Atoms and the Periodic Table CH2 by Owen Borville October 7, 2025
The atom is the smallest quantity of matter that still retains the properties of matter. An element is a substance that cannot be broken down into two or more simpler substances. (elements of the Periodic Table such as copper, silver, gold, hydrogen, and helium).
John Dalton, an 18th to 19th century English scientist, said that atoms that make up all matter are tiny, indivisible particles. Dividing an atom further produces subatomic particles. The nature, number, and arrangement of subatomic particles determine the properties of atoms, which in turn determine the properties of all material things.
Scientists in the late 1800s were researching radiation, the emission and transmission of energy in the form of waves. A cathode ray tube was commonly used that had two metal plates sealed inside a glass tube from which most of the air had been removed.
When metal plates are connected to a high-voltage source, the negatively charged plate or cathode emits an invisible ray. The cathode ray is drawn to the anode where it passes through a small hole. Although invisible, the path is revealed when the ray strikes a phosphor coated surface producing a bright light. A phosphor is a substance that has luminescence.
Through research, scientists discovered that like charges repel each other and opposite charges attract one another. British scientist J.J. Thompson (1856-1940) noted the rays were repelled by a plate bearing a negative charge, and attracted to a plate bearing a positive charge.
J.J. Thompson proposed that the rays were actually a stream of negatively charged particles (called electrons). By changing the electric field and measuring the degree of deflection of cathode rays, Thompson determined the charge to mass ration of electrons to be 1.76 x 10^8 C/g. (C is a coulomb, the derived SI unit of electric charge).
American scientist R.A. Millikan (1868-1953) determined the charge on an electron by examining the motion of tiny oil drops. The charge was determined to be -1.6022 x 10^-19 C.
By knowing the charge, he was then able to use Thompson's charge-to-mass ratio to determine the mass of an electron.
Mass of an electron = charge/(charge/mass) = (-1.6022 x 10^-19 C)/(-1.76 x 10^8 C/g) = 9.10 x 10^-28g
German scientist Wilhelm Rontgen (1845-1923) discovered x-rays as they were not deflected by magnetic or electric fields, so they could not consist of charged particles.
French scientist Antoine Becquerel (1852-1908) discovered radioactivity, which is the spontaneous emission of radiation. Radioactive substances, such as uranium, can produce three types of radiation.
Alpha (α) rays contain positively charged particles, called α particles. Beta rays or β particles, are electrons so they are deflected away from the negatively charged plate.
Gamma (γ) rays have no charge and are unaffected by external electric or magnetic field.
New Zealand scientist Earnest Rutherford (1871-1937) used alpha (α) particles to prove the structure of atoms. During research, the majority of particles penetrated the gold foil undeflected. Sometimes, (α) particles were deflected at a large angle. Sometimes, (α) particles bounced back in the direction from which they had come.
Rutherford proposed a new model for the atom, a nuclear model, where positive charge is concentrated in the nucleus. The nucleus accounts for most of an atom's mass and is an extremely dense central core within the atom.
A typical atomic radius is about 100 pm. A typical nucleus has a radius of about 5 x 10^-3 pm. 1 pm = 1 x 10^-12 m.
Protons are positively charged particles found in the nucleus. Neutrons are electronically neutral particles found in the nucleus. Neutrons are slightly larger than protons. Electrons are negatively charged particles distributed around the nucleus.
Electron Mass = 9.10 x 10^-28 g AMU Mass = 5.48 x 10^-4 Charge = -1.6 x 10^-19 Charge Unit = -1
Proton Mass = 1.67 x 10^-24 g AMU Mass = 1.0073 Charge = +1.6 x 10^-19 Charge Unit = +1
Neutron Mass = 1.67 x 10^-24 g AMU Mass = 1.0086 Charge = 0 Charge Unit = 0
All atoms can be identified by the number of protons and neutrons they contain. The atomic number (Z) is the number of protons in the nucleus. Atoms are neutral, so Z is also the number of electrons. Protons determine the identity of the element. Ex. the atomic number of boron is 5, so every boron atom has 5 protons.
The mass number (A) is the total number of protons and neutrons. Protons and neutrons together are called nucleons. The element symbol is the large letter on the Periodic table for each element.
Most elements have two or more isotopes, which are atoms of the same element that have the same atomic number (Z) but different mass numbers (A). Isotopes of the same element usually have similar chemical properties.
Ex. All hydrogen atoms have one proton, but some isotopes of hydrogen atoms have zero neutrons (called protium) , some hydrogen atoms have one neutron (called deuterium), and some hydrogen atoms have two neutrons (called tritium).
When writing the symbols for elements including their atomic number, the elements symbol is written in large letters, the atomic number is show at the lower left subscript of the symbol, and the mass is shown at the upper left superscript of the symbol. Atoms are neutral, so the atomic number equals the number of protons and number of electrons.
The nucleus is a small portion of the total volume of an atom. However, the nucleus contains most of the atom's mass. The stability of the atomic nucleus can be related to density. The highest known element density is 22.6 g/cm^3 (Osmium). The average atomic nucleus is about 9 trillion times as dense as the densest element.
The principle factor for nuclear stability is neutron to proton ratio (n/p). There are more stable nuclei with 2, 8, 20, 50, 82, 126 protons or neutrons (so called magic numbers). There are more stable nuclei with even numbers of protons or neutrons. All elements with atomic numbers greater than 83 are radioactive (and unstable). All isotopes of elements Tc and Pm are radioactive. There are only four stable isotopes with an odd number of protons and neutrons. If protons only are an even number, there are 53 stable isotopes. If neutrons only are an even number, there are 50 stable isotopes. If both protons and neutrons are an even number, there are 164 stable isotopes.
On a "belt of stability" graph, the number of neutrons are plotted versus the number of protons in various isotopes. Stable nuclei are located in a lenticular area of the graph known as the belt of stability. The most radioactive nuclei are outside the belt. Above the belt of stability, the nuclei have higher neutron-to proton ratio.
Atomic mass is the mass of an atom in atomic mass units (amu). One (1) amu = 1/12 the mass of a carbon 12 atom. The average atomic mass on the periodic table represents the average mass of all the naturally occurring mixture of isotopes of an element. This is why atomic mass often has decimal numbers showing the average mass number. The isotopic mass or mass of each isotope is multiplied by its percent natural abundance to find the average mass. Then each isotope's contribution is added together to find the average atomic mass.
Measuring atomic mass: The most direct and most accurate method for determining atomic and molecular masses is mass spectrometry, using a mass spectrometer.
The periodic table is a chart in which elements having similar chemical and physical properties are grouped together.
The atom is the smallest quantity of matter that still retains the properties of matter. An element is a substance that cannot be broken down into two or more simpler substances. (elements of the Periodic Table such as copper, silver, gold, hydrogen, and helium).
John Dalton, an 18th to 19th century English scientist, said that atoms that make up all matter are tiny, indivisible particles. Dividing an atom further produces subatomic particles. The nature, number, and arrangement of subatomic particles determine the properties of atoms, which in turn determine the properties of all material things.
Scientists in the late 1800s were researching radiation, the emission and transmission of energy in the form of waves. A cathode ray tube was commonly used that had two metal plates sealed inside a glass tube from which most of the air had been removed.
When metal plates are connected to a high-voltage source, the negatively charged plate or cathode emits an invisible ray. The cathode ray is drawn to the anode where it passes through a small hole. Although invisible, the path is revealed when the ray strikes a phosphor coated surface producing a bright light. A phosphor is a substance that has luminescence.
Through research, scientists discovered that like charges repel each other and opposite charges attract one another. British scientist J.J. Thompson (1856-1940) noted the rays were repelled by a plate bearing a negative charge, and attracted to a plate bearing a positive charge.
J.J. Thompson proposed that the rays were actually a stream of negatively charged particles (called electrons). By changing the electric field and measuring the degree of deflection of cathode rays, Thompson determined the charge to mass ration of electrons to be 1.76 x 10^8 C/g. (C is a coulomb, the derived SI unit of electric charge).
American scientist R.A. Millikan (1868-1953) determined the charge on an electron by examining the motion of tiny oil drops. The charge was determined to be -1.6022 x 10^-19 C.
By knowing the charge, he was then able to use Thompson's charge-to-mass ratio to determine the mass of an electron.
Mass of an electron = charge/(charge/mass) = (-1.6022 x 10^-19 C)/(-1.76 x 10^8 C/g) = 9.10 x 10^-28g
German scientist Wilhelm Rontgen (1845-1923) discovered x-rays as they were not deflected by magnetic or electric fields, so they could not consist of charged particles.
French scientist Antoine Becquerel (1852-1908) discovered radioactivity, which is the spontaneous emission of radiation. Radioactive substances, such as uranium, can produce three types of radiation.
Alpha (α) rays contain positively charged particles, called α particles. Beta rays or β particles, are electrons so they are deflected away from the negatively charged plate.
Gamma (γ) rays have no charge and are unaffected by external electric or magnetic field.
New Zealand scientist Earnest Rutherford (1871-1937) used alpha (α) particles to prove the structure of atoms. During research, the majority of particles penetrated the gold foil undeflected. Sometimes, (α) particles were deflected at a large angle. Sometimes, (α) particles bounced back in the direction from which they had come.
Rutherford proposed a new model for the atom, a nuclear model, where positive charge is concentrated in the nucleus. The nucleus accounts for most of an atom's mass and is an extremely dense central core within the atom.
A typical atomic radius is about 100 pm. A typical nucleus has a radius of about 5 x 10^-3 pm. 1 pm = 1 x 10^-12 m.
Protons are positively charged particles found in the nucleus. Neutrons are electronically neutral particles found in the nucleus. Neutrons are slightly larger than protons. Electrons are negatively charged particles distributed around the nucleus.
Electron Mass = 9.10 x 10^-28 g AMU Mass = 5.48 x 10^-4 Charge = -1.6 x 10^-19 Charge Unit = -1
Proton Mass = 1.67 x 10^-24 g AMU Mass = 1.0073 Charge = +1.6 x 10^-19 Charge Unit = +1
Neutron Mass = 1.67 x 10^-24 g AMU Mass = 1.0086 Charge = 0 Charge Unit = 0
All atoms can be identified by the number of protons and neutrons they contain. The atomic number (Z) is the number of protons in the nucleus. Atoms are neutral, so Z is also the number of electrons. Protons determine the identity of the element. Ex. the atomic number of boron is 5, so every boron atom has 5 protons.
The mass number (A) is the total number of protons and neutrons. Protons and neutrons together are called nucleons. The element symbol is the large letter on the Periodic table for each element.
Most elements have two or more isotopes, which are atoms of the same element that have the same atomic number (Z) but different mass numbers (A). Isotopes of the same element usually have similar chemical properties.
Ex. All hydrogen atoms have one proton, but some isotopes of hydrogen atoms have zero neutrons (called protium) , some hydrogen atoms have one neutron (called deuterium), and some hydrogen atoms have two neutrons (called tritium).
When writing the symbols for elements including their atomic number, the elements symbol is written in large letters, the atomic number is show at the lower left subscript of the symbol, and the mass is shown at the upper left superscript of the symbol. Atoms are neutral, so the atomic number equals the number of protons and number of electrons.
The nucleus is a small portion of the total volume of an atom. However, the nucleus contains most of the atom's mass. The stability of the atomic nucleus can be related to density. The highest known element density is 22.6 g/cm^3 (Osmium). The average atomic nucleus is about 9 trillion times as dense as the densest element.
The principle factor for nuclear stability is neutron to proton ratio (n/p). There are more stable nuclei with 2, 8, 20, 50, 82, 126 protons or neutrons (so called magic numbers). There are more stable nuclei with even numbers of protons or neutrons. All elements with atomic numbers greater than 83 are radioactive (and unstable). All isotopes of elements Tc and Pm are radioactive. There are only four stable isotopes with an odd number of protons and neutrons. If protons only are an even number, there are 53 stable isotopes. If neutrons only are an even number, there are 50 stable isotopes. If both protons and neutrons are an even number, there are 164 stable isotopes.
On a "belt of stability" graph, the number of neutrons are plotted versus the number of protons in various isotopes. Stable nuclei are located in a lenticular area of the graph known as the belt of stability. The most radioactive nuclei are outside the belt. Above the belt of stability, the nuclei have higher neutron-to proton ratio.
Atomic mass is the mass of an atom in atomic mass units (amu). One (1) amu = 1/12 the mass of a carbon 12 atom. The average atomic mass on the periodic table represents the average mass of all the naturally occurring mixture of isotopes of an element. This is why atomic mass often has decimal numbers showing the average mass number. The isotopic mass or mass of each isotope is multiplied by its percent natural abundance to find the average mass. Then each isotope's contribution is added together to find the average atomic mass.
Measuring atomic mass: The most direct and most accurate method for determining atomic and molecular masses is mass spectrometry, using a mass spectrometer.
The periodic table is a chart in which elements having similar chemical and physical properties are grouped together.
Elements are arranged in periods, which are horizontal rows numbered from one to seven top to bottom and in order of increasing atomic number. Elements can be categorized as metals, non-metals, or metalloids. Metals are good conductors of heat and electricity. Non-metals are poor conductors of heat or electricity. Metalloids have intermediate properties between metals and non-metals.
A vertical column is known as a group, numbered left to right.
Group 1A elements (Li, Na, K, Rb, Cs, and Fr) on the left side of the table are called alkali metals.
Group 2A elements (Be, Mg, Ca, Sr, Ba, and Ra) are called alkaline earth metals.
Group 6A elements (O, S, Se, Te, Po) are called chalcogens.
Group 7A elements (F, Cl, Br, I, At) are called halogens.
Group 8A elements (He, Ne, Ar, Kr, Xe, Rn) are called the noble gases.
Groups 1B and 3B-8B (3-12) in the center of the table are called the transition elements or transition metals.
The mole is defined as the amount of substance that contains as many elementary entities as there are atoms in exactly 12 grams of carbon 12. This experimentally determined number is called Avogadro's number Nₐ or NA, named after the Italian scientist Amedeo Avogadro (1776-1856).
Nₐ = 6.0221415 x 10^23, which is commonly rounded to 6.022 x 10^23
1 mole = 6.022 x 10^23 atoms. This is used a conversion factor in calculating the number of atoms from moles and vise versa.
Conversion factor = (6.022 x 10^23 atoms of substance)/ (1 mol substance) or (1 mol substance)/(6.022 x 10^23 atoms of substance)
The Molar Mass of a substance is the mass in grams of one mole of the substance. The mass of a mole of carbon-12 is exactly 12g. The mass of one carbon-12 atom is 12 amu. The mass of one mole of carbon-12 is exactly 12 grams.
Molar mass is usually expressed in units of grams per mole (g/mol) to help in cancellation of units in calculations of conversion units.
Molar mass is the conversion factor from mass to moles, and vise versa. Avogadro's constant converts from moles to atoms.
List of Elements
1 Hydrogen, H
2 Helium, He
3 Lithium, Li
4 Beryllium, Be
5 Boron, B
6 Carbon, C
7 Nitrogen, N
8 Oxygen, O
9 Fluorine, F
10 Neon, Ne
11 Sodium, Na
12 Magnesium, Mg
13 Aluminum, Al
14 Silicon, Si
15 Phosphorus, P
16 Sulfur, S
17 Chlorine, Cl
18 Argon, Ar
19 Potassium, K
20 Calcium, Ca
21 Scandium, Sc
22 Titanium, Ti
23 Vanadium, V
24 Chromium, Cr
25 Manganese, Mn
26 Iron, Fe
27 Cobalt, Co
28 Nickel, Ni
29 Copper, Cu
30 Zinc, Zn
31 Gallium, Ga
32 Germanium, Ge
33 Arsenic, As
34 Selenium, Se
35 Bromine, Br
36 Krypton, Kr
37 Rubidium, Rb
38 Strontium, Sr
39 Yttrium, Y
40 Zirconium, Zr
41 Niobium, Nb
42 Molybdenum, Mo
43 Technetium, Tc
44 Ruthenium, Ru
45 Rhodium, Rh
46 Palladium, Pd
47 Silver, Ag
48 Cadmium, Cd
49 Indium, In
50 Tin, Sn
51 Antimony, Sb
52 Tellurium, Te
53 Iodine, I
54 Xenon, Xe
55 Cesium, Cs
56 Barium, Ba
72 Hafnium, Hf
73 Tantalum, Ta
74 Tungsten, W
75 Rhenium, Re
76 Osmium, Os
77 Iridium, Ir
78 Platinum, Pt
79 Gold, Au
80 Mercury, Hg
81 Thallium, Tl
82 Lead, Pb
83 Bismuth, Bi
84 Polonium, Po
85 Astatine, At
86 Radon, Rn
87 Francium, Fr
88 Radium, Ra
89 Actinium, Ac
90 Thorium, Th
91 Protactinium, Pa
92 Uranium, U
A vertical column is known as a group, numbered left to right.
Group 1A elements (Li, Na, K, Rb, Cs, and Fr) on the left side of the table are called alkali metals.
Group 2A elements (Be, Mg, Ca, Sr, Ba, and Ra) are called alkaline earth metals.
Group 6A elements (O, S, Se, Te, Po) are called chalcogens.
Group 7A elements (F, Cl, Br, I, At) are called halogens.
Group 8A elements (He, Ne, Ar, Kr, Xe, Rn) are called the noble gases.
Groups 1B and 3B-8B (3-12) in the center of the table are called the transition elements or transition metals.
The mole is defined as the amount of substance that contains as many elementary entities as there are atoms in exactly 12 grams of carbon 12. This experimentally determined number is called Avogadro's number Nₐ or NA, named after the Italian scientist Amedeo Avogadro (1776-1856).
Nₐ = 6.0221415 x 10^23, which is commonly rounded to 6.022 x 10^23
1 mole = 6.022 x 10^23 atoms. This is used a conversion factor in calculating the number of atoms from moles and vise versa.
Conversion factor = (6.022 x 10^23 atoms of substance)/ (1 mol substance) or (1 mol substance)/(6.022 x 10^23 atoms of substance)
The Molar Mass of a substance is the mass in grams of one mole of the substance. The mass of a mole of carbon-12 is exactly 12g. The mass of one carbon-12 atom is 12 amu. The mass of one mole of carbon-12 is exactly 12 grams.
Molar mass is usually expressed in units of grams per mole (g/mol) to help in cancellation of units in calculations of conversion units.
Molar mass is the conversion factor from mass to moles, and vise versa. Avogadro's constant converts from moles to atoms.
List of Elements
1 Hydrogen, H
2 Helium, He
3 Lithium, Li
4 Beryllium, Be
5 Boron, B
6 Carbon, C
7 Nitrogen, N
8 Oxygen, O
9 Fluorine, F
10 Neon, Ne
11 Sodium, Na
12 Magnesium, Mg
13 Aluminum, Al
14 Silicon, Si
15 Phosphorus, P
16 Sulfur, S
17 Chlorine, Cl
18 Argon, Ar
19 Potassium, K
20 Calcium, Ca
21 Scandium, Sc
22 Titanium, Ti
23 Vanadium, V
24 Chromium, Cr
25 Manganese, Mn
26 Iron, Fe
27 Cobalt, Co
28 Nickel, Ni
29 Copper, Cu
30 Zinc, Zn
31 Gallium, Ga
32 Germanium, Ge
33 Arsenic, As
34 Selenium, Se
35 Bromine, Br
36 Krypton, Kr
37 Rubidium, Rb
38 Strontium, Sr
39 Yttrium, Y
40 Zirconium, Zr
41 Niobium, Nb
42 Molybdenum, Mo
43 Technetium, Tc
44 Ruthenium, Ru
45 Rhodium, Rh
46 Palladium, Pd
47 Silver, Ag
48 Cadmium, Cd
49 Indium, In
50 Tin, Sn
51 Antimony, Sb
52 Tellurium, Te
53 Iodine, I
54 Xenon, Xe
55 Cesium, Cs
56 Barium, Ba
72 Hafnium, Hf
73 Tantalum, Ta
74 Tungsten, W
75 Rhenium, Re
76 Osmium, Os
77 Iridium, Ir
78 Platinum, Pt
79 Gold, Au
80 Mercury, Hg
81 Thallium, Tl
82 Lead, Pb
83 Bismuth, Bi
84 Polonium, Po
85 Astatine, At
86 Radon, Rn
87 Francium, Fr
88 Radium, Ra
89 Actinium, Ac
90 Thorium, Th
91 Protactinium, Pa
92 Uranium, U