- Group 16 (VI) elements form -2 ions (e.g., O2-)
Ion Formation: #
- When an atom loses electrons, it forms a positive ion (cation)
- When an atom gains electrons, it forms a negative ion (anion)
- The charge on an ion is written as a superscript after the symbol
Polyatomic Ions #
These are ions made up of more than one atom:
- Sulfate: SO42-
- Carbonate: CO32-
- Nitrate: NO3–
- Hydroxide: OH–
Important Atmospheric Gases #
Composition of Air #
These are the common gases that are found in the atmsophere and their percentages in the atnmosphere
- Nitrogen (N2): 78%
- Oxygen (O2): 21%
- Argon (Ar): 0.9%
- Carbon dioxide (CO2): 0.04%
Nitrogen (N2) is the most abundant gas in Earth’s atmosphere, making up approximately 78% of the air we breathe. This is an important fact to remember for exams.
Photosynthesis #
Photosynthesis is the process where plants use Carbob Dioxide, Water and Sunlight to make Oxygen and Glucose
Chemical Equation for Photosynthesis: #
6CO2 + 6H2O → C6H12O6 + 6O2
Products of Photosynthesis #
- Glucose (C6H12O6)
- Oxygen (O2)
Key Concepts to Remember #
1. Counting Atoms in Molecules: #
- Always add up all atoms from the subscripts
- For example, H2SO4 contains 2 H atoms, 1 S atom, and 4 O atoms (7 atoms total)
2. Identifying Ions: #
- Cations (positive ions) are typically metals
- Anions (negative ions) are typically non-metals
3. Diatomic Molecules: #
- Remember the seven diatomic elements: H2, N2, F2, O2, I2, Cl2, Br2
- Helpful memory phrase: “Have No Fear Of Ice Cold Beer” (Hydrogen, Nitrogen, Fluorine, Oxygen, Iodine, Chlorine, Bromine)
4. Reading Ionic Charges: #
- The charge is written as a superscript (e.g., Na+, Cl–)
- For ions like SO42-, the charge applies to the entire group of atoms
Test Questions #
A list of symbols and formulae is shown.
Br2, Cl–, CO2, H2, K+, N2, O2, SO42-
Answer the following questions about these symbols and formulae. Each symbol or formula may be used once, more than once or not at all.
State which symbol or formula represents:
- a molecule containing only five atoms
- a diatomic molecule of an element in Group VII of the Periodic Table
- an ion formed when an atom gains one electron
- an anion
- a gas that occupies 78% of atmospheric air
- a Group 1 cation
- a product of photosynthesis
Chemical Symbols and Formulae – Test Answers #
Test Question Answers with Explanations #
The question provides the following list of symbols and formulae:
Br2, Cl–, CO2, H2, K+, N2, O2, SO42-
-
Question (a): A molecule containing only five atoms.
Answer: SO42-
Explanation: To identify the molecule with exactly 5 atoms, we need to count the total number of atoms in each formula:
- Br2: 2 bromine atoms = 2 atoms total
- Cl–: 1 chlorine atom = 1 atom total
- CO2: 1 carbon atom + 2 oxygen atoms = 3 atoms total
- H2: 2 hydrogen atoms = 2 atoms total
- K+: 1 potassium atom = 1 atom total
- N2: 2 nitrogen atoms = 2 atoms total
- O2: 2 oxygen atoms = 2 atoms total
- SO42-: 1 sulfur atom + 4 oxygen atoms = 5 atoms total
Therefore, SO42- is the only formula that contains exactly 5 atoms.
-
Question (b): A diatomic molecule of an element in Group VII of the Periodic Table.
Answer: Br2
Explanation: A diatomic molecule contains two atoms of the same element. The elements in Group VII (or Group 17 in the modern periodic table) are the halogens: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At).
From the given list, only Br2 represents a diatomic molecule of an element in Group VII. Br2 is a molecule of bromine containing two bromine atoms. Bromine is in Group VII of the Periodic Table.
Note: While Cl– contains the chlorine element which is in Group VII, it is an ion, not a diatomic molecule.
-
Question (c): An ion formed when an atom gains one electron.
Answer: Cl–
Explanation: When an atom gains an electron, it forms a negative ion (anion) with a -1 charge. From the given list, Cl– is the only ion that has formed by gaining one electron.
Chlorine atoms have 7 electrons in their outer shell. They gain 1 more electron to achieve a stable octet (8 electrons), resulting in the Cl– ion with a -1 charge.
K+ is formed when a potassium atom loses one electron (not gains one).
SO42- is a polyatomic ion with a -2 charge, but it’s not formed by a single atom gaining one electron.
-
Question (d): An anion.
Answer: Either Cl– or SO42-
Explanation: An anion is a negatively charged ion. From the given list, there are two anions:
- Cl–: This is the chloride ion with a -1 charge.
- SO42-: This is the sulfate ion with a -2 charge.
Both are correct answers as they are both anions. However, in an exam situation, you would typically be expected to give one answer, and either would be acceptable unless the question specifically asked for “a polyatomic anion” (then SO42-) or “a monatomic anion” (then Cl–).
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Question (e): A gas that occupies 78% of atmospheric air.
Answer: N2
Explanation: Earth’s atmosphere consists of approximately:
- 78% Nitrogen (N2)
- 21% Oxygen (O2)
- 0.9% Argon (Ar)
- 0.04% Carbon dioxide (CO2)
- Small amounts of other gases
Nitrogen (N2) is the most abundant gas in our atmosphere, making up approximately 78% of the air. Therefore, N2 is the gas that occupies 78% of atmospheric air.
-
Question (f): A Group 1 cation.
Answer: K+
Explanation: Group 1 elements (also known as alkali metals) include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr).
When Group 1 elements form ions, they lose one electron to form +1 cations. From the given list, K+ (potassium ion) is the only Group 1 cation present.
K (potassium) is in Group 1 of the periodic table, and K+ is the ion formed when potassium loses one electron.
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Question (g): A product of photosynthesis.
Answer: O2
Explanation: Photosynthesis is the process by which plants, algae, and some bacteria convert carbon dioxide and water into glucose and oxygen using sunlight. The overall equation for photosynthesis is:
6CO2 + 6H2O + sunlight → C6H12O6 + 6O2
The products of photosynthesis are:
- Glucose (C6H12O6)
- Oxygen (O2)
From the given list, O2 (oxygen) is a product of photosynthesis.
Exam Tips: #
- Always read the question carefully – notice that in question (a) it asks for a molecule with “only five atoms” so you need to count the total atoms in each formula.
- Pay attention to the specific terminology: “diatomic molecule,” “anion,” “cation,” etc. These terms have specific meanings in chemistry.
- Remember that ions have charges (shown as superscripts), while molecules don’t.
- For questions like (d) asking for “an anion,” there might be multiple correct answers in the list (both Cl– and SO42- are anions). In an exam, give one unless specifically asked for all possible answers.
