Elements and Atoms

Elements And The Periodic Table. Physical And Chemical Properties. Chemical Compounds. Composition Of Atoms. Intensive And Extensive Properties.

Are there 100% pure substances? What is inside these pure substances?.For example, let’s consider pure gold. What is inside gold? Can gold be separated into a smaller substance?.What is inside the water we drink?.

Gold and water are very different categories of substances. Gold is considered an element whereas water is considered a compound.Thus a pure substance can be either an element or a compound.A compound has two or more elements chemically united in exact proportions.

Water is made up of two parts with the element hydrogen and one part with the element oxygen. Salt is made up of one part of sodium and one part of chlorine.Ammonia is made up of one part of nitrogen and four parts of hydrogen.

This composition does not change, regardless of whether the water comes from a faucet in the United States, the Yangtze River in China, or the ice caps on Mars. A chemical method needs to be used to obtain elements out of compounds.Most elements, once they interact with one or more other elements, form a compound.

This is a summary of what we have studied so far. Matter is either a mixture or a pure substance.Mixtures are either homogeneous or heterogeneous.Mixtures, can be separated into pure substances by using physical means.Pure substances are either Compounds or simply elements.

So what really are elements? How many elements are there in the world? There are 118 elements in the modern periodic table. Everything in this world, including you and me, is made out of these elements.Chemists use the alphabet to represent these elements. The first alphabet of the symbol for an element is always upper-case, but the 2nd alphabet is never upper-case.

The symbols usually correspond to their actual name. However, some symbols for some elements are derived from their Latin names. Examples are aurum for gold, ferrum for iron, natrium for sodium.

Let’s revisit some of the questions we looked at before. Why do some substances produce bubbles? Why do some substances release gas once they are mixed with other substances? Why do some explode when combined with some other chemicals?.

To figure this out, let’s look at two other things.Substances can also be identified by their properties as well as by their composition.What are the properties of matter? The properties of matter are are Color, melting point, boiling point, and density. These are physical properties.

A physical property can be measured and observed without changing the composition or identity of a substance.For example, we can measure the melting point of ice by heating a block of ice and recording the temperature at which the ice is converted to water. Water differs from ice only in appearance. They both have the same composition. Therefore this is a physical change. We can freeze the water to recover the original ice. Therefore, the melting temperature of water is a physical-attribute.

On the other hand, a chemical-property brings about a change in structure and composition. In other words we carry out a chemical change.The statement “Hydrogen gas burns in oxygen gas to form water” describes a chemical-property. In this case this chemical change is called burning. After the change, the original substances, hydrogen and oxygen gas, will have vanished and a chemically different substance, water, will have taken their place. We cannot recover hydrogen and oxygen from water by a physical change such as boiling it or freezing it.

For example every time we boil an egg, we bring about a chemical change. When subjected to heating, the yolk and the egg white undergo reactions that alter not only their physical appearance but their chemical makeup as well. Once eaten, the egg is changed again, by enzymes. This digestive process is another example of a chemical change. What happens during such a process depends on the chemical properties of the specific enzymes and of the food involved.

So what really is the structure of a compound? A compound is made of many elements. We group different types of atoms using the term element. In the 5th century B C the Greek philosopher Democritus expressed an idea.He said that all matter consists of very small indivisible particles. Many people did not accept that idea. However it became the foundation of atom. It was in 1808 another scientist came up with a precise definition of the indivisible constituents of matter.

The scientist Dalton’s idea marked the beginning of modern chemistry. He suggested the following. Elements are made of extremely small particles called atoms. The atoms of a given element are identical in size, mass and other properties. The atoms of different elements differ in size, mass and other properties.

They cannot be subdivided, created or destroyed. Atoms of different elements combine in simple whole-number ratios to form chemical compounds. In a chemical reaction the following can happen to atoms. They can be combined together. They can be separated from each other. They can be rearranged among each other.

Are atoms unbreakable?.A scientist called J J Thomson investigated a beam called a cathode ray using a Crookes tube.It is a sealed glass container. Inside, there is a vacuum and there are two electrodes.When a voltage is applied across the electrodes, cathode rays are generated. They hit the glass at the opposite end of the tube and they give a glow at that spot.

He discovered that the path rays could be changed by an electric field. He concluded that these rays were composed of very light-weight negatively charged particles. He measured their mass-to-charge ratio. He found it was 1800 times smaller than of the hydrogen. Hydrogen is the smallest atom. If electrons are smaller than the smallest atoms, they must be parts of atoms.

Thomson came up with Plum Pudding representation. He said that atoms are breakable. An atom has structure. Electrons suspended in a positively charged electric field. It must have positive charge to equate the negative charge of electrons and make the atom neutral. The atom’s mass is because of it’s electrons.An atom is mostly empty space when compared with the size of an electron.This leads us to the Rutherford’s Gold foil experiment.

How can you prove something is empty?.He used a foil made out of large atoms as the target. It was very thin foil. He then used a very small particles as bullets to shoot at the this foil. The particles had high energy.If they were very small in weight, then the electrons of the atom might affect it. Therefore they had to be heavier than electrons. Hence he used alpha particles as the bullet and a thin gold foil. Alpha particles have a mass of 4 atomic mass units. Gold has a mass of 197 atomic mass units. Gold is very malleable.

About 0.01% of the particles bounced off the gold foil.About 2% of the particles went through the foil but were deflected by large angles.Over 98% of the alpha particles went directly through the gold foil without deflecting.

The Modern Atom.We know atoms are composed of three main pieces - protons, neutrons and electrons. The nucleus contains protons and neutrons. The nucleus is only about 10^-13 cm in diameter.

The electrons move outside the nucleus. Their average distance is about 10^-8 cm. Therefore, the radius of the atom is about 10^5 times larger than the radius of the nucleus.

The study of chemistry depends heavily on measurement. For instance, chemists use measurements to compare the properties of different substances and to assess changes resulting from an experiment. But how do we measure properties of matter? Elements and compounds are extremely small. What units of measurement can be used?.

All measurable properties of matter can be categorized two categories. Extensive and intensive properties.Extensive properties of matter deal with the quantity such as mass, length, and volume.Intensive properties of matter don't depend on the amount of matter being considered. A good such example is temperature.

To do this, the General Conference of Weights and Measures, the international authority on units, proposes a metric system called the International System of Units.Represented in the table is the seven SI units. You might already be familiar with some of these.

All units in the metric system are related to the fundamental unit by a mathematics power of 10. The power of 10 is indicated by a prefix. The prefixes are always the same, regardless of the fundamental unit. One example is Kilogram, which measures mass and Kilometer, which measures distance. Now that we know more about atoms, their size and composition, we can study chemistry further.