What Is Atomic Mass? Reference Table and How to Calculate the Mass of an Atom Atomic mass is foundational knowledge that helps students understand how elements are represented and used in Chemistry exercises. If they only memorise the table, students can easily confuse atomic mass, mass number, molecular mass and molar mass. This article helps you understand the concept, look up atomic masses in the table, learn how to calculate the mass of an atom and avoid common mistakes. Let’s explore it together so you can build a firmer grasp of Chemistry from the fundamentals. What is atomic mass? Before using an atomic mass table, students need to understand what the numbers in it represent. If they only memorise them, they can easily make mistakes when calculating molecular mass or tackling problems involving isotopes. A simple definition of atomic mass Atomic mass is the relative mass of an atom, compared with a very small standard unit. Because atoms are so tiny, we don’t use grams in the usual way to describe the mass of individual atoms. Put simply, atomic mass tells you how many times heavier an atom is than the atomic mass unit. For example, hydrogen usually has an atomic mass of 1, carbon 12 and oxygen 16. These values come up frequently in atomic mass exercises in Chemistry. What are u, amu and đvC? Atomic mass is measured in atomic mass units, not grams. Several different symbols are in use, which can easily confuse students, because older textbooks and the new curriculum use different names. Textbooks following Vietnam’s previous curriculum usually used đvC (đơn vị cacbon, or “carbon unit”). The 2018 General Education Curriculum (Natural Sciences Grade 7 and Chemistry Grade 10) has moved to amu, in line with international usage. In international materials, the symbol u or the dalton (Da) is also used. The key point to remember: đvC, amu and u are essentially the same; only the name has changed over time. All three are equal to 1/12 of the mass of one carbon-12 atom, so the values in the atomic mass table stay the same; only the unit label changes. If students come across older materials that use đvC, they should understand it as the former notation. When working from current textbooks, they should use amu to match the questions and answers. In bilingual learning settings, this quantity is also known as relative atomic mass. Understanding both the Vietnamese and English terms helps students read international materials more easily and keep up with different curricula. Why is carbon-12 used as the standard? Carbon-12 was chosen as the agreed reference point because it is stable and well suited to building a scale of atomic masses. By convention, 1 u is equal to 1/12 of the mass of one carbon-12 atom. Thanks to this convention, the atomic mass table can represent the masses of different elements on a consistent basis. A detailed look at the concept of atomic mass and the units you need to know How is atomic mass different from mass number, molecular mass and molar mass? When learning about atomic mass in Chemistry, students can easily confuse it with mass number, molecular mass and molar mass. These concepts all relate to “mass”, but they are used in different situations. Understanding them correctly from the start helps students look up atomic masses more accurately and avoid mistakes in exercises. Atomic mass vs mass number Atomic mass tells you the relative mass of an atom. It is the value usually used when calculating molecular mass or working through basic Chemistry problems. Mass number, on the other hand, belongs to the nucleus of the atom. It tells you the total number of protons and neutrons in the nucleus. A quick way to tell them apart: Atomic mass: refers to the relative mass of an atom. Mass number: refers to the total number of protons and neutrons. How to spot it in a question: if the question asks about relative mass, use atomic mass; if it asks about protons and neutrons, use mass number. For example, carbon-12 has 6 protons and 6 neutrons, so its mass number is 12. This value is close to the atomic mass of carbon, but the two concepts are not exactly the same. Atomic mass vs molecular mass Atomic mass is used for a single atom. Molecular mass is used for a molecule or compound, so you need to add up the atomic masses of all the atoms in the formula. For example, oxygen has a commonly used atomic mass of 16. But water, H₂O, doesn’t have a molecular mass of 16, because the formula contains 2 H atoms and 1 O atom. Calculating H₂O: H has an atomic mass of 1. O has an atomic mass of 16. H₂O = 2 × 1 + 16 = 18. Where students often go wrong is the subscript number below the symbol. If they forget to multiply by this subscript, their molecular mass will be wrong even if they remember the atomic mass table correctly. Atomic mass vs molar mass In school exercises, atomic mass and molar mass usually have the same numerical value, but they differ in meaning and unit. A simple way to understand it: Atomic mass: the relative mass of one atom. Molar mass: the mass of 1 mole of a substance. Units: atomic mass usually uses u or đvC; molar mass uses g/mol. For example, the atomic mass of O is 16. The molar mass of O atoms is 16 g/mol. The two values look the same, but one describes a single atom while the other describes an amount of substance made up of a huge number of particles. A detailed comparison of four easily confused concepts when learning about atomic mass Full table of atomic masses for all 118 elements When students first learn Chemistry, they don’t need to memorise the whole periodic table. What matters more is knowing which elements come up frequently in exercises. This is a more practical way to learn, as most molecular mass calculations at the early stage revolve around a small number of familiar elements. Full atomic mass table for reference The atomic mass table below covers all 118 elements. The values follow the rounding commonly used in the school curriculum. No. Element Symbol Atomic mass 1 Hiđro H 1 2 Heli He 4 3 Liti Li 7 4 Beri Be 9 5 Bo B 11 6 Cacbon C 12 7 Nitơ N 14 8 Oxi O 16 9 Flo F 19 10 Neon Ne 20 11 Natri Na 23 12 Magie Mg 24 13 Nhôm Al 27 14 Silic Si 28 15 Photpho P 31 16 Lưu huỳnh S 32 17 Clo Cl 35,5 18 Argon Ar 40 19 Kali K 39 20 Canxi Ca 40 21 Scandi Sc 45 22 Titan Ti 48 23 Vanadi V 51 24 Crom Cr 52 25 Mangan Mn 55 26 Sắt Fe 56 27 Coban Co 59 28 Niken Ni 59 29 Đồng Cu 64 30 Kẽm Zn 65 31 Gali Ga 70 32 Gecmani Ge 73 33 Asen As 75 34 Selen Se 79 35 Brom Br 80 36 Krypton Kr 84 37 Rubidi Rb 85 38 Stronti Sr 88 39 Yttri Y 89 40 Zirconi Zr 91 41 Niobi Nb 93 42 Molypden Mo 96 43 Tecneti Tc 98 44 Rutheni Ru 101 45 Rhodi Rh 103 46 Paladi Pd 106 47 Bạc Ag 108 48 Cadmi Cd 112 49 Indi In 115 50 Thiếc Sn 119 51 Antimon Sb 122 52 Telua Te 128 53 Iốt I 127 54 Xenon Xe 131 55 Xêsi Cs 133 56 Bari Ba 137 57 Lantan La 139 58 Xeri Ce 140 59 Praseodymi Pr 141 60 Neodymi Nd 144 61 Promethi Pm 145 62 Samari Sm 150 63 Europi Eu 152 64 Gadolini Gd 157 65 Terbi Tb 159 66 Dysprosi Dy 163 67 Holmi Ho 165 68 Erbi Er 167 69 Thuli Tm 169 70 Ytterbi Yb 173 71 Luteti Lu 175 72 Hafni Hf 178 73 Tantal Ta 181 74 Wolfram W 184 75 Rheni Re 186 76 Osmi Os 190 77 Iridi Ir 192 78 Platin Pt 195 79 Vàng Au 197 80 Thủy ngân Hg 201 81 Tali Tl 204 82 Chì Pb 207 83 Bitmut Bi 209 84 Poloni Po 209 85 Astatin At 210 86 Radon Rn 222 87 Franxi Fr 223 88 Radi Ra 226 89 Actini Ac 227 90 Thori Th 232 91 Protactini Pa 231 92 Urani U 238 93 Neptuni Np 237 94 Plutoni Pu 244 95 Americi Am 243 96 Curi Cm 247 97 Berkeli Bk 247 98 Californi Cf 251 99 Einsteini Es 252 100 Fermi Fm 257 101 Mendelevi Md 258 102 Nobeli No 259 103 Lawrenci Lr 262 104 Rutherfordi Rf 267 105 Dubni Db 268 106 Seaborgi Sg 269 107 Bohri Bh 270 108 Hassi Hs 269 109 Meitneri Mt 278 110 Darmstadti Ds 281 111 Roentgeni Rg 282 112 Copernici Cn 285 113 Nihoni Nh 286 114 Flerovi Fl 289 115 Moscovi Mc 290 116 Livermori Lv 293 117 Tennessine Ts 294 118 Oganesson Og 294 Notes: The values in the table have been rounded in the way commonly used in the school curriculum (chlorine is kept at 35.5). For radioactive and synthetic elements (Tc, Pm and from polonium onwards), the figure shown is the mass number of the most stable or best-known isotope, not an average atomic mass (these elements almost never appear in school exercises). For example, to calculate the molecular mass of CaCO₃, students only need to look up Ca = 40, C = 12 and O = 16. Then calculate: 40 + 12 + 3 × 16 = 100. When do you need the extended atomic mass table? The extended table is needed when a question includes less common elements, such as Ag, Pb, Hg, Br or transition elements. At this level, students shouldn’t try to memorise everything. A better approach is knowing how to look up the right symbol, read the element’s name correctly and choose the value the question requires. With atomic mass in Chemistry, looking up the table is only the first step. Students still need to understand the chemical formula before calculating. If they misread a subscript in the formula, the result will still be wrong even if they looked up the table correctly. A note on rounded values in exercises Some elements have atomic masses that aren’t whole numbers. Chlorine is usually taken as 35.5. Some more advanced tables may give more precise values, such as 55.845 for Fe or 63.546 for Cu. However, school exercises usually use rounded values. If the question already gives atomic masses, use the figures in the question first. This is an important principle, especially when the question is testing calculation skills rather than the ability to remember the table. How to calculate atomic mass and the mass of an atom When doing Chemistry exercises, students usually don’t need to weigh an atom in grams. That value is too small to picture in everyday life. That’s why atomic mass in Chemistry is usually used as a relative comparison, making calculations simpler. However, in some exercises, the question may ask students to convert from atomic mass to the actual mass of an atom. In that case, students need to remember the relationship between the unit u and grams. Formula for the mass of one atom in grams By the convention commonly used in Chemistry: 1 u ≈ 1.6605 × 10⁻²⁴ grams From this, the formula for the mass of one atom is: Mass of one atom = atomic mass × 1.6605 × 10⁻²⁴ grams When applying the formula, students should note: Take the atomic mass from the atomic mass table or from the data in the question. If the question gives its own figures, use those first. Don’t confuse the mass of one atom with molar mass. The result in grams is usually very small, so it can be written as a power of 10. Example: calculating the mass of one oxygen atom Oxygen has a commonly used atomic mass of 16. The mass of one oxygen atom is calculated as follows: m(O) = 16 × 1.6605 × 10⁻²⁴ m(O) ≈ 2.6568 × 10⁻²³ grams This number is far smaller than the masses we are familiar with in everyday life. That isn’t a calculation error. Atoms are extremely small, so the mass of one atom in grams is also very small. The key thing for students to grasp is the calculation process: Identify the element to calculate. Look up the element’s atomic mass. Multiply by 1.6605 × 10⁻²⁴ grams. Check the final unit. An easy-to-follow process for calculating the mass of an atom in grams When do students need to calculate in grams? Not every exercise requires converting atomic mass into grams. In basic exercises, students usually only need to look up atomic masses to calculate the molecular mass of a compound. Calculating in grams usually comes up when: The question asks for the actual mass of one atom. The exercise aims to help students understand the meaning of the unit u. An advanced exercise involves the very small mass of atoms. The teacher asks for the result to be converted into grams. Before calculating, students should read the question carefully. If it asks for “atomic mass”, only the relative value is needed. If it asks for “the mass of one atom”, convert to grams using the formula above. What is average atomic mass? Average atomic mass is the average mass of the isotopes of the same element. This is why some elements in the atomic mass table don’t have whole-number values. At a basic level, students usually remember familiar values such as H = 1, C = 12 and O = 16. However, when they study atomic mass in Chemistry in more depth, they will come across values such as Cl = 35.5. This number doesn’t represent an individual chlorine atom; it is an average value based on the chlorine isotopes found in nature. Why is there an average atomic mass? An element can exist in several isotopic forms. These isotopes have the same number of protons but different numbers of neutrons. As a result, their masses aren’t exactly the same. When calculating average atomic mass, two factors need to be considered: The mass or mass number of each isotope. The percentage of each isotope found in nature. This helps explain why many values in the atomic mass table aren’t whole numbers. It also helps students avoid confusing average atomic mass with the mass number of a specific isotope. Formula for average atomic mass The commonly used formula is: Average atomic mass = (A₁ × a₁ + A₂ × a₂ + ...) / 100 Where: A₁, A₂ are the mass numbers or atomic masses of each isotope. a₁, a₂ are the percentages of each isotope. When working through a problem, students need to check the percentage format carefully. If the question gives proportions as %, divide by 100. If it gives them as decimals, don’t divide again. Example: calculating average atomic mass Suppose element X has two isotopes: Isotope X-10 makes up 20%. Isotope X-11 makes up 80%. The average atomic mass of X is: (10 × 20 + 11 × 80) / 100 = 10.8 So the average atomic mass of X is 10.8. For this type of problem, students should identify each isotope correctly first, then substitute the numbers into the formula to avoid mixing up the data. A worked example of how to calculate average atomic mass Common types of atomic mass exercises Once they understand the concept, students need to know how to use atomic mass in exercises. This step makes sure their knowledge goes beyond memorising the atomic mass table. Atomic mass exercises in Chemistry usually fall into three main types: calculating molecular mass, finding an unknown element and calculating average atomic mass. Each type has its own method, but all start with one important step: reading the chemical formula correctly. Exercise type Main method Common mistake Calculating molecular mass Add up the atomic masses in the molecule Forgetting to multiply by the subscript Finding an unknown element Work backwards from the molecular mass Guessing the element by instinct Calculating average atomic mass Multiply each isotope's mass by its percentage Forgetting to divide by 100 Type 1: Calculating the molecular mass of a compound This is the most fundamental type. Students need to identify which elements make up the compound. Then they count the number of atoms of each element and add up the corresponding atomic masses. For example, water has the formula H₂O. This molecule contains 2 H atoms and 1 O atom. If H has an atomic mass of 1 and O has an atomic mass of 16, we calculate: H₂O = 2 × 1 + 16 = 18 For carbon dioxide gas, the formula is CO₂: CO₂ = 12 + 2 × 16 = 44 The key thing to watch is the small subscript below the symbol. In CO₂, the 2 only applies to O, not to C. For CaCO₃, students need to include Ca, C and all 3 O atoms. Type 2: Finding an element from its molecular mass This type requires students to reason backwards. The question usually gives the formula of a compound and its molecular mass. The task is to find the missing element. For example, a compound has the formula XO and a molecular mass of 40. O has an atomic mass of 16. So: Atomic mass of X + 16 = 40 Atomic mass of X = 40 − 16 = 24 Looking up the atomic mass table, the element with an atomic mass close to 24 is Mg. So X is magnesium. For this type, students shouldn’t guess the element straight away. A more reliable approach is to set up the calculation first, then check it against the table. If the formula contains several elements, separate out the known part first. Type 3: Calculating average atomic mass Average atomic mass problems usually come up when an element has several isotopes. The question gives the mass of each isotope and its corresponding percentage. The method is to multiply the mass of each isotope by its percentage. Then add the results together and divide by 100. For example, suppose element X has two isotopes: Isotope Mass Proportion X1 10 20% X2 11 80% The average atomic mass of X is: (10 × 20 + 11 × 80) / 100 = 10.8 A common mistake in this type is forgetting to divide by 100. Some students also confuse percentages with numbers of atoms. If the question gives proportions as decimals, substitute them directly as decimals instead of dividing again. 5 common mistakes students make Forgetting to multiply by the subscript in a chemical formula: For example, H₂O must include 2 H atoms. If you only calculate 1 + 16, the result will be wrong. Confusing atomic mass with molecular mass: Atomic mass is for one atom. Molecular mass is the total atomic mass of the whole molecule. Using the wrong rounded value: Some elements have special values in school exercises; for example, Cl is usually taken as 35.5. Don’t round values yourself unless the question asks you to. Missing groups in brackets: For Ca(OH)₂, students need to calculate: Ca + 2 × O + 2 × H. The 2 outside the bracket applies to both O and H. Not prioritising the data in the question: If the question gives its own atomic masses, use those figures. The table should only be used when the question doesn’t provide other data. Common types of atomic mass exercises and mistakes to avoid How to learn the atomic mass table effectively Learning the atomic mass table shouldn’t just mean reading a long list over and over. That approach can help students remember quickly, but they forget just as quickly. When they face real exercises, they may still confuse symbols, forget to multiply by subscripts or use the wrong atomic mass. A more effective approach is to learn in small groups, use the values straight away in exercises and test yourself regularly. With atomic mass in Chemistry, the goal isn’t to remember as much as possible. The goal is to know when to use the table and to calculate correctly when you meet a chemical formula. Learn by groups of common elements At the early stage, students don’t need to learn the whole periodic table. They should start with the elements that appear most often in basic exercises. Groups to prioritise: H, C, N, O: common in simple compounds and organic compounds. Na, Mg, Al, K, Ca: familiar metals in school exercises. S, Cl: often found in salts, acids and inorganic compounds. Fe, Cu, Zn, Ba: common in more advanced exercises. Learning by group helps students see the atomic mass table more systematically. They no longer have to memorise each element in isolation. Instead, they understand which elements often appear together in chemical formulas. For example, once they remember H = 1, O = 16 and C = 12, students can practise straight away with H₂O, CO₂ or H₂CO₃. When they add Ca = 40, they can move on to CaCO₃ or Ca(OH)₂. Link the table to exercises instead of memorising mechanically A common mistake is when students know atomic masses by heart but still calculate molecular masses incorrectly. The reason usually isn’t memory, but how they use the table in exercises. When learning a group of elements, students should do a few short calculations straight away: H₂O = 2 × 1 + 16 = 18 CO₂ = 12 + 2 × 16 = 44 CaCO₃ = 40 + 12 + 3 × 16 = 100 Ca(OH)₂ = 40 + 2 × (16 + 1) = 74 This approach helps students practise three skills at once: Reading chemical symbols correctly. Spotting the subscripts below symbols. Knowing when to multiply a whole group in brackets. As a result, the atomic mass table is no longer a separate thing to memorise. It becomes a tool for solving problems. This is also a more lasting way to learn, because students remember through using it. Use flashcards and short self-tests Flashcards are well suited to learning the atomic mass table. Each card should have just one element to avoid overload. A simple method: Front: the symbol, e.g. O. Back: the name and atomic mass, e.g. Oxygen — 16. Study 5–10 cards at a time. After answering, set yourself a small calculation with that element. For example, when you draw the H and O cards, you can calculate H₂O. When you draw Ca, C and O, you can try calculating CaCO₃. Elements you know well should be moved to a light-review pile. Elements that are often confused, such as Na = 23, Mg = 24 and Cl = 35.5, should be kept in the pile for more frequent testing. Learning the atomic mass table effectively isn’t about studying once for a long time. What matters is studying in short bursts, using what you learn straight away and repeating the areas that are still weak. The easiest and most effective ways for students to learn atomic masses Effective Chemistry learning starts with understanding the fundamentals For many students, Chemistry becomes difficult not because the formulas are too long. The difficulty usually starts when they memorise before understanding the fundamentals. Atomic mass is a clear example. If students only memorise the atomic mass table, they may get a few familiar exercises right. But when a question is phrased differently, introduces a new compound or asks for average atomic mass, they can easily get stuck. When they understand why each element has its own mass value, calculating molecular mass or working with data becomes much clearer. An effective way to learn atomic mass in Chemistry should move from the concept, to the reference table, then to applied exercises. The table is only a tool. What matters is that students know when to use it, read formulas correctly and check each step. At Westlink, the spirit of learning Science goes beyond memorising answers. Students are encouraged to ask questions, observe, explain and apply knowledge to new situations. Parents can find out more about the curriculum and teaching approach at Westlink to understand how their child builds a stronger foundation in science. Understanding atomic mass correctly doesn’t just help students remember the table faster; it also helps them read formulas, calculate molecular mass and approach isotope problems logically. When Chemistry is learnt from the fundamentals, children become more confident with new types of questions instead of just following familiar patterns. At Westlink, students are encouraged to observe, ask questions and explain problems through scientific thinking. If you’re interested, you can book a school tour to meet our academic team and find out more about your child’s learning pathway.
