atoms

Raisin bread

After Eugen Goldstein's 1886 discovery that atoms had positive charges, Thomson imagined that atoms looked like pieces of raisin bread, a structure in which clumps of small, negatively charged electrons (the "raisins") were scattered inside a smear of positive charges. In 1908, Ernest Rutherford, a former student of Thomson's, proved Thomson's raisin bread structure incorrect.

Line spectra

An emission spectrum of light that contains very sharply defined lines. Line spectra are given off when matter is heated or excited in some way, and each line corresponds to a wavelength of light given off during an electron transition from an excited state to the ground state.

Weight

Another important characteristic of an atom is its weight, or atomic mass. The weight of an atom is roughly determined by the total number of protons and neutrons in the atom. While protons and neutrons are about the same size, the electron is more than 1,800 times smaller than the two. Thus the electrons' weight is inconsequential in determining the weight of an atom - it's like comparing the weight of a flea to the weight of an elephant.

Size

Atoms are extremely small. One hydrogen atom (the smallest atom known) is approximately 5 x 10-8 mm in diameter. To put that in perspective, it would take almost 20 million hydrogen atoms to make a line as long as this dash -. Most of the space taken up by an atom is actually empty because the electron spins at a very far distance from the nucleus. For example, if we were to draw a hydrogen atom to scale and used a 1-cm proton the atom's electron would spin at a distance of ~0.5 km from the nucleus. In other words, the atom would be larger than a football field!

Atoms are small

Atoms are so tiny that 20 million hydrogen atoms could fit on this dash -. In spite of their incredibly small size, scientists have come to an accurate understanding of atomic structure.

Number

Atoms of different elements are distinguished from each other by their number of protons (the number of protons is constant for all atoms of a single element; the number of neutrons and electrons can vary under some circumstances). To identify this important characteristic of atoms, the term atomic number (z) is used to describe the number of protons in an atom. For example, z = 1 for hydrogen and z = 2 for helium.

Finger-print of atoms

Different elements give off distinct colors when heated? This is because electrons "jump" away from the nucleus when they are excited. When the electrons "fall" back, they release energy in the form of light. Anyone who has seen the glow of neon lights has seen this process in action.

Niels Bohr proposed yet another modification

Ernest Rutherford's view of the atom consisted of a dense nucleus surrounded by freely spinning electrons. In 1913, the Danish physicist Niels Bohr proposed yet another modification to the theory of atomic structure based on a curious phenomenon called line spectra. When matter is heated, it gives off light. For example, turning on an ordinary light bulb causes an electric current to flow through a metal filament that heats the filament. This heats the filament and produces light. The electrical energy absorbed by the filament excites the atoms' electrons, causing them to "wiggle." This absorbed energy is eventually released from the atoms in the form of light.

Chadwick discovers the neutron

In 1932, James Chadwick discovered a third type of subatomic particle, which he named the neutron. Neutrons help stabilize the protons in the atom's nucleus. Because the nucleus is so tightly packed together, the positively charged protons would tend to repel each other normally. Neutrons help to reduce the repulsion between protons and stabilize the atom's nucleus. Neutrons always reside in the nucleus of atoms and they are about the same size as protons. However, neutrons do not have any electrical charge; they are electrically neutral.

Each electron shell has limits for the number of electrons a shell can hold.

Not only did Bohr predict that electrons would occupy specific energy levels, he also predicted that those levels had limits to the number of electrons each could hold. Under Bohr's theory, the maximum capacity of the first (or innermost) electron shell is two electrons. For any element with more than two electrons, the extra electrons will reside in additional electron shells. For example, in the ground state configuration of lithium (which has three electrons) two electrons occupy the first shell and one electron occupies the second shell.

Rutherford proposes a planetary model of an atom

Rutherford performed a series of experiments with radioactive alpha particles. While it was unclear at the time what the alpha particle was, it was known to be very tiny. Rutherford fired tiny alpha particles at solid objects such as gold foil. He found that while most of the alpha particles passed right through the gold foil, a small number of alpha particles passed through at an angle (as if they had bumped up against something) and some bounced straight back like a tennis ball hitting a wall. Rutherford's experiments suggested that gold foil, and matter in general, had holes in it! These holes allowed most of the alpha particles to pass directly through, while a small number ricocheted off or bounced straight back because they hit a solid object. In 1911, Rutherford proposed a revolutionary view of the atom. He suggested that the atom consisted of a small, dense core of positively charged particles in the center (or nucleus) of the atom, surrounded by a swirling ring of electrons. The nucleus was so dense that the alpha particles would bounce off of it, but the electrons were so tiny, and spread out at such great distances, that the alpha particles would pass right through this area of the atom. Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the center and the electrons revolving around the nucleus.

Isotopes

The number of neutrons in an atom can also vary. Two atoms of the same element that contain different numbers of neutrons are called isotopes. For example, normally hydrogen contains no neutrons. An isotope of hydrogen does exist that contains one neutron (commonly called deuterium). The atomic number (z) is the same in both isotopes; however the atomic mass increases by one in deuterium as the atom is made heavier by the extra neutron.

Cathode ray tube and electrons

Thomson's notion of the electron came from his work with a nineteenth century scientific curiosity: the cathode ray tube. For years scientists had known that if an electric current was passed through a vacuum tube, a stream of glowing material could be seen; however, no one could explain why. Thomson found that the mysterious glowing stream would bend toward a positively charged electric plate. Thomson theorized, and was later proven correct, that the stream was in fact made up of small particles, pieces of atoms that carried a negative charge. These particles were later named electrons.

Electron shells

To Bohr, the line spectra phenomenon showed that atoms could not emit energy continuously, but only in very precise quantities (he described the energy emitted as quantized). Because the emitted light was due to the movement of electrons, Bohr suggested that electrons could not move continuously in the atom (as Rutherford had suggested) but only in precise steps. Bohr hypothesized that electrons occupy specific energy levels. When an atom is excited, such as during heating, electrons can jump to higher levels. When the electrons fall back to lower energy levels, precise quanta of energy are released as specific wavelengths (lines) of light. Under Bohr's theory, an electron's energy levels (also called electron shells) can be imagined as concentric circles around the nucleus. Normally, electrons exist in the ground state, meaning they occupy the lowest energy level possible (the electron shell closest to the nucleus). When an electron is excited by adding energy to an atom (for example, when it is heated), the electron will absorb energy, "jump" to a higher energy level, and spin in the higher energy level. After a short time, this electron will spontaneously "fall" back to a lower energy level, giving off a quantum of light energy. Key to Bohr's theory was the fact that the electron could only "jump" and "fall" to precise energy levels, as measured in electron volts, or eV, thus emitting a limited spectrum of light.

J J Thomson's contribution

Until the final years of the nineteenth century, the accepted model of the atom resembled that of a billiard ball - a small, solid sphere. In 1897, J. J. Thomson dramatically changed the modern view of the atom with his discovery of the electron. Thomson's work suggested that the atom was not an "indivisible" particle as John Dalton had suggested but a jigsaw puzzle made of smaller pieces.

Ions

When the number of electrons changes in an atom, the electrical charge changes. If an atom gains electrons, it picks up an imbalance of negatively charged particles and therefore becomes negative. If an atom loses electrons, the balance between positive and negative charges is shifted in the opposite direction and the atom becomes positive. In either case, the magnitude (+1, +2, -1, -2, etc.) of the electrical charge will correspond to the number of electrons gained or lost. Atoms that carry electrical charges are called ions (regardless of whether they are positive or negative).