Atoms, Elements, and the Periodic Table

Atoms, Elements, and the Periodic Table Chapter Four

Structure of Matter Section One

If you crushed a sugar cube, you would find a number of small fragments that were still sugar. If you crushed a fragment, you would find a number of small particles that were still sugar. How long could you divide the sugar until the particles were no longer sugar?

This was the approach that Democritus took in explaining the atom. He said that you could take a pair of shears and cut a piece of copper in two and sometime, you would reach a piece that couldn’t be cut anymore. He named this particle an atom meaning indivisible.

Over two thousand years later, Lavoisier used this concept to develop the Law of Conservation of Matter.

The Law of Conservation of Matter states that matter is neither created nor destroyed in ordinary chemical or physical reactions.

Dalton’s Atomic TheoryIn 1808, an English chemistry teacher named John Dalton proposed the atomic theory. From data gathered in his student’s experiments, he explained the theories mentioned above and laid the foundation for understanding the atom.

Dalton’s Atomic Theory can be summed up in the following statements:All matter is composed of small particles called atoms.Atoms of the same element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and properties.

From the beginning, we all know that atoms are very small. However it has taken some good science to develop a model of the atom.

Gases at atmospheric pressure do not conduct electricity well, however, gases at very low pressures do conduct. Scientists at the beginning of the twentieth century found that a glowing current will pass from a negatively charged cathode to a positively charged anode in a glass tube called a cathode ray. This stream was called a cathode ray and the device was called a cathode ray tube.

John Thompson found that the cathode ray could cause a paddle wheel to roll along rails through the tube. This indicated that the ray was made of a particles. Cathode rays are deflected by a magnetic field and the rays were deflected away from a negatively charged object. This would indicated that they carry a negative charge.

Later, scientists established that the particle was the smallest known to man with a mass of 1/2000 of a hydrogen atom. It was named the electron.

Thompson developed the “chocolate chip cookie” model in which he envisioned atoms as being a ball of positive dough with embedded electron “chips”. Overall, the atom would have a neutral charge.

In 1911, New Zealander Earnest Rutherford and his associates Hans Geiger and Ernest Marsden bombarded a thin gold foil with alpha particles emitted from a radioactive source. They expected an evenly charged force field in the foil so they planned that the particles would pass straight through. When the detector was studied, they were greatly surprised to find about 1 in 8000 particles bounced straight back! Rutherford exclaimed that this would be like firing a 15 inch artillery shell into a piece of tissue paper and have it bounce back.

Rutherford concluded that atoms must contain a very small, dense, positively charged nucleus. The nucleus of the atom is very small. If it were the size of a marble, the atom would be larger than a football field. He named the positively charged particles that made up the nucleus protons.

Eventually, a student of Rutherford’s, James Chadwick, discovered a nucleur particle that lacked a charge called a neutron.

Later in the twentieth century, Niels Bohr established a model of the atom in which the electrons orbited the nucleus in fixed orbits like the planets around the sun.

More recent studies have shown that electrons don’t occupy orbits like planets. Instead, they move in spaces forming an electron cloud. The higher the energy of the electron, the farther from the nucleus it is.

Studies have also found smaller subatomic particles in the nucleus called quarks.

The Simplest Matter Section Two

The basic makeup of the earth is simple substances made of only one kind of atom called elements.

Of the total 115 elements, about 90 elements occur naturally on earth and the remainder are man-made.

The elements are charted on the periodic table. Atoms with similar energies make up rows or periods. Atoms with similar properties make up columns or groups.

When the atoms are arranged this way, certain properties reoccur periodically giving the chart its name.

The atomic number of an element is the number of protons in the nucleus. This number identifies the element. It will be found near the top of each grid on the periodic table. For instance, hydrogen is 1, lithium is 3, carbon is 6, and silver is 47.

The relative atomic mass is the number that is usually shown with several decimals. When you round it off to a whole number you get the mass number which is the number of protons and neutrons in the nucleus.

Some atoms of the same element have different numbers of neutrons. They are called isotopes and usually have special properties. The relative atomic mass is an average of the mass of an elements isotopes so it usually includes decimals.

Metals make up most of the chart on the right side.

Metals: • Have a shiny surface luster • Conduct heat and electricity • Are malleable • Are ductile • Most are solid at room temperature • Most are denser that other substances

Nonmetals make up the far left side of the chart and lack the properties of metals.

Metalloids are found between metals and nonmetals and have special properties. They are sometimes known as semiconductors.

Elements are divided into three groups: • Metals • Nonmetals • Metalloids

Compounds and Mixtures Section Three

A pure substanceis only made of one kind of particle. This would mean that it is an element or a compound.

A chemical compound is a pure substance that is made of two or more elements that are chemically combined. Because of this definition, we know that the substances that make up a compound change into a new identity and cannot be separated physically.

Compounds have a definite chemical formula that does not change.H2O = WaterCO2 = Carbon Dioxide