Atoms and Elements

In the early 1900s, Hans Geiger and Ernest Marsden, working under Ernest Rutherford at the University of Manchester, fired a beam of alpha particles at a very thin metal foil, usually gold. According to Wikipedia's article on the Geiger-Marsden experiments, most of the particles passed straight through, but a small fraction were deflected sharply, some "in all directions, sometimes more than 90°". Rutherford concluded that the atom's positive charge and most of its mass sit in a tiny nucleus, with the electrons spread around it. That picture, a small dense centre and a lot of empty space, is the one this chapter fills in. It explains what an atom is built from, why the number of protons decides which element you have, and what an isotope is.

The Atom and Its Three Particles

Wikipedia's Atom article opens: "Atoms are the basic particles of the chemical elements and the fundamental building blocks of matter." It describes an atom as a nucleus "of protons and generally neutrons", surrounded by electrons. Chapter 1 called an element "a chemical substance made up of a particular kind of atom"; this chapter says what "kind" means.

The Daltons (Da) in the cards are units of mass for single atoms and particles, where one dalton is roughly the mass of one proton or neutron. Two things follow from the numbers. First, the nucleus holds nearly all of an atom's mass, because the electrons are about 1836 times lighter than a proton. Second, protons and neutrons are so close in mass that, for counting purposes, each can be treated as "1".

The Atom article says the total number of protons and neutrons in an atom, which it calls "nucleons", is "the mass number". In an uncharged atom, the Atomic number article says, the number of electrons equals the number of protons, so the positive and negative charges cancel. Atoms that have gained or lost electrons are called ions and are the subject of Chapter 4 (this forward pointer is mine, from the course outline).

Atomic Number: What Defines an Element

The Atom article says: "The chemical elements are distinguished from each other by the number of protons that are in their atoms." That number is the atomic number, symbol Z. The Atomic number article puts it this way: it "can be used to uniquely identify ordinary chemical elements", and it is "the number of protons found in the nucleus of every atom of that element".

So an atom with 6 protons is carbon, whatever else it contains, and an atom with 17 protons is chlorine. Change the number of protons and you have a different element; change the number of electrons or neutrons and you do not.

Isotopes

Because the proton count fixes the element but the neutron count is free to vary, one element can have atoms of several kinds. The Atom article says: "Atoms with the same number of protons but a different number of neutrons are called isotopes of the same element." The Isotope article names an isotope by writing the element name, a hyphen and the mass number, for example carbon-12 and carbon-14, and in symbols as ¹²C and ¹⁴C.

The names and mass numbers come from the Isotope article. The proton counts (6 and 17) are the atomic numbers of carbon and chlorine, which I took from general knowledge and the periodic table rather than from the pages I read (check); the neutron column is my own subtraction, A - Z, done with Python.

The Isotope article says isotopes have the same atomic number but different mass numbers "due to different numbers of neutrons in their nuclei", and that "all isotopes of a given element have virtually the same chemical properties, they have different atomic masses and physical properties". The article identifies carbon-14 as radioactive and mentions radiocarbon dating; it gives no half-life in the part I read, so none is given here.

Relative Atomic Mass: A Weighted Average

A sample of an element usually contains a mixture of isotopes, so the mass quoted for the element is an average. Wikipedia's Relative atomic mass article defines it as "the ratio of the average mass of atoms of a chemical element in a given sample to the atomic mass constant", and says that for a given sample it is the "weighted arithmetic mean" of the masses of the individual atoms, all isotopes included. "Weighted" means each isotope counts in proportion to how common it is.

The Isotope article gives chlorine as an example: a sample "contains 75.8% chlorine-35 and 24.2% chlorine-37, giving an average atomic mass of 35.5 daltons". The Relative atomic mass article has no chlorine example, but it works through silicon, which has three isotopes:

The Relative atomic mass article says the quantity is a ratio, so it has no unit; the Isotope article quotes the chlorine figure in daltons. For school purposes the two are used interchangeably, which I state from general knowledge (check).

Linking Back: Atoms and the States of Matter

Chapter 1 described solids, liquids and gases by how their particles are arranged, and flagged that picture as general knowledge. The particles in question are atoms, or groups of atoms. A change of state, such as ice melting, rearranges them without changing what they are, and that is why it is a physical change. A chemical change, by contrast, rearranges the electrons that hold atoms together (the Chemistry article describes reactions this way) and produces new substances. The Atom article's States section, which might have connected atoms to states of matter directly, fell in the part of the page I did not read (the page was cut off), so this link is the course's own framing (check).

A Short History Note

The Geiger-Marsden experiments, described at the start of the chapter, were run between 1906 and 1913 according to the article. In 1909, Geiger and Marsden reported that some alpha particles scattered by more than 90 degrees, which the earlier "plum pudding" model by Thomson could not explain. Only a compact central charge could cause the rare large deflections. The article says nothing, in the part I read, about the later discovery that the nucleus contains neutrons, so this chapter does not date that step.

Where to Read More

• Encyclopaedias. Wikipedia's Atom, Isotope, Relative atomic mass, Atomic number, Proton, Neutron, Electron and Geiger-Marsden experiments articles are the sources used in this chapter. I read only the first 100,000 characters of Atom (of about 252,000), Isotope (of about 148,000), Proton (of about 151,000), Neutron (of about 275,000), Electron (of about 272,000) and Geiger-Marsden experiments (of about 163,000). I read the Relative atomic mass and Atomic number articles in full.
• A school textbook. The periodic table in any chemistry textbook lists atomic numbers and relative atomic masses for every element.
• Practical. Pick any element from a periodic table and work out its proton, neutron and electron counts for one isotope, using the rules above.

Exercises