First 20 Elements of the Periodic Table: Names, Symbols & Atomic Numbers

A popular and easy mnemonic to remember the first 20 elements is using a sentence where the first letter(s) of each word correspond to the element's symbol. For example: “Happy Henry Likes Beer But Could Not Obtain Four New Nachos. Mighty All-Silicon Protectors Shield Clever Arthur's Kingdom of Camelot.” Creating your own funny or memorable sentence can also be a very effective learning strategy.

The first 20 elements can be broadly categorised based on their properties:

• Metals: These are good conductors of heat and electricity. This group includes Lithium (Li), Beryllium (Be), Sodium (Na), Magnesium (Mg), Aluminum (Al), Potassium (K), and Calcium (Ca).
• Nonmetals: These are generally poor conductors. This group is the largest and includes Hydrogen (H), Helium (He), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Neon (Ne), Phosphorus (P), Sulfur (S), Chlorine (Cl), and Argon (Ar).
• Metalloids: These have properties intermediate between metals and nonmetals. Among the first 20 elements, Boron (B) and Silicon (Si) are considered metalloids.

Studying the first 20 elements is crucial because they form the basis for understanding fundamental chemical principles. They include the most abundant elements in the universe (Hydrogen, Helium) and the essential elements for life (Carbon, Nitrogen, Oxygen, Phosphorus, Calcium). These elements perfectly demonstrate core concepts like atomic structure, electronic configuration, and periodic trends (like atomic size and reactivity) in a simple, predictable way, which provides the foundation for studying the rest of the periodic table.

The first 20 elements have numerous applications in daily life and industry:

• Hydrogen (H): Used as a clean fuel and in the production of ammonia.
• Carbon (C): The basis of all organic life; forms fuels like coal and is found in materials like diamonds and graphite.
• Oxygen (O): Essential for respiration and combustion.
• Sodium (Na): Found in common salt (NaCl) and used in streetlights.
• Aluminum (Al): Used in cans, foils, and airplane parts due to its low density and corrosion resistance.
• Silicon (Si): The basis of modern electronics, used in computer chips and solar cells.
• Calcium (Ca): Crucial for strong bones and teeth, and used to make cement and plaster.

The electronic configuration is the key to an element's address in the periodic table. The period number corresponds to the highest principal energy level (shell) occupied by electrons. For instance, Lithium (1s²2s¹) has its outermost electron in the 2nd shell, so it's in Period 2. The group number is determined by the number of valence electrons (electrons in the outermost shell). For example, both Lithium (2s¹) and Sodium (3s¹) have one valence electron, placing them in Group 1 (the alkali metals).

Hydrogen's position is unique and debated due to its dual nature. With one valence electron (1s¹), it resembles the Group 1 alkali metals (like Li, Na, K). However, it is a gas, not a metal. It can also gain one electron to achieve a full shell, similar to the Group 17 halogens (like F, Cl). Because it doesn't perfectly fit into either group, it is often placed separately at the top-centre or in both groups in different versions of the periodic table, highlighting its distinct properties.

This is explained by the Aufbau principle and orbital energy levels. After Argon (atomic number 18), the third electron shell (n=3) is not completely full (the 3d subshell is empty). However, the 4s orbital is at a lower energy level than the 3d orbital. Therefore, electrons begin filling the 4s orbital before the 3d orbital. Potassium ([Ar] 4s¹) has one electron in the 4s orbital, and Calcium ([Ar] 4s²) has two. Since the outermost shell is now the fourth shell (n=4), they begin a new period, Period 4.

Moving from left to right across Period 3 (from Na to Cl) demonstrates clear periodic trends:

• Atomic Size: Decreases because the increasing nuclear charge pulls electrons closer to the nucleus.
• Metallic Character: Decreases significantly. Sodium (Na) and Magnesium (Mg) are reactive metals, Aluminum (Al) is a less reactive metal, Silicon (Si) is a metalloid, and Phosphorus (P), Sulfur (S), and Chlorine (Cl) are nonmetals.
• Reactivity: The trend is complex. Metals on the left (like Sodium) are highly reactive by losing electrons, while nonmetals on the right (like Chlorine) are highly reactive by gaining electrons. Argon at the end is unreactive.