Approximately 13.7 billion years ago, the Universe was born with the Big Bang. Starting as a singularity (a single point in space), the Universe undergoes an exponential expansion to form an incredibly dense and high-energy quark-gluon plasma. As the Universe expands, the temperature drops. About one second after the Big Bang, the Universe has cooled to a temperature of 10,000,000,000 K, and quarks begin to bind together to form protons and neutrons. About ten seconds after the Big Bang, antimatter stops being created, electrons and positrons finish annihilating each other, and the Universe is dominated by matter and high-energy photons. About three minutes after the Big Bang, the temperature has cooled to 1,000,000,000 K, and protons and neutrons begin to combine into atomic nuclei (a process known as nucleosynthesis). Seventeen minutes later, the Universe is too cool to continue fusing atomic nuclei together to create heavier elements. At this point, the Universe consists of a plasma of atomic nuclei (mostly hydrogen, helium, and deuterium, with trace amounts of lithium and beryllium) and free electrons. About 380,000 years after the Big Bang, the temperature has cooled to 4000 K, and free electrons are moving slow enough for atomic nuclei to capture them, forming actual hydrogen, helium, and deuterium atoms. About 150 million years after the Big Bang, the first stars and galaxies begin to form. Our sun and solar system form about 9 billion years after the Big Bang (4.6 billion years ago).

Protons, neutrons, and electrons are the subatomic particles that make up atoms. Protons are positively charged particles that have a mass of 1.6726 × 10-27 kg, and neutrons are neutral particles (no electrical charge) that have a mass of 1.6929 × 10-27 kg. Protons and neutrons are approximately the same size… both on the order of 2 × 10-15 m across. Electrons are negatively charged particles that have a mass of 9.11 × 10-31 kg. A proton is about 1836 times as massive as an electron and a neutron is about 1839 times as massive. Electrons are so small that, so far, we have been unable to estimate their size (and may even be point particles with no physical size).

The basic structure of an atom is a nucleus, consisting of protons and neutrons bound together, surrounded by a cloud of electrons. Because electrons are negatively charged and protons are positively charged, electrons and protons are attracted to each other. This is what holds the electrons of an atom close to the nucleus of the atom (otherwise, the electrons would simply fly away). However, because electrons are negatively charged, they repel each other, just like how the north poles of two magnets repel each other while their north and south poles attract each other. This keeps the electrons from getting too close together. Positively charged protons also repel each other, but they are held together in the atom’s nucleus along with neutrons by the strong nuclear force.

There are many different types of atoms. The type of atom is determined by the number of protons in the atom’s nucleus. An atom with one proton in its nucleus is a hydrogen atom, an atom with two protons in its nucleus is a helium atom, and an atom with three protons in its nucleus is a lithium atom. (A carbon atom has six protons in its nucleus, an oxygen atom has eight protons, and a plutonium atom has 94 protons!) You will learn more about the different types of atoms listed in the periodic table later in this unit.

The smallest and lightest atom is the hydrogen atom. It consists of one proton orbited by a single electron. One of the largest and heaviest atoms is the plutonium atom (it is the heaviest naturally occurring atom). Plutonium-244 consists of a nucleus with 94 protons and 150 neutrons orbited by 94 electrons. Neutrons help hold the nucleus together. Without them, the protons in the nucleus would be pushed apart by their positive charges. When the number of protons and electrons in an atom are equal, then the atom is neutral (has no electrical charge).

Relative Scale of Atoms

Imagine that a proton were the size of a marble (about 12.7 mm in diameter). Then the nucleus of an oxygen atom would be about the size of a table tennis (ping pong) ball. If you placed the oxygen nucleus in an empty field, the outermost electron in the oxygen atom would be almost five football fields away (435 m). And while no one knows the physical size of an electron, it would be much smaller than the smallest speck of dust.

If this oxygen atom were part of a water (H2O) molecule, then the water molecule would be almost one mile across (about 1.5 km) and the next nearest water molecule would be about 2.8 km away in a liquid state. It would take a stack of 250,000 water molecules in a liquid state to equal the thickness of one sheet of paper. The thickness of this sheet of paper would be about 700,000 km (435,000 miles), which is almost 17.5 times the distance around the Earth or more than 1.8 times the distance from the Earth to the Moon. By comparison, the nucleus of a gold atom would be slightly smaller than a softball (about 9 cm) and the outermost electron in the gold atom would be about 1 km away. If gold atoms were stacked end-to-end (with no space between them), it would take 370,000 atoms to equal the thickness of one sheet of paper.

Since over 99.97% of its mass is in its nucleus, an oxygen atom is almost entirely empty space. This is true for a gold atom, and all other atoms, as well. Matter (even solid matter) is not nearly as solid as we might think. Except maybe in a nuclear reaction (see nuclear fusion below), objects and atoms do not actually touch each other. When two objects seem to touch, the nuclei of the atoms in one object are actually just being repelled (pushed against) by the nuclei of the atoms in the other object because of electromagnetic forces.

Most types of atom can exist in several different forms. If the number of protons and electrons is not equal, then the atom has an electrical charge and is called an “ion.” The hydrogen atom is unique in that it naturally exists as a positive ion and a negative ion. The hydrogen atom also has three naturally occurring “isotopes.” Hydrogen-1 is the most common isotope (hydrogen-1 represents 99.9885% of all hydrogen atoms on Earth). It has one proton in its nucleus. Hydrogen-2 (commonly known as deuterium) is a hydrogen atom since it still has one proton in its nucleus, but it also has one neutron. Hydrogen-3 is very rare. It has one proton and two neutrons in its nucleus. Basically, ions vary based on the number of electrons the atom has and isotopes vary based on the number of neutrons the atom has, and the type of atom is based on its number of protons.

Some isotopes are radioactive. This means that the isotope is unstable and will decay (breakdown) over time into more stable isotopes. How quickly a radioactive isotope decays is measured by its “half-life.” Well known radioactive isotopes include carbon-14 (used in radiocarbon dating to date archaeological finds) and uranium-235 and plutonium-239 (both used as fissile material in nuclear weapons).

The electrons in an atom are attracted to the protons in the atom’s nucleus by the electromagnetic force. At one time, scientists believed that these electrons traveled in circular orbits around the nucleus, much like planets circle the sun in a solar system. However, this model (known as the Bohr model and introduced in 1913) could not fully explain many of the observed behavior of atoms. It was not until 1924, when it was first proposed that electrons behave as both particles and waves, that the modern model of the atom was developed.

Classical mechanics, which is what students study in physics through high school and even most of college, is typically sufficient to describe the behavior of macroscopic objects. It is not until you start to get down to the scale of atoms and molecules that classical mechanics becomes insufficient and quantum mechanics becomes necessary. While any detailed discussion of quantum mechanics is well beyond the scope of this unit, a basic familiarity with a few key concepts is important. First, subatomic particles behave as both particles and waves. Second, electrons do not exist in specific positions; the position of an electron can only be described by a probability distribution and, instead of existing in a circular orbit, an electron exists in a cloud of likely positions. Third, the state of an electron is determined by a set of quantum numbers: the energy (n), orbital angular momentum (l), magnetic moment (m), and spin (s) of the electron. Fourth, because these electron properties are quantized (restricted to specific, discrete values), an electron cannot have just any amount of energy… it must have a specific energy level so that n is whole number. And fifth, no two electrons can share the same four quantum numbers in one atom.

The physical region in space where an electron can be found around an atom’s nucleus is called an atomic orbital. Each atomic orbital corresponds to a specific energy level. The higher the energy level, the farther away from the nucleus that an electron is likely to be, and the less energy it takes to separate the electron from the atom. When we talk about the “radius” of an atom, we are talking about the distance the outermost electrons (the electrons with the highest energy level) are from the nucleus.

The atoms created by the Big Bang were hydrogen (H-1), helium (He-4), and deuterium (H-2), with trace amounts of lithium (Li) and beryllium (Be). It is estimated that, 1 million years after the Big Bang, the Universe consisted of about 75% hydrogen, 25% helium, and 0.01% deuterium. So where did the heavier atoms, such as carbon, oxygen, or iron come from? To create a different type of atom, you have to change the number of protons in the atom’s nucleus. This requires a nuclear reaction.

Nuclear fission takes the nucleus of a heavy atom and splits it to create two lighter atoms. Splitting an atom’s nucleus takes a tremendous amount of energy. The strong nuclear force that holds protons and neutrons together is incredibly strong (much, much, much stronger than the gravitational force that holds us to the planet Earth, or the electromagnetic force that holds water molecules together and electrons to the nucleus in an atom). Nuclear fission is the reaction that powers our nuclear power plants and most of our nuclear weapons.

Nuclear fusion takes the nuclei of two light atoms and fuses them together to create one heavier atom. Pushing two atomic nuclei together takes a tremendous amount of energy. The nucleus of an atom is positively charged. As two nuclei get closer together, the electromagnetic force pushing them apart gets increasingly stronger. It takes the massive pressures found deep inside a star to push two atomic nuclei together. But once the two nuclei come into “contact” with each other, the strong nuclear force will fuse them together. Nuclear fusion is the reaction that powers active stars, including our Sun. Heavier atoms like carbon, oxygen, and iron were created inside stars and then spread throughout the Universe when those stars explode in supernovas.