Chemistry Basics – Atoms, Molecules, Elements, Compounds, and Mixtures

In this article, let us familiarize with some of the the fundamentals of chemistry. Chemistry is the study of the composition, structure, properties, and transformation of matter [matter is defined as any substance with rest size and mass (occupying space) and forms particles] .Chemistry is primarily concerned with atoms (a fundamental element in chemistry) and their interactions with other atoms.

Let’s explore some fundamental concepts in chemistry: atoms, molecules, elements, compounds, and mixtures.

Atoms

An atom is a basic unit of matter that consists of a central nucleus surrounded by negatively charged electrons. The nucleus contains protons and neutrons. Electrons revolve around the nucleus in different orbits.

Subatomic particles

The constituent particles of an atom are called subatomic particles. They mainly include protons, electrons and neutrons. The electron is the least massive of these particles at 9.11×10−31 kg with a negative charge. Protons have a positive charge and Neutrons have no electrical charge.

Discoveries:

•     Proton by Ernest Rutherford in 1918.
•     Electron by J.J. Thomson in 1897.
•     Neutron by James Chadwick in 1932.

Nucleus

The central part of an atom is called nucleus. Particles present inside the nucleus are called nucleons and they include mainly protons and neutrons. Due to the presence of protons nucleus has a positive charge.

Molecules

Molecules are made of atoms of one or more elements. Some molecules are made only by one type of atoms (two oxygen atoms bond together to form O2 molecule) while molecules like protein are made up of atoms from different elements.

Elements

A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number. Carbon, Oxygen, Silicon, Arsenic, Aluminum, Iron, Copper, Gold, Mercury etc. are all examples of elements.

Note :

• Hydrogen and Helium are the most abundant elements in the universe.
• Iron is the most abundant element (by mass) in the earth.
• Oxygen is the most common element in the earth’s crust.
• The 8 most abundant elements in Earth’s crust (by mass) are the following :

1.   46.6% Oxygen (O)
2.   27.7% Silicon (Si)
3.   8.1% Aluminum (Al)
4.   5.0% Iron (Fe)
5.   3.6% Calcium (Ca)
6.   2.8% Sodium (Na)
7.   2.6% Potassium (K)
8.   2.1% Magnesium (Mg)

Periodic Table

Dmitri Ivanovich Mendeleev is the father of periodic table. The first detailed form of the periodic table was developed by Mendeleev (based on mass number) but later Henry Gwyn Jeffrey’s Moseley made a new periodic table based on atomic number. Mosley is called the father of modern periodic table.

Natural elements

The elements which occur in naturally on earth are called natural elements.

Eg: Oxygen, Nitrogen, Carbon etc.

Synthetic or man-made elements

A synthetic element is a chemical element that does not occur naturally on the earth. These are prepared by artificially and are unstable. The first synthetic element made was Technetium.

Compounds

Compounds contain more than one kind of atoms (more on atoms, later). It cannot be separated into constituent atoms by simple methods.

Eg: common salt (NaCl), Sodium carbonate (Na2CO3).

Mixtures

Mixtures are a combination of two or more substances, which when combined, each substance retains its own chemical identity. Examples of Mixtures include :

• sand and water.
• salt and water.
• sugar and salt.

Mixtures are classified into two main types:

• Homogeneous mixtures: The components are evenly distributed, and the mixture appears uniform, such as saltwater.
• Heterogeneous mixtures: The components are not evenly distributed, and different parts of the mixture can be seen, such as a salad.

Chemistry Basics MCQs

Here are some multiple-choice questions (MCQs) related to the basics of chemistry, focusing on atoms, molecules, elements, compounds, and mixtures:

1. What is the smallest unit of an element that retains the properties of that element?

• A) Molecule
• B) Atom
• C) Compound
• D) Mixture

Answer: B) Atom

2. Which of the following is a compound?

• A) Oxygen (O₂)
• B) Water (H₂O)
• C) Nitrogen (N₂)
• D) Hydrogen (H₂)

Answer: B) Water (H₂O)

3. Which of the following statements is true about elements?

• A) They can be broken down into simpler substances by chemical means.
• B) They consist of atoms with the same number of protons.
• C) They are always found in a combined state.
• D) They are composed of different types of atoms.

Answer: B) They consist of atoms with the same number of protons.

4. Which of the following is an example of a mixture?

• A) Table salt (NaCl)
• B) Pure water (H₂O)
• C) Air
• D) Carbon dioxide (CO₂)

Answer: C) Air

5. Which of the following is true about a molecule?

• A) It is always made up of two or more atoms of the same element.
• B) It is the smallest unit of a compound that retains the properties of the compound.
• C) It is always a combination of different elements.
• D) It cannot be broken down by chemical means.

Answer: B) It is the smallest unit of a compound that retains the properties of the compound.

6. What is the main difference between a compound and a mixture?

• A) Compounds are chemically combined, while mixtures are physically combined.
• B) Compounds can be separated by physical means, while mixtures cannot.
• C) Compounds are always homogeneous, while mixtures are always heterogeneous.
• D) Compounds always consist of elements, while mixtures consist of molecules.

Answer: A) Compounds are chemically combined, while mixtures are physically combined.

7. Which of the following is NOT a characteristic of a mixture?

• A) It can be separated into its components by physical means.
• B) It has a fixed composition.
• C) Its components retain their original properties.
• D) It can be homogeneous or heterogeneous.

Answer: B) It has a fixed composition.

8. Which of the following is a characteristic of an element?

• A) It can be broken down into simpler substances.
• B) It consists of only one type of atom.
• C) It is always found in a gaseous state.
• D) It always exists as a single atom.

Answer: B) It consists of only one type of atom.

9. Which of the following correctly describes a compound?

• A) It consists of two or more elements chemically bonded together.
• B) It can be separated into its elements by physical means.
• C) It is made up of only one type of atom.
• D) It always exists in a gaseous state.

Answer: A) It consists of two or more elements chemically bonded together.

10. What happens when a chemical reaction occurs between elements to form a compound?

• A) The atoms of the elements are destroyed.
• B) The elements retain their original properties.
• C) The atoms of the elements are rearranged to form a new substance.
• D) The elements physically combine to form a mixture.

Answer: C) The atoms of the elements are rearranged to form a new substance.

These questions cover fundamental concepts in chemistry and should be helpful for anyone studying the basics of the subject.