Chapter 3 Section 2

The Elements—One Kind of Atom The Simplest Matter 2 The Elements—One Kind of Atom An element is matter made of only one kind of atom. At least 118 elements are known and about 90 of them occur naturally on Earth. Examples of naturally occurring elements include the oxygen and nitrogen in the air you breathe and the metals gold, silver, aluminum, and iron.

The Elements—One Kind of Atom The Simplest Matter 2 The Elements—One Kind of Atom The other elements are known as synthetic elements. These elements have been made in nuclear reactions by scientists with machines called particle accelerators. Some synthetic elements have important uses in medical testing and are found in smoke detectors and heart pacemaker batteries.

The Periodic Table— Charting the Elements The Simplest Matter 2 The Periodic Table— Charting the Elements Chemists have created a chart called the periodic table of the elements to help them organize and display the elements. Each element is represented by a chemical symbol that contains one to three letters. The symbols are a form of chemical shorthand.

The Periodic Table— Charting the Elements The Simplest Matter 2 The Periodic Table— Charting the Elements

The Periodic Table— Charting the Elements The Simplest Matter 2 The Periodic Table— Charting the Elements The elements are organized on the periodic table by their properties. There are rows and columns that represent relationships between the elements. The rows in the table are called periods. The elements in a row have the same number of energy levels.

The Periodic Table— Charting the Elements The Simplest Matter 2 The Periodic Table— Charting the Elements The columns are called groups. The elements in each group have similar properties related to their structure. They also tend to form similar properties related to their structure. They also tend to form similar bonds.

Identifying Characteristics The Simplest Matter 2 Identifying Characteristics Each element is different and has unique properties. These differences can be described in part by looking at the relationships between the atomic particles in each element.

Number of Protons and Neutrons The Simplest Matter 2 Number of Protons and Neutrons Cl is the symbol for chlorine. The top number is the element’s atomic number.

Number of Protons and Neutrons The Simplest Matter 2 Number of Protons and Neutrons It tells you the number of protons in the nucleus of each atom of that element. Every atom of chlorine, for example, has 17 protons in its nucleus.

The Simplest Matter 2 Isotopes Although the number of protons changes from element to element, every atom of the same element has the same number of protons. However, the number of neutrons can vary even for one element. These are called isotopes (I suh tohps), which are atoms of the same element that have different numbers of neutrons.

The Simplest Matter 2 Isotopes An atom’s mass number is the number of protons plus the number of neutrons it contains. Hydrogen has three isotopes with mass numbers of 1, 2, and 3.

The Simplest Matter 2 Isotopes Each hydrogen atom always has one proton, but in each isotope the number of neutrons is different.

The Simplest Matter 2 Atomic Mass The calculation of atomic mass takes into account the different isotopes of the element.

The Simplest Matter 2 Atomic Mass About 76 percent of chlorine atoms are chlorine-35 and about 24 percent are chlorine-37. The weighted average mass of all chlorine atoms is 35.45 u.

The Simplest Matter 2 Atomic Mass The atomic mass is the weighted average mass of the isotopes of an element. The atomic mass is the number found below the element symbol. The unit that scientists use for atomic mass is called the atomic mass unit, which is given the symbol u. It is defined as 1/12 the mass of a carbon-12 atom.

Classification of Elements The Simplest Matter 2 Classification of Elements Elements fall into three general categories—metals, metalloids (ME tuh loydz), and nonmetals. Metals generally have a shiny or metallic luster and are good conductors of heat and electricity. All metals, except mercury, are solids at room temperature.

Classification of Elements The Simplest Matter 2 Classification of Elements Metals are malleable (MAL yuh bul), which means they can be bent and pounded into various shapes. Metals are also ductile, which means they can be drawn into wires without breaking. Most of the elements are metals.

The Simplest Matter 2 Other Elements Nonmetals are elements that are usually dull in appearance. Most are poor conductors of heat and electricity. Many are gases at room temperature, and bromine is a liquid.

The Simplest Matter 2 Other Elements The solid nonmetals are generally brittle, meaning they cannot change shape easily without breaking. The nonmetals are essential to the chemicals of life. More than 97 percent of your body is made up of various nonmetals. Except for hydrogen, the nonmetals are found on the right side of the periodic table.

The Simplest Matter 2 Other Elements Metalloids are elements that have characteristics of metals and nonmetals. All metalloids are solids at room temperature. Some metalloids are shiny and many are conductors, but they are not as good at conducting heat and electricity as metals are.

Section Check 2 Question 1 What is the term for matter composed of only a single kind of atom? A. compound B. element C. mixture D. substance

Answer 2 The correct answer is B. Gold is an example of an element. Section Check 2 Answer The correct answer is B. Gold is an example of an element.

Section Check 2 Question 2 What is the name of the chart, composed by chemists, that arranges all the known elements, their properties, and their symbols?

Section Check 2 Answer The chart is known as the periodic table.

Section Check 2 Question 3 Every atom of the same element has the same number of protons. However, a given atom may have more neutrons than another atom of the same element. Atoms of an element with different numbers of neutrons are known as _______?

Section Check 2 Answer The answer is isotopes. Chlorine-35 and chlorine-37 are examples of isotopes.