Atoms, Elements, and the Periodic Table

Structure of the Atom Atoms are the smallest particles into which matter can be divided and still maintain its characteristics. Atoms are the building blocks of the universe. There are 92 different kinds of atoms that occur naturally. These building blocks can be joined together to create all of the different kinds of matter we can observe. All atoms share the same basic structure. Over the years, scientists have designed many different models for this structure. Each one was the best model at the time, but as new evidence was discovered, the models were revised. Scientists will probably revise the current model as more evidence becomes available. ? What’s inside of an atom?

Early Atomic Models “Atoms are indivisible.” - Democritus The Greek scientist Democritus was the first person to say that all matter is made of atoms, but he also said that atoms are indivisible. Atoms of an element are alike. In the early 1800’s, the English scientist John Dalton developed his Atomic Theory of Matter. It said four things: (1) All matter is made of atoms. (2) Atoms of an element are alike. (3) Atoms of different elements are different. (4) Atoms of different elements join together to make compounds. Different elements have different atoms. + = Positively-charged matrix Atoms join to make compounds. - John Dalton Negatively- charged electron - J. J. Thomson In the late 1800’s, J. J. Thomson discovered that atoms were made of smaller particles. He discovered the electron and devised the “chocolate-chip cookie” model of the atom.

Recent Atomic Models Protons are in the nucleus. - Rutherford In the early 1900’s, scientists discovered the positively-charged proton. In 1911, Rutherford discovered the nucleus of the atom, where protons are located. Atoms are mostly empty space. Niels Bohr changed Rutherford’s model in 1913. Bohr said that electrons move around the nucleus in fixed energy levels. Electrons can move from one energy level to another. Bohr’s model has been called the “solar system” model of the atom. Electrons orbit the nucleus. - Niels Bohr Negative electron cloud The current model of atomic structure is called the “electron cloud” model. Electrons move within an energy level in an ever-changing path, not a fixed orbit. Neutrons bond with protons in the nucleus. Neutral neutrons are in nucleus with positive protons. - electron cloud model

Atomic Number and Mass Number All atoms of an element have the same number of protons, and different elements have different numbers of protons in their atoms. The atomic number of an atom is the number of protons it has. If the atom is neutral, the atomic number is also the number of electrons. Atoms of the same element may have different numbers of neutrons. Atoms with extra or missing neutrons are called isotopes. An atom’s mass number is the number of protons plus neutrons. Different isotopes of an element have different mass numbers. Mass number of this neutral atom is 14. This is the sum of the protons plus neutrons. Since there are 6 protons, there must be 14 - 6 = 8 neutrons. (protons are blue, electrons are yellow, and neutrons are green) Atomic number of this neutral atom = 6. This means that it has 6 protons and 6 electrons.

Elements An element is matter that is made of only one kind of atom. All of the atoms of an element are alike. There are 92 naturally occurring elements. Elements can be identified by their properties. Some properties are color, texture, density, malleability, ductility, ability to dissolve in water, and ability to conduct heat or electricity. At room temperature, most elements are solids, although a few are liquids and some are gases. Chemical symbols are abbreviations for the names of elements. You can also use a chemical symbol to represent one atom of an element. Chemical symbols are the same all over the world, no matter what language is spoken. Na Cl Chemical symbol for the element Chlorine, used in bleach and water treatment. Chemical symbol for the element Sodium, used in sodium vapor lamps.

Compounds The atoms of most elements are very reactive. They chemically join together with atoms of other elements to form compounds. The properties of compounds are different than the properties of the elements that make the compound. When elements join to make compounds, they always join in the same proportion. H2O is water, not HO or H3O ! Compounds are separated into two groups, depending on how their atoms are joined: (1) Molecular compounds are made of atoms that share electrons. (2) Ionic compounds are made of atoms that gain or lose electrons. Chemical formulas represent compounds. They show the kind and number of atoms in the compound. NH3 H is symbol for Hydrogen O is symbol for Oxygen H2O N is symbol for Nitrogen H is symbol for Hydrogen Subscript 3 means there are 3 Hydrogen atoms No subscript means one Oxygen atom Subscript 2 means that there are 2 Hydrogen atoms No subscript means one Nitrogen atoms Water molecule Ammonia Molecule

Classification of Elements Pure substances are either compounds or elements. Compounds can be broken down into simpler substances, but elements cannot because they are made of only one kind of atom. In the last three centuries, new technologies led scientists to try to identify all of the existing elements and group them into similar categories. Currently, scientists have identified AND name112 different elements. 92 of them occur naturally, and 20 are man-made. Scientists build super-heavy atoms to test the limits of atomic structure. During the eighteenth and nineteenth centuries, scientists noticed that some elements behaved like other elements. By comparing the behaviors of elements, scientists identified several groups of elements. Other scientists of that time were able to determine the relative atomic masses of different elements. These discoveries led to the development of the Periodic Table of the Elements, which is an important tool of chemists.

The First Periodic Table Dmitri Mendeleev is known as the father of the Periodic Table. He was born in Russia in 1834. As were many other scientists of the time, Mendeleev was interested in discovering if a pattern existed relating the known elements. Mendeleev discovered that the physical and chemical properties of the elements occurred in a regular repeating pattern—the Periodic Law. He published his Periodic Table of the Elements in 1869, the first table that successfully summarized the relationships between the elements. In this table, he arranged the known elements in ascending order according to their relative atomic masses, beginning with Hydrogen. Mendeleev also arranged the elements in columns according to their physical and chemical properties. Mendeleev’s table pointed out some errors in accepted atomic weights, and also predicted the existence and properties of elements not yet discovered. Mendeleev’s table did not include any of the noble gases, because none of them had been discovered.

Mendeleev’s 1st Table H Li Be B C N O F Na Mg Al Si P S Cl K Ca --- Ti   Li Be B C N O F Na Mg Al Si P S Cl K Ca --- Ti V Cr Mn Fe, Co, Ni Cu Zn As Se Br Ru, Rh, Pd Ag Cd In Sn Sb Te I Cs Ba

Early Version of Mendeleev’s Table

Revision of Mendeleev’s Table In Mendeleev’s table, there were some elements that appeared to be in the wrong group if arranged strictly by increasing atomic weight. As more elements were discovered after Mendeleev’s death, a few more “misplaced” elements appeared. Some elements, such as were placed in the wrong groups because of multiple valence numbers or errors in the calculation of their atomic weights. In 1914, Henry Moseley discovered the significance of atomic number, the number of protons in the nucleus of an atom. When the Periodic Table was revised, arranging the elements by ascending atomic number, the inconsistencies and errors in the table vanished. The Modern Periodic Table of the Elements has been modified so that the elements are now arranged by atomic number, rather than atomic weight. The Table has seven horizontal rows, called periods, and a number of vertical columns called groups or families. Elements in the same period have the same number of occupied energy levels. Elements in the same group or family have similar properties.