is a neutron?
Neutrons are the other principal component of the nucleus of an atom (along with protons). The neutron is neutral in charge and has a mass roughly similar to that of a proton. Atoms of the same element that contain different numbers of neutrons will generally still have the same behavior as one another in terms of chemical reactivity properties. Both protons and neutrons are, in fact, made up of even smaller particles, but chemistry doesn’t usually deal with these even smaller bits.
What were some early models for the atom?
Experiments suggested that atoms were actually made up of smaller particles, which motivated the development of new models involving protons, neutrons, and electrons. One was Thomson’s Plum Pudding Model, which described the atom as a positively charged “pudding” filled with negatively charged electrons. Rutherford later proposed the idea of a positively charged nucleus, but couldn’t explain why electrons didn’t just fall into it. A Danish physicist named Niels Bohr proposed the idea that electrons travel around the nucleus in specific orbits and advanced the atomic theory to a point very close to where it is today.
How did scientists determine that atoms consist of electrons, neutrons, and protons?
Originally atoms were thought to be the smallest unit of matter, but in the late nineteenth century experiments allowed scientists to finally probe inside atoms. Some of these first experiments were carried out by the British physicist J. J. Thomson, who discovered the electron. He noticed that the rays (actually rays of electrons, though he didn’t know it at the time) were deflected by electrically charged plates and concluded that these rays must consist of charged particles that were much smaller than atoms themselves.
Thomson’s first graduate student, Ernest Rutherford, continued to investigate the nature of the atom. In the early twentieth century, Rutherford carried out a now-famous experiment in which radioactive particles were shot through extremely thin gold foil. While some bounced off of the nuclei in different directions, most of the particles actually passed through the foil undeflected. Rutherford interpreted this as an indication that the atoms making up the foil must consist of mostly empty space. Over his career, he developed the picture of the atom as a positively charged center surrounded by electrons, and he also proposed that there must be neutral particles (neutrons) to explain the different isotopes of a given element.
What is the current model for the atom?
The current model for the atom consists of negatively charged electrons orbiting a positively charged nucleus. The nucleus consists of neutrons and protons that are very tightly bound to each other by a strong force. Orbiting electrons behave something like a cloud surrounding the nucleus, and we can’t be sure quite where they are at any given time. The electrons are also very lightweight compared to the nucleus, and they move much, much faster.
What fraction of atoms are empty space?
The fraction of an atom that is occupied by empty space is very large. In fact, over 99.9% of atoms are empty space! The protons, neutrons, and electrons are incredibly small, and the atom occupies such a relatively large effective volume because of the delocalized electron cloud around the nucleus.
What is an atomic mass unit?
One atomic mass unit has a mass of 1.66 × 10−27 kg, which is about the mass of a single proton or neutron. These units are convenient, since when the mass of atoms is expressed in atomic mass units, their masses come out to values that are very close to integers. The values of mass given on the periodic table are expressed in atomic mass units.
What is an isotope?
Isotopes are atoms with the same number of protons and electrons, but with different numbers of neutrons. Since the numbers of protons and electrons effectively determine the reactivity of an atom, isotopes have the same basic chemical properties and are the same element. They have different masses, though, since they have different numbers of neutrons.
The various isotopes of an element are usually present in relatively fixed ratios throughout nature, but in some cases the ratio can depend on the environment or molecule in which they are found. For example, the element carbon most commonly exists with 6 protons, 6 neutrons, and 6 electrons (referred to as Carbon–12, the total number of particles in the nucleus). A small fraction, however, have 6 protons, 7 neutrons, and 6 electrons (Carbon–13). Roughly 99% of carbon atoms have 6 neutrons, but most of the remaining 1% have 7 neutrons.
Below is a table of the breakdown of the isotopic abundance of several common elements:
TRENDS IN REACTIVITY AND THE PERIODIC TABLE
What was the law of triads?
A scientist named Johann Döbereiner discovered trends in the reactivity of groups of elements. Certain sets of three elements, such as lithium, sodium, and potassium, showed similar chemical properties, and Döbereiner noticed that the average of the atomic masses of the atoms of the heaviest and lightest elements in the triad gave the atomic mass of the midweight element. For example, the mean of the atomic masses of lithium and potassium is (3 + 19)/2 = 11, which is the atomic mass of sodium. Due to differing numbers of neutrons in each element and the existence of different isotopes, this law isn’t always strictly true, but it does tend to work well, especially for lighter elements. For reasons we hope to explain in the coming questions, this trend plays an important role in the structure of the periodic table.
What was the law of octaves?
The law of octaves was put forth by the British chemist John Newlands. He noticed that, when the elements were listed in order of increasing atomic weight, elements with similar properties occurred every eight elements. The trend was named “the law of octaves” by analogy to musical scales, and it was the first realization of the relationship between atomic masses and a repeating pattern of elemental properties. This periodicity has since been explained in detail as chemists have gained a better understanding of atomic structure, and the law of octaves played a crucial role in the development of the periodic table chemists use today.
How was the modern periodic table developed?
A French geologist named Alexandre Béguyer de Chancourtois is actually the first on record to list all of the elements in order of increasing atomic mass. His first version contained sixty-two elements and they were placed in columns that wrapped around a cylinder; however, there were a variety of issues with this first attempt that were later improved upon. Newlands made the next significant advance, publishing the elements in columns of those with similar properties, which brought the description close to the version chemists use today.
The modern version of the periodic table was proposed by a Russian scientist named Dmitri Mendeleev in 1869. His table was the first to lay out the elements in order of increasing atomic mass in columns of elements with similar reactivity. Elements on the table appeared periodically, essentially in accordance with the law of octaves, hence the name “the periodic table.” Mendeleev’s table had to include some blank spaces so that the elements were each in the proper column according to reactivity. As more elements were discovered, the blank spaces in the table were eventually filled in, validating Mendeleev’s table.
Which elements are most abundant?
