The rather steady increase of atomic weights through the periodic table was explained when physicists managed to split atoms into three component particles.
The exploration of atomic structure began in 1911, when Ernest Rutherford, a New Zealander who worked in Canada and England, discovered that atoms had a dense central
nucleus that contained positively charged particles, which he named
protons. (See Table
1 .) It was soon established that each chemical element was characterized by a specific number of protons in each atom. A hydrogen atom has 1 proton, helium has 2, lithium has 3, and so forth through the periodic table. The
atomic number is the number of protons for each element.
|
Subatomic Particle
|
Mass Units
|
Electric Charge
|
Atomic Location
|
|
Proton
|
1.0073
|
+1
|
Nucleus
|
|
Neutron
|
1.0087
|
0
|
Nucleus
|
|
Electron
|
0.0005
|
−1
|
Orbital
|
|
Except for the simplest hydrogen atom with a single proton as its entire nucleus, all atoms contain
neutrons (particles that are electrically neutral) in addition to protons. For most of the light elements, the numbers of protons and neutrons in the nucleus are nearly equal. Table
2 shows the most common nucleus for each element with the atomic weight rounded to the nearest integer. You can see that the rounded-off atomic weights are the sum of the protons and neutrons for each atom. The sum of the protons and neutrons is the
mass number of an atom.
|
TABLE 2
|
Nuclear Structure of the Lightest Elements
|
|
Element
|
Atomic Number
|
Protons
|
Neutrons
|
Atomic Weight
|
|
Hydrogen
|
1
|
1
|
0
|
1
|
|
Helium
|
2
|
2
|
2
|
4
|
|
Lithium
|
3
|
3
|
4
|
7
|
|
Beryllium
|
4
|
4
|
5
|
9
|
|
Boron
|
5
|
5
|
6
|
11
|
|
Carbon
|
6
|
6
|
6
|
12
|
|
Nitrogen
|
7
|
7
|
7
|
14
|
|
Oxygen
|
8
|
8
|
8
|
16
|
|
John Dalton's idea that atomic weights were multiples of hydrogen weight was premature, but near the truth. The series of elements of increasing atomic weights is generated by adding
nucleons, the two types of particles comprising the nucleus.
Elements
Atomic Structure
