Lithium Oxygen. Water Molecule

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1 Elements and atoms Elements are the simple substances, like hydrogen, copper, and iron, that make up all matter in the universe. There are 118 known elements, which are organized on the periodic table. An atom is the smallest part of a substance that has the properties of that substance. Atoms of elements have unique numbers of protons, neutrons, and electrons. Protons and neutrons are found in the center of the atoms, whereas electrons are arranged at specific distances from the nucleus. The number of protons, neutrons, and electrons give atoms their properties. For example, lithium, which is a metal, has three protons, four neutrons, and three electrons. On the other hand, oxygen, which is a gas, has eight protons, eight neutrons, and eight electrons. Lithium Oxygen Charge: Atomic particles have charges. Protons have a +1 charge (positive), and electrons have a -1 charge (negative). Neutrons do not have a charge (neutral). Each atom has the same number of protons and electrons, because each positively charged proton attracts one negatively charged electron. Compound: A compound is a substance that is made when two or more types of atoms bond together. For example, the bonding of two hydrogen atoms to one oxygen atom forms the compound we call water. Molecule: A group of atoms that form the smallest unit of a compound is called a molecule. For example, a glass of water has billions of atoms of hydrogen and oxygen. Each molecule of water has two hydrogen atoms to one oxygen atom. Water Molecule

2 Information on the periodic table The periodic table organizes elements according to their atomic number, the way their electrons are arranged, and their physical/chemical properties. Each box on the periodic table contains information for that element, including its name, symbol, atomic number, and atomic mass. Atomic Number: The atomic number is the number of protons and electrons in the nucleus. For example, oxygen has eight protons and eight electrons. Atomic Mass: The atomic mass is the number of protons plus neutrons in the nucleus. For example, oxygen has an atomic mass of We round the mass to 16, and since oxygen has eight protons, it must have 8 neutrons (16-8=8). So why isn t atomic mass a whole number? Masses are not whole numbers because they are the average mass of atoms of each element. Most oxygen atoms have eight neutrons, but a few only have seven neutrons, so the average mass of oxygen is only instead of 16. Atomic Symbol: The atomic symbol is a one or two letter abbreviation for each element that is used on the periodic table and chemical formulas. For example, oxygen is "O"; nitrogen is "N"; and xenon is "Xe". If an element has a two letter symbol, the second letter is always lowercase. Element name Atomic number: and electrons Atomic symbol Atomic mass: and neutrons

3 How are electrons arranged? Electrons are at specific distances from the nucleus in clouds that are called shells and subshells. The first shell can hold only two electrons. The second can hold eight, and the third can hold 18. For example, lithium has three electrons. The first two electrons fill the first shell, and that leaves one extra electron that will go into the second shell. Oxygen, on the other hand, has eight electrons. Two fill the first shell, and the other six go into the second shell. The electrons in the outermost shell of an atom are called valence electrons. Lithium has one valence electron, and oxygen has six. 2 nd Shell 1 st Shell Atomic number: and electrons The table below shows the electron configurations of four elements. Element Symbol Atomic Number (total # of electrons) in the first shell in the second shell in the third shell Hydrogen H Helium He Fluorine F Potassium Na

4 Combining atoms What makes atoms combine with each other in unique ways? For example, why does one sodium atom combine with one chlorine atom to make table salt? Why not two or even three sodium atoms for each chlorine? The answer lies in the desire of atoms to fill their outermost electron shell. They do this by accepting or donating electrons. So in the case of sodium chloride, chlorine needs one electron to fill its outermost shell, and sodium wants to give away one electron. In the diagrams below, notice how each atom gets a full outermost shell by gaining or losing one electron. Source: Lithium Fluorine Source:

5 Organization of the periodic table Groups: The vertical columns on the periodic table are called groups. Elements in each group have the same number of valence electrons. Group 1A elements (H, Li, Na, etc.) have one valence electron; group 7A elements (F, Cl, etc.) have seven valence electrons, etc. Periods: Metals, nonmetals, and metalloids Metals: Most of the elements on the periodic table are metals. Metals are good conductors of heat and electricity. They can be drawn into wire, and can be hammered into shapes. Electrons can easily move from one atom of a metal to another. Metals are solid at room temperature (with the exception of mercury). Some metals tarnish (or rust) when they are exposed to oxygen. Nonmetals: Nonmetals can be solids, liquids, or gases at room temperature. Nonmetals do not conduct electricity, and are poor conductors of heat. Solid nonmetals are usually brittle, and have lower melting points than metals. Metalloids: Metalloids are seven elements that have some of the qualities of metals and some of the qualities of nonmetals. Metalloids are usually brittle, and are poor conductors of heat and electricity. Metalloids are often used as semiconductors in the electronics industry. Some metalloids have a metallic luster.