Learning Outcomes
- Define atomic and mass numbers.
- Determine the number of protons, neutrons, and electrons in an atom.
- Identify the charge and relative mass of subatomic particles.
- Label the location of subatomic particles in the atom.
- Determine the mass of an atom based on its subatomic particles.
- Write A/Z and symbol-mass format for an atom.
Atoms are the fundamental building blocks of all matter and are composed of protons, neutrons, and electrons. Because atoms are electrically neutral, the number of positively charged protons must be equal to the number of negatively charged electrons. Since neutrons do not affect the charge, the number of neutrons is not dependent on the number of protons and will vary even among atoms of the same element.
The atomic number is the number of protons in an atom, and isotopes have the same atomic number but differ in the number of neutrons. If you're seeing this message, it means we're having trouble loading external resources on our website. The atomic number or proton number is defined as the total number of protons in the nucleus and is given the symbol Z.The number of electrons in an electrically-neutral atom is the same as the atomic number. The total electrical charge of the nucleus is therefore +Ze.
Atomic Number
Atomic Number Of Carbon
The atomic number (represented by the letter Z)of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and any atom with a different number of protons would be a different element. The periodic table (see figure below) displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on.
Since atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Helium, with two protons, will have two electrons. In the chemical classroom, the proton count will always be equivalent to an atom's atomic number. This value will not change unless the nucleus decays or is bombarded (nuclear physics).
Mass Number
Experimental data showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number (represented by the letter A)is defined as the total number of protons and neutrons in an atom. Consider the table below, which shows data from the first six elements of the periodic table.
Name | Symbol | Atomic Number (Z) | Protons | Neutrons | Electrons | Mass Number (A) (rounded to two decimals) |
---|---|---|---|---|---|---|
hydrogen | (ce{H}) | 1 | 1 | 0 | 1 | 1.01 |
helium | (ce{He}) | 2 | 2 | 2 | 2 | 4.00 |
lithium | (ce{Li}) | 3 | 3 | 4 | 3 | 6.94 |
beryllium | (ce{Be}) | 4 | 4 | 5 | 4 | 9.01 |
boron | (ce{B}) | 5 | 5 | 6 | 5 | 10.18 |
carbon | (ce{C}) | 6 | 6 | 6 | 6 | 12.01 |
Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since (2 + 2 = 4), we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.
Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.
[text{Number of neutrons} = text{ rounded mass number} - text{atomic number}]
Atoms of the element chromium (left( ce{Cr} right)) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:
[52 - 24 = 28 : text{neutrons in a chromium atom}]
The composition of any atom can be illustrated with a shorthand notation called A/Z format. Both the atomic number and mass are written to the left of the chemical symbol. The 'A' value is written as a superscript while the 'Z' value is written as a subscript. For an example of this notation, look to the chromium atom shown below:
[ce{^{52}_{24}Cr}]
Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr-52. In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values.
Example (PageIndex{1})
Calculate each of the three subatomic particles and give specific group or period names for each atom.
- mercury
- platinum
- bromine
Solutions
- Hg (transition metal)- has 80 electrons, 80 protons, and 121 neutrons
- Pt (transition metal)- has 78 electrons, 78 protons, and 117 neutrons
- Br (halogen)- has 35 electrons, 35 protons, and 45 neutrons
Example (PageIndex{2})
Write both A/Z and symbol-mass formats for the atoms in Example (PageIndex{1}).
Solutions
- (ce{^{201}_{80}Hg}) and Hg-201
- (ce{^{195}_{78}Pt}) and Pt-195
- (ce{^{80}_{35}Br}) and Br-80
Example (PageIndex{3}) Mac pages numbers and keynote.
Identify the elements based on the statements below.
- Which element has 25 protons?
- Which element has 0 neutrons?
- Which element has 83 electrons?
Identify the elements based on the statements below.
- Which element has 25 protons?
- Which element has 0 neutrons?
- Which element has 83 electrons?
Printable List Of Element Names
Solutions
Atomic Number Of Sodium
a. manganese
b. hydrogen
c. bismuth
Need More Practice?
- Turn to section 3.E of this OER and answer questions #1-#2, #4, and #8.
Contributors and Attributions
CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.
Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)
Click a column header, such as Name, to sort the table by that item.
SEENotes at the bottom of the Table.
No. | Atomic weight | Name | Sym. | M.P. (°C) | B.P. (°C) | Density* (g/cm3) | Earth crust (%)* | Discovery (Year) | Group* | Electron configuration | Ionization energy (eV) | |
---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | 1.008 | Hydrogen | H | -259 | -253 | 0.09 | 0.14 | 1776 | 1 | 1s1 | 13.60 | |
2 | 4.003 | Helium | He | -272 | -269 | 0.18 | 1895 | 18 | 1s2 | 24.59 | ||
3 | 6.941 | Lithium | Li | 180 | 1,347 | 0.53 | 1817 | 1 | [He] 2s1 | 5.39 | ||
4 | 9.012 | Beryllium | Be | 1,278 | 2,970 | 1.85 | 1797 | 2 | [He] 2s2 | 9.32 | ||
5 | 10.811 | Boron | B | 2,300 | 2,550 | 2.34 | 1808 | 13 | [He] 2s2 2p1 | 8.30 | ||
6 | 12.011 | Carbon | C | 3,500 | 4,827 | 2.26 | 0.09 | ancient | 14 | [He] 2s2 2p2 | 11.26 | |
7 | 14.007 | Nitrogen | N | -210 | -196 | 1.25 | 1772 | 15 | [He] 2s2 2p3 | 14.53 | ||
8 | 15.999 | Oxygen | O | -218 | -183 | 1.43 | 46.71 | 1774 | 16 | [He] 2s2 2p4 | 13.62 | |
9 | 18.998 | Fluorine | F | -220 | -188 | 1.70 | 0.03 | 1886 | 17 | [He] 2s2 2p5 | 17.42 | |
10 | 20.180 | Neon | Ne | -249 | -246 | 0.90 | 1898 | 18 | [He] 2s2 2p6 | 21.56 | ||
11 | 22.990 | Sodium | Na | 98 | 883 | 0.97 | 2.75 | 1807 | 1 | [Ne] 3s1 | 5.14 | |
12 | 24.305 | Magnesium | Mg | 639 | 1,090 | 1.74 | 2.08 | 1755 | 2 | [Ne] 3s2 | 7.65 | |
13 | 26.982 | Aluminum | Al | 660 | 2,467 | 2.70 | 8.07 | 1825 | 13 | [Ne] 3s2 3p1 | 5.99 | |
14 | 28.086 | Silicon | Si | 1,410 | 2,355 | 2.33 | 27.69 | 1824 | 14 | [Ne] 3s2 3p2 | 8.15 | |
15 | 30.974 | Phosphorus | P | 44 | 280 | 1.82 | 0.13 | 1669 | 15 | [Ne] 3s2 3p3 | 10.49 | |
16 | 32.065 | Sulfur | S | 113 | 445 | 2.07 | 0.05 | ancient | 16 | [Ne] 3s2 3p4 | 10.36 | |
17 | 35.453 | Chlorine | Cl | -101 | -35 | 3.21 | 0.05 | 1774 | 17 | [Ne] 3s2 3p5 | 12.97 | |
18 | 39.948 | Argon | Ar | -189 | -186 | 1.78 | 1894 | 18 | [Ne] 3s2 3p6 | 15.76 | ||
19 | 39.098 | Potassium | K | 64 | 774 | 0.86 | 2.58 | 1807 | 1 | [Ar] 4s1 | 4.34 | |
20 | 40.078 | Calcium | Ca | 839 | 1,484 | 1.55 | 3.65 | 1808 | 2 | [Ar] 4s2 | 6.11 | |
21 | 44.956 | Scandium | Sc | 1,539 | 2,832 | 2.99 | 1879 | 3 | [Ar] 3d1 4s2 | 6.56 | ||
22 | 47.867 | Titanium | Ti | 1,660 | 3,287 | 4.54 | 0.62 | 1791 | 4 | [Ar] 3d2 4s2 | 6.83 | |
23 | 50.942 | Vanadium | V | 1,890 | 3,380 | 6.11 | 1830 | 5 | [Ar] 3d3 4s2 | 6.75 | ||
24 | 51.996 | Chromium | Cr | 1,857 | 2,672 | 7.19 | 0.04 | 1797 | 6 | [Ar] 3d5 4s1 | 6.77 | |
25 | 54.938 | Manganese | Mn | 1,245 | 1,962 | 7.43 | 0.09 | 1774 | 7 | [Ar] 3d5 4s2 | 7.43 | |
26 | 55.845 | Iron | Fe | 1,535 | 2,750 | 7.87 | 5.05 | ancient | 8 | [Ar] 3d6 4s2 | 7.90 | |
27 | 58.933 | Cobalt | Co | 1,495 | 2,870 | 8.90 | 1735 | 9 | [Ar] 3d7 4s2 | 7.88 | ||
28 | 58.693 | Nickel | Ni | 1,453 | 2,732 | 8.90 | 0.02 | 1751 | 10 | [Ar] 3d8 4s2 | 7.64 | |
29 | 63.546 | Copper | Cu | 1,083 | 2,567 | 8.96 | ancient | 11 | [Ar] 3d10 4s1 | 7.73 | ||
30 | 65.390 | Zinc | Zn | 420 | 907 | 7.13 | ancient | 12 | [Ar] 3d10 4s2 | 9.39 | ||
31 | 69.723 | Gallium | Ga | 30 | 2,403 | 5.91 | 1875 | 13 | [Ar] 3d10 4s2 4p1 | 6.00 | ||
32 | 72.640 | Germanium | Ge | 937 | 2,830 | 5.32 | 1886 | 14 | [Ar] 3d10 4s2 4p2 | 7.90 | ||
33 | 74.922 | Arsenic | As | 81 | 613 | 5.72 | ancient | 15 | [Ar] 3d10 4s2 4p3 | 9.79 | ||
34 | 78.960 | Selenium | Se | 217 | 685 | 4.79 | 1817 | 16 | [Ar] 3d10 4s2 4p4 | 9.75 | ||
35 | 79.904 | Bromine | Br | -7 | 59 | 3.12 | 1826 | 17 | [Ar] 3d10 4s2 4p5 | 11.81 | ||
36 | 83.800 | Krypton | Kr | -157 | -153 | 3.75 | 1898 | 18 | [Ar] 3d10 4s2 4p6 | 14.00 | ||
37 | 85.468 | Rubidium | Rb | 39 | 688 | 1.63 | 1861 | 1 | [Kr] 5s1 | 4.18 | ||
38 | 87.620 | Strontium | Sr | 769 | 1,384 | 2.54 | 1790 | 2 | [Kr] 5s2 | 5.69 | ||
39 | 88.906 | Yttrium | Y | 1,523 | 3,337 | 4.47 | 1794 | 3 | [Kr] 4d1 5s2 | 6.22 | ||
40 | 91.224 | Zirconium | Zr | 1,852 | 4,377 | 6.51 | 0.03 | 1789 | 4 | [Kr] 4d2 5s2 | 6.63 | |
41 | 92.906 | Niobium | Nb | 2,468 | 4,927 | 8.57 | 1801 | 5 | [Kr] 4d4 5s1 | 6.76 | ||
42 | 95.940 | Molybdenum | Mo | 2,617 | 4,612 | 10.22 | 1781 | 6 | [Kr] 4d5 5s1 | 7.09 | ||
43 | * | 98.000 | Technetium | Tc | 2,200 | 4,877 | 11.50 | 1937 | 7 | [Kr] 4d5 5s2 | 7.28 | |
44 | 101.070 | Ruthenium | Ru | 2,250 | 3,900 | 12.37 | 1844 | 8 | [Kr] 4d7 5s1 | 7.36 | ||
45 | 102.906 | Rhodium | Rh | 1,966 | 3,727 | 12.41 | 1803 | 9 | [Kr] 4d8 5s1 | 7.46 | ||
46 | 106.420 | Palladium | Pd | 1,552 | 2,927 | 12.02 | 1803 | 10 | [Kr] 4d10 | 8.34 | ||
47 | 107.868 | Silver | Ag | 962 | 2,212 | 10.50 | ancient | 11 | [Kr] 4d10 5s1 | 7.58 | ||
48 | 112.411 | Cadmium | Cd | 321 | 765 | 8.65 | 1817 | 12 | [Kr] 4d10 5s2 | 8.99 | ||
49 | 114.818 | Indium | In | 157 | 2,000 | 7.31 | 1863 | 13 | [Kr] 4d10 5s2 5p1 | 5.79 | ||
50 | 118.710 | Tin | Sn | 232 | 2,270 | 7.31 | ancient | 14 | [Kr] 4d10 5s2 5p2 | 7.34 | ||
51 | 121.760 | Antimony | Sb | 630 | 1,750 | 6.68 | ancient | 15 | [Kr] 4d10 5s2 5p3 | 8.61 | ||
52 | 127.600 | Tellurium | Te | 449 | 990 | 6.24 | 1783 | 16 | [Kr] 4d10 5s2 5p4 | 9.01 | ||
53 | 126.905 | Iodine | I | 114 | 184 | 4.93 | 1811 | 17 | [Kr] 4d10 5s2 5p5 | 10.45 | ||
54 | 131.293 | Xenon | Xe | -112 | -108 | 5.90 | 1898 | 18 | [Kr] 4d10 5s2 5p6 | 12.13 | ||
55 | 132.906 | Cesium | Cs | 29 | 678 | 1.87 | 1860 | 1 | [Xe] 6s1 | 3.89 | ||
56 | 137.327 | Barium | Ba | 725 | 1,140 | 3.59 | 0.05 | 1808 | 2 | [Xe] 6s2 | 5.21 | |
57 | 138.906 | Lanthanum | La | 920 | 3,469 | 6.15 | 1839 | 3 | [Xe] 5d1 6s2 | 5.58 | ||
58 | 140.116 | Cerium | Ce | 795 | 3,257 | 6.77 | 1803 | 101 | [Xe] 4f1 5d1 6s2 | 5.54 | ||
59 | 140.908 | Praseodymium | Pr | 935 | 3,127 | 6.77 | 1885 | 101 | [Xe] 4f3 6s2 | 5.47 | ||
60 | 144.240 | Neodymium | Nd | 1,010 | 3,127 | 7.01 | 1885 | 101 | [Xe] 4f4 6s2 | 5.53 | ||
61 | * | 145.000 | Promethium | Pm | 1,100 | 3,000 | 7.30 | 1945 | 101 | [Xe] 4f5 6s2 | 5.58 | |
62 | 150.360 | Samarium | Sm | 1,072 | 1,900 | 7.52 | 1879 | 101 | [Xe] 4f6 6s2 | 5.64 | ||
63 | 151.964 | Europium | Eu | 822 | 1,597 | 5.24 | 1901 | 101 | [Xe] 4f7 6s2 | 5.67 | ||
64 | 157.250 | Gadolinium | Gd | 1,311 | 3,233 | 7.90 | 1880 | 101 | [Xe] 4f7 5d1 6s2 | 6.15 | ||
65 | 158.925 | Terbium | Tb | 1,360 | 3,041 | 8.23 | 1843 | 101 | [Xe] 4f9 6s2 | 5.86 | ||
66 | 162.500 | Dysprosium | Dy | 1,412 | 2,562 | 8.55 | 1886 | 101 | [Xe] 4f10 6s2 | 5.94 | ||
67 | 164.930 | Holmium | Ho | 1,470 | 2,720 | 8.80 | 1867 | 101 | [Xe] 4f11 6s2 | 6.02 | ||
68 | 167.259 | Erbium | Er | 1,522 | 2,510 | 9.07 | 1842 | 101 | [Xe] 4f12 6s2 | 6.11 | ||
69 | 168.934 | Thulium | Tm | 1,545 | 1,727 | 9.32 | 1879 | 101 | [Xe] 4f13 6s2 | 6.18 | ||
70 | 173.040 | Ytterbium | Yb | 824 | 1,466 | 6.90 | 1878 | 101 | [Xe] 4f14 6s2 | 6.25 | ||
71 | 174.967 | Lutetium | Lu | 1,656 | 3,315 | 9.84 | 1907 | 101 | [Xe] 4f14 5d1 6s2 | 5.43 | ||
72 | 178.490 | Hafnium | Hf | 2,150 | 5,400 | 13.31 | 1923 | 4 | [Xe] 4f14 5d2 6s2 | 6.83 | ||
73 | 180.948 | Tantalum | Ta | 2,996 | 5,425 | 16.65 | 1802 | 5 | [Xe] 4f14 5d3 6s2 | 7.55 | ||
74 | 183.840 | Tungsten | W | 3,410 | 5,660 | 19.35 | 1783 | 6 | [Xe] 4f14 5d4 6s2 | 7.86 | ||
75 | 186.207 | Rhenium | Re | 3,180 | 5,627 | 21.04 | 1925 | 7 | [Xe] 4f14 5d5 6s2 | 7.83 | ||
76 | 190.230 | Osmium | Os | 3,045 | 5,027 | 22.60 | 1803 | 8 | [Xe] 4f14 5d6 6s2 | 8.44 | ||
77 | 192.217 | Iridium | Ir | 2,410 | 4,527 | 22.40 | 1803 | 9 | [Xe] 4f14 5d7 6s2 | 8.97 | ||
78 | 195.078 | Platinum | Pt | 1,772 | 3,827 | 21.45 | 1735 | 10 | [Xe] 4f14 5d9 6s1 | 8.96 | ||
79 | 196.967 | Gold | Au | 1,064 | 2,807 | 19.32 | ancient | 11 | [Xe] 4f14 5d10 6s1 | 9.23 | ||
80 | 200.590 | Mercury | Hg | -39 | 357 | 13.55 | ancient | 12 | [Xe] 4f14 5d10 6s2 | 10.44 | ||
81 | 204.383 | Thallium | Tl | 303 | 1,457 | 11.85 | 1861 | 13 | [Xe] 4f14 5d10 6s2 6p1 | 6.11 | ||
82 | 207.200 | Lead | Pb | 327 | 1,740 | 11.35 | ancient | 14 | [Xe] 4f14 5d10 6s2 6p2 | 7.42 | ||
83 | 208.980 | Bismuth | Bi | 271 | 1,560 | 9.75 | ancient | 15 | [Xe] 4f14 5d10 6s2 6p3 | 7.29 | ||
84 | * | 209.000 | Polonium | Po | 254 | 962 | 9.30 | 1898 | 16 | [Xe] 4f14 5d10 6s2 6p4 | 8.42 | |
85 | * | 210.000 | Astatine | At | 302 | 337 | 0.00 | 1940 | 17 | [Xe] 4f14 5d10 6s2 6p5 | 9.30 | |
86 | * | 222.000 | Radon | Rn | -71 | -62 | 9.73 | 1900 | 18 | [Xe] 4f14 5d10 6s2 6p6 | 10.75 | |
87 | * | 223.000 | Francium | Fr | 27 | 677 | 0.00 | 1939 | 1 | [Rn] 7s1 | 4.07 | |
88 | * | 226.000 | Radium | Ra | 700 | 1,737 | 5.50 | 1898 | 2 | [Rn] 7s2 | 5.28 | |
89 | * | 227.000 | Actinium | Ac | 1,050 | 3,200 | 10.07 | 1899 | 3 | [Rn] 6d1 7s2 | 5.17 | |
90 | 232.038 | Thorium | Th | 1,750 | 4,790 | 11.72 | 1829 | 102 | [Rn] 6d2 7s2 | 6.31 | ||
91 | 231.036 | Protactinium | Pa | 1,568 | 0 | 15.40 | 1913 | 102 | [Rn] 5f2 6d1 7s2 | 5.89 | ||
92 | 238.029 | Uranium | U | 1,132 | 3,818 | 18.95 | 1789 | 102 | [Rn] 5f3 6d1 7s2 | 6.19 | ||
93 | * | 237.000 | Neptunium | Np | 640 | 3,902 | 20.20 | 1940 | 102 | [Rn] 5f4 6d1 7s2 | 6.27 | |
94 | * | 244.000 | Plutonium | Pu | 640 | 3,235 | 19.84 | 1940 | 102 | [Rn] 5f6 7s2 | 6.03 | |
95 | * | 243.000 | Americium | Am | 994 | 2,607 | 13.67 | 1944 | 102 | [Rn] 5f7 7s2 | 5.97 | |
96 | * | 247.000 | Curium | Cm | 1,340 | 0 | 13.50 | 1944 | 102 | 5.99 | ||
97 | * | 247.000 | Berkelium | Bk | 986 | 0 | 14.78 | 1949 | 102 | 6.20 | ||
98 | * | 251.000 | Californium | Cf | 900 | 0 | 15.10 | 1950 | 102 | 6.28 | ||
99 | * | 252.000 | Einsteinium | Es | 860 | 0 | 0.00 | 1952 | 102 | 6.42 | ||
100 | * | 257.000 | Fermium | Fm | 1,527 | 0 | 0.00 | 1952 | 102 | 6.50 | ||
101 | * | 258.000 | Mendelevium | Md | 0 | 0 | 0.00 | 1955 | 102 | 6.58 | ||
102 | * | 259.000 | Nobelium | No | 827 | 0 | 0.00 | 1958 | 102 | 6.65 | ||
103 | * | 262.000 | Lawrencium | Lr | 1,627 | 0 | 0.00 | 1961 | 102 | 4.90 | ||
104 | * | 261.000 | Rutherfordium | Rf | 0 | 0 | 0.00 | 1964 | 4 | 0.00 | ||
105 | * | 262.000 | Dubnium | Db | 0 | 0 | 0.00 | 1967 | 5 | 0.00 | ||
106 | * | 266.000 | Seaborgium | Sg | 0 | 0 | 0.00 | 1974 | 6 | 0.00 | ||
107 | * | 264.000 | Bohrium | Bh | 0 | 0 | 0.00 | 1981 | 7 | 0.00 | ||
108 | * | 277.000 | Hassium | Hs | 0 | 0 | 0.00 | 1984 | 8 | 0.00 | ||
109 | * | 268.000 | Meitnerium | Mt | 0 | 0 | 0.00 | 1982 | 9 | 0.00 | ||
No. | Atomic weight | Name | Sym. | M.P. (°C) | B.P. (°C) | Density* (g/cm3) | Earth crust (%)* | Discovery (Year) | Group* | Electron configuration | Ionization energy (eV) |
Notes:
• Density of elements with boiling points below 0°C is given in g/l. In a sorted list, these elements are shown before other elements that have boiling points >0°C.
• Earth crust composition average values are from a report by F. W. Clarke and H. S. Washington, 1924. Elemental composition of crustal rocks differ between different localities (see article).
• Group: There are only 18 groups in the periodic table that constitute the columns of the table. Lanthanoids and Actinoids are numbered as 101 and 102 to separate them in sorting by group.
• The elements marked with an asterisk (in the 2nd column) have no stable nuclides. For these elements the weight value shown represents the mass number of the longest-lived isotope of the element.
Abbreviations and Definitions:
No. - Atomic Number; M.P. - melting point; B.P. - boiling point
Atomic number: The number of protons in an atom. Each element is uniquely defined by its atomic number.
Atomic mass: The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus. Atomic mass is measured in Atomic Mass Units (amu) which are scaled relative to carbon, 12C, that is taken as a standard element with an atomic mass of 12. This isotope of carbon has 6 protons and 6 neutrons. Thus, each proton and neutron has a mass of about 1 amu.
Isotope: Atoms of the same element with the same atomic number, but different number of neutrons. Isotope of an element is defined by the sum of the number of protons and neutrons in its nucleus. Elements have more than one isotope with varying numbers of neutrons. For example, there are two common isotopes of carbon, 12C and 13C which have 6 and 7 neutrons respectively. The abundances of different isotopes of elements vary in nature depending on the source of materials. For relative abundances of isotopes in nature see reference on Atomic Weights and Isotopic Compositions.
Atomic weight: Atomic weight values represent weighted average of the masses of all naturally occurring isotopes of an element. The values shown here are based on the IUPAC Commission determinations (Pure Appl. Chem. 73:667-683, 2001). The elements marked with an asterisk have no stable nuclides. For these elements the weight value shown represents the mass number of the longest-lived isotope of the element.
Electron configuration: See next page for explanation of electron configuration of atoms.
Ionization energy (IE): The energy required to remove the outermost electron from an atom or a positive ion in its ground level. The table lists only the first IE in eV units. To convert to kJ/mol multiply by 96.4869. Reference: NIST Reference Table on Ground states and ionization energies for the neutral atoms. IE decreases going down a column of the periodic table, and increases from left to right in a row. Thus, alkali metals have the lowest IE in a period and Rare gases have the highest.
Other resources related to the Periodic Table
- Chemical Evolution of the Universe