Periodic table of first 20 elements covers Hydrogen (H, atomic number 1) to Calcium (Ca, atomic number 20). These 20 elements include all the building blocks of organic chemistry (C, H, O, N), essential biological elements (Na, K, Ca, P, S, Cl), noble gases (He, Ne, Ar), and common metals (Na, Mg, Al, K, Ca). Every chemistry student must memorise these first 20 elements — their symbols, atomic numbers, atomic masses, electron configurations, valencies, and basic properties. This guide provides the complete list with a memory trick, element-by-element details, electron configurations, valency chart, and exam-ready FAQs.
First 20 elements: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
They span 4 periods and include groups 1, 2, 13–18
7 metals, 9 non-metals, 2 metalloids, and 3 noble gases in the first 20
Lightest element: Hydrogen (1.008 amu); heaviest of the 20: Calcium (40.08 amu)
Noble gases (He, Ne, Ar) have full outer shells and valency 0
Alkali metals (Li, Na, K) have 1 valence electron and are highly reactive
Fluorine is the most electronegative element; Helium has the smallest atomic radius
Carbon (Z=6) is the basis of all organic chemistry and life
Na and K symbols come from Latin: Natrium and Kalium
Electron shells fill in order: K (max 2) → L (max 8) → M (max 8) → N
No. | Symbol | Element | Atomic Mass | Group | Period | Type 1 | H | Hydrogen | 1.008 | 1 | 1 | Non-metal 2 | He | Helium | 4.003 | 18 | 1 | Noble gas 3 | Li | Lithium | 6.941 | 1 | 2 | Alkali metal 4 | Be | Beryllium | 9.012 | 2 | 2 | Alkaline earth metal 5 | B | Boron | 10.81 | 13 | 2 | Metalloid 6 | C | Carbon | 12.01 | 14 | 2 | Non-metal 7 | N | Nitrogen | 14.01 | 15 | 2 | Non-metal 8 | O | Oxygen | 16.00 | 16 | 2 | Non-metal 9 | F | Fluorine | 19.00 | 17 | 2 | Halogen 10 | Ne | Neon | 20.18 | 18 | 2 | Noble gas 11 | Na | Sodium | 22.99 | 1 | 3 | Alkali metal 12 | Mg | Magnesium | 24.31 | 2 | 3 | Alkaline earth metal 13 | Al | Aluminium | 26.98 | 13 | 3 | Metal 14 | Si | Silicon | 28.09 | 14 | 3 | Metalloid 15 | P | Phosphorus | 30.97 | 15 | 3 | Non-metal 16 | S | Sulphur | 32.07 | 16 | 3 | Non-metal 17 | Cl | Chlorine | 35.45 | 17 | 3 | Halogen 18 | Ar | Argon | 39.95 | 18 | 3 | Noble gas 19 | K | Potassium | 39.10 | 1 | 4 | Alkali metal 20 | Ca | Calcium | 40.08 | 2 | 4 | Alkaline earth metal
Use this popular mnemonic sentence to memorise the first 20 elements in order:
"Happy Henry Lives Beside Boron Cottage, Near Our Friend Nelly Nancy Mg Allen Silly Patrick Stays Close Arthur King Ca"
H — Happy — Hydrogen He — Henry — Helium Li — Lives — Lithium Be — Beside — Beryllium B — Boron — Boron C — Cottage — Carbon N — Near — Nitrogen O — Our — Oxygen F — Friend — Fluorine Ne — Nelly — Neon Na — Nancy — Sodium Mg — Mg — Magnesium Al — Allen — Aluminium Si — Silly — Silicon P — Patrick — Phosphorus S — Stays — Sulphur Cl — Close — Chlorine Ar — Arthur — Argon K — King — Potassium Ca — Ca — Calcium
Alternative short mnemonic (first letters): H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca "Hi He Lies Because Boron Can Not Oxidise Fluorine. New Nations Might Also Sign Peace Security Clause. Argentina Knows Canada."
No. | Element | Symbol | Electron Configuration | Shells (K, L, M, N) | Valence Electrons 1 | Hydrogen | H | 1s¹ | 1 | 1 2 | Helium | He | 1s² | 2 | 2 (stable) 3 | Lithium | Li | 1s² 2s¹ | 2, 1 | 1 4 | Beryllium | Be | 1s² 2s² | 2, 2 | 2 5 | Boron | B | 1s² 2s² 2p¹ | 2, 3 | 3 6 | Carbon | C | 1s² 2s² 2p² | 2, 4 | 4 7 | Nitrogen | N | 1s² 2s² 2p³ | 2, 5 | 5 8 | Oxygen | O | 1s² 2s² 2p⁴ | 2, 6 | 6 9 | Fluorine | F | 1s² 2s² 2p⁵ | 2, 7 | 7 10 | Neon | Ne | 1s² 2s² 2p⁶ | 2, 8 | 8 (stable) 11 | Sodium | Na | 1s² 2s² 2p⁶ 3s¹ | 2, 8, 1 | 1 12 | Magnesium | Mg | 1s² 2s² 2p⁶ 3s² | 2, 8, 2 | 2 13 | Aluminium | Al | 1s² 2s² 2p⁶ 3s² 3p¹ | 2, 8, 3 | 3 14 | Silicon | Si | 1s² 2s² 2p⁶ 3s² 3p² | 2, 8, 4 | 4 15 | Phosphorus | P | 1s² 2s² 2p⁶ 3s² 3p³ | 2, 8, 5 | 5 16 | Sulphur | S | 1s² 2s² 2p⁶ 3s² 3p⁴ | 2, 8, 6 | 6 17 | Chlorine | Cl | 1s² 2s² 2p⁶ 3s² 3p⁵ | 2, 8, 7 | 7 18 | Argon | Ar | 1s² 2s² 2p⁶ 3s² 3p⁶ | 2, 8, 8 | 8 (stable) 19 | Potassium | K | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ | 2, 8, 8, 1 | 1 20 | Calcium | Ca | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² | 2, 8, 8, 2 | 2
Pattern: Elements 1–2 fill the K shell (max 2), elements 3–10 fill the L shell (max 8), elements 11–18 fill the M shell (max 8 in this range), elements 19–20 start the N shell.
No. | Element | Symbol | Valence Electrons | Valency 1 | Hydrogen | H | 1 | 1 2 | Helium | He | 2 | 0 (noble gas) 3 | Lithium | Li | 1 | 1 4 | Beryllium | Be | 2 | 2 5 | Boron | B | 3 | 3 6 | Carbon | C | 4 | 4 7 | Nitrogen | N | 5 | 3 (also 5) 8 | Oxygen | O | 6 | 2 9 | Fluorine | F | 7 | 1 10 | Neon | Ne | 8 | 0 (noble gas) 11 | Sodium | Na | 1 | 1 12 | Magnesium | Mg | 2 | 2 13 | Aluminium | Al | 3 | 3 14 | Silicon | Si | 4 | 4 15 | Phosphorus | P | 5 | 3 (also 5) 16 | Sulphur | S | 6 | 2 (also 4, 6) 17 | Chlorine | Cl | 7 | 1 (also 3, 5, 7) 18 | Argon | Ar | 8 | 0 (noble gas) 19 | Potassium | K | 1 | 1 20 | Calcium | Ca | 2 | 2
How to find valency: • If valence electrons ≤ 4: valency = valence electrons (element tends to lose electrons) • If valence electrons > 4: valency = 8 − valence electrons (element tends to gain electrons) • Noble gases (He, Ne, Ar): valency = 0 (already have stable, full outer shell)
Hydrogen (H) • Atomic number: 1 | Atomic mass: 1.008 | State: Gas • Lightest element in the universe | Makes up ~75% of all matter by mass • Burns in air: 2H₂ + O₂ → 2H₂O
Helium (He) • Atomic number: 2 | Atomic mass: 4.003 | State: Gas • Noble gas — completely inert | Second most abundant element in the universe • Used in balloons, cryogenics, and MRI machines
Lithium (Li) • Atomic number: 3 | Atomic mass: 6.941 | State: Solid • Lightest metal | Stored in oil (reacts with air and water) • Used in rechargeable batteries (Li-ion)
Beryllium (Be) • Atomic number: 4 | Atomic mass: 9.012 | State: Solid • Hard, lightweight metal | Toxic if inhaled as dust • Used in aerospace alloys and X-ray windows
Boron (B) • Atomic number: 5 | Atomic mass: 10.81 | State: Solid • Metalloid (properties of both metals and non-metals) • Used in borosilicate glass, detergents (borax)
Carbon (C) • Atomic number: 6 | Atomic mass: 12.01 | State: Solid • Basis of all organic chemistry and life on Earth • Allotropes: diamond, graphite, fullerene, graphene
Nitrogen (N) • Atomic number: 7 | Atomic mass: 14.01 | State: Gas • Makes up 78% of Earth's atmosphere • Essential for proteins, DNA, and amino acids
Oxygen (O) • Atomic number: 8 | Atomic mass: 16.00 | State: Gas • Makes up 21% of Earth's atmosphere • Essential for respiration and combustion
Fluorine (F) • Atomic number: 9 | Atomic mass: 19.00 | State: Gas • Most reactive element and strongest oxidiser • Most electronegative element (EN = 4.0 on Pauling scale)
Neon (Ne) • Atomic number: 10 | Atomic mass: 20.18 | State: Gas • Noble gas — inert | Produces red-orange glow in discharge tubes • Used in neon signs and advertising
Sodium (Na) • Atomic number: 11 | Atomic mass: 22.99 | State: Solid • Soft, silvery alkali metal | Reacts violently with water: 2Na + 2H₂O → 2NaOH + H₂ • Symbol from Latin 'Natrium' | Found in table salt (NaCl)
Magnesium (Mg) • Atomic number: 12 | Atomic mass: 24.31 | State: Solid • Light, silvery metal | Burns with bright white flame in air • Essential for chlorophyll in plants and bones in humans
Aluminium (Al) • Atomic number: 13 | Atomic mass: 26.98 | State: Solid • Most abundant metal in Earth's crust | Light, strong, corrosion-resistant • Used in aircraft, cans, foil, and electrical wiring
Silicon (Si) • Atomic number: 14 | Atomic mass: 28.09 | State: Solid • Metalloid | Second most abundant element in Earth's crust (after oxygen) • Basis of semiconductor technology — used in computer chips
Phosphorus (P) • Atomic number: 15 | Atomic mass: 30.97 | State: Solid • Exists as white phosphorus (toxic, glows) and red phosphorus (stable, used in matches) • Essential for DNA, RNA, ATP, and bones (as calcium phosphate)
Sulphur (S) • Atomic number: 16 | Atomic mass: 32.07 | State: Solid • Yellow, brittle non-metal | Burns with blue flame producing SO₂ • Used in sulphuric acid (H₂SO₄), fertilisers, and vulcanisation of rubber
Chlorine (Cl) • Atomic number: 17 | Atomic mass: 35.45 | State: Gas • Yellow-green, pungent halogen | Strong disinfectant • Used in water purification, PVC production, and bleach (NaClO)
Argon (Ar) • Atomic number: 18 | Atomic mass: 39.95 | State: Gas • Noble gas — completely inert | Third most abundant gas in atmosphere (~0.93%) • Used in welding, light bulbs, and double-glazed windows
Potassium (K) • Atomic number: 19 | Atomic mass: 39.10 | State: Solid • Soft alkali metal | Reacts violently with water (even more than sodium) • Symbol from Latin 'Kalium' | Essential for nerve function and muscle contraction
Calcium (Ca) • Atomic number: 20 | Atomic mass: 40.08 | State: Solid • Alkaline earth metal | Fifth most abundant element in Earth's crust • Essential for bones, teeth, and muscle contraction | Found in limestone (CaCO₃)
The first 20 elements can be classified as:
Metals (7 elements): • Lithium (Li), Beryllium (Be), Sodium (Na), Magnesium (Mg), Aluminium (Al), Potassium (K), Calcium (Ca) • Properties: shiny, good conductors of heat and electricity, malleable, ductile, tend to lose electrons
Non-Metals (9 elements): • Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Phosphorus (P), Sulphur (S), Chlorine (Cl) • Properties: dull, poor conductors, brittle (if solid), tend to gain electrons
Metalloids (2 elements): • Boron (B), Silicon (Si) • Properties: intermediate between metals and non-metals, semiconductors
Noble Gases (3 elements): • Helium (He), Neon (Ne), Argon (Ar) • Properties: colourless, odourless, extremely unreactive, full outer electron shells
Note: Noble gases are technically non-metals, but they are often classified separately due to their complete inertness and full outer shells.
The first 20 elements span 4 periods and several groups:
Period 1 (2 elements): H, He • K shell fills (max 2 electrons)
Period 2 (8 elements): Li, Be, B, C, N, O, F, Ne • L shell fills (max 8 electrons)
Period 3 (8 elements): Na, Mg, Al, Si, P, S, Cl, Ar • M shell fills (max 8 electrons in this range)
Period 4 (first 2 elements): K, Ca • N shell starts filling
Groups represented: • Group 1 (Alkali metals): H, Li, Na, K — 1 valence electron, valency 1 • Group 2 (Alkaline earth metals): Be, Mg, Ca — 2 valence electrons, valency 2 • Group 13: B, Al — 3 valence electrons, valency 3 • Group 14: C, Si — 4 valence electrons, valency 4 • Group 15: N, P — 5 valence electrons, valency 3 or 5 • Group 16: O, S — 6 valence electrons, valency 2 • Group 17 (Halogens): F, Cl — 7 valence electrons, valency 1 • Group 18 (Noble gases): He, Ne, Ar — full shell, valency 0
Trend: As you move left to right across a period, elements change from metals to metalloids to non-metals to noble gases.
The first 20 elements in order are: Hydrogen (H), Helium (He), Lithium (Li), Beryllium (Be), Boron (B), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Neon (Ne), Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P), Sulphur (S), Chlorine (Cl), Argon (Ar), Potassium (K), and Calcium (Ca). They span atomic numbers 1 to 20.
Use this mnemonic: 'Hi He Lies Because Boron Can Not Oxidise Fluorine. New Nations Might Also Sign Peace Security Clause. Argentina Knows Canada.' Each first letter or word corresponds to an element: H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca. Repeating this sentence a few times helps memorise all 20 in order.
The electron configurations are: H (1s¹), He (1s²), Li (2,1), Be (2,2), B (2,3), C (2,4), N (2,5), O (2,6), F (2,7), Ne (2,8), Na (2,8,1), Mg (2,8,2), Al (2,8,3), Si (2,8,4), P (2,8,5), S (2,8,6), Cl (2,8,7), Ar (2,8,8), K (2,8,8,1), Ca (2,8,8,2). Shells fill in order: K shell (max 2), L shell (max 8), M shell (max 8), then N shell.
Valencies: H(1), He(0), Li(1), Be(2), B(3), C(4), N(3), O(2), F(1), Ne(0), Na(1), Mg(2), Al(3), Si(4), P(3 or 5), S(2, 4, or 6), Cl(1, 3, 5, or 7), Ar(0), K(1), Ca(2). Rule: if valence electrons ≤ 4, valency = valence electrons. If > 4, valency = 8 − valence electrons. Noble gases have valency 0.
The metals among the first 20 elements are: Lithium (Li), Beryllium (Be), Sodium (Na), Magnesium (Mg), Aluminium (Al), Potassium (K), and Calcium (Ca) — 7 metals in total. Li, Na, and K are alkali metals (Group 1). Be, Mg, and Ca are alkaline earth metals (Group 2). Al is a post-transition metal (Group 13).
The non-metals are: Hydrogen (H), Carbon (C), Nitrogen (N), Oxygen (O), Fluorine (F), Phosphorus (P), Sulphur (S), and Chlorine (Cl) — 8 non-metals. Additionally, there are 3 noble gases: Helium (He), Neon (Ne), and Argon (Ar), which are also non-metals but classified separately. The 2 metalloids are Boron (B) and Silicon (Si).
Potassium's symbol is K from its Latin name 'Kalium,' derived from the Arabic word 'al-qalyah' (plant ashes, from which potassium was first isolated). P was already taken by Phosphorus (atomic number 15). Similarly, Sodium's symbol is Na from its Latin name 'Natrium.' Many elements have symbols from their Latin or Greek names rather than their English names.
The noble gases in the first 20 elements are Helium (He, atomic number 2), Neon (Ne, atomic number 10), and Argon (Ar, atomic number 18). All belong to Group 18. They have completely filled outer electron shells (He: 2, Ne: 2,8, Ar: 2,8,8), making them extremely stable and unreactive. Their valency is 0.
Hybridization and Structure of Ammonia (NH3)
Learn the hybridization of the central nitrogen atom in Ammonia (NH3). Understand why its shape is trigonal pyramidal and not tetrahedral.
Oxidation Number of Nitrogen in NH3
Learn how to calculate the oxidation number (state) of Nitrogen in the Ammonia (NH3) molecule using simple redox rules.
Electron Configuration of Nickel (Ni)
Learn the electron configuration of Nickel (Ni, Z=28). Understand the ground state configuration [Ar] 3d8 4s2 and its exceptions.
Hybridization of [Ni(CN)4]2- Ion
Understand the dsp2 hybridization of the tetracyanonickelate(II) ion. Learn about strong field ligands and square planar geometry.
Hybridization of Ni(CO)₄
Learn the hybridization of Ni(CO)4 — Nickel tetracarbonyl. Understand why it is sp3 hybridized, tetrahedral, and diamagnetic.
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