It's Elementary

An element is a pure substance: It cannot be broken down into simpler substances. Because an element is pure, every atom of that element is like every other atom of that element with the exact same physical and chemical properties.

Atoms, the smallest units of matter, are made of subatomic particles: protons, neutrons and electrons. Electrons surround the nucleus and are negatively charged particles with almost no mass. Protons and neutrons, at the center of the atom, do have mass. Protons are positively charged particles, while neutrons are neutral.

In most atoms, the number of electrons and protons are the same, balancing the atom's electrical charge. Sometimes, however, a few of the electrons can become shared with another atom. This creates a new molecule called a compound. Compounds have the atoms of two or more different elements.

Hydrogen bromide is a chemical compound. It’s formed from atoms of hydrogen and bromide, two

• electrons

• elements

• neutrons

• nuclei

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Elements are grouped together into categories based on their physical and chemical properties. The two main categories are metals and non-metals. Non-metals include metalloids and noble gases.

Iron, gold, and copper are three examples of metals. They share some common properties: They are all malleable, which means they can be flattened or hammered into different shapes; they are all ductile, which means they can be drawn out into wire; they are all good conductors of electricity and heat; and they are all shiny. Some properties that may vary for each metal include boiling point, density, solubility and color. Mercury, for instance, is a metal that is liquid at room temperature, while lead is an especially dense metal.

Non-metals are not shiny; they're not malleable or ductile; and most are poor conductors of heat and electricity.

Silver is an element that can conduct electricity and heat and is shiny. Silver is most likely

• a metal

• a non-metal

• a liquid

• not malleable

Each element on the periodic table has its own box and chemical symbol. The chemical symbol of hydrogen is H, and gold is Au. The little number at the top of the box is the element's atomic number, which is the number of protons that one atom of that element has. The little number at the bottom is the atomic mass of this element, which is the combined number of protons and neutrons.

Each horizontal row is called a period. The properties of the elements in each row follow a repeating, or periodic, pattern from left to right. In every row the first elements are metals, followed by non-metals, metalloids and, finally, noble gases.

Each column is called a group. Elements in a group share similar chemical and physical properties. For example, all the elements in group 1, like lithium and sodium, are very reactive metals.

On the periodic table, gold has the atomic number 79. This means that

• an atom of gold has 79 protons and electrons combined

• gold can be found in 79 isotopes

• gold was the 79th element discovered

• an atom of gold has 79 protons

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Groups 17 and 18 on the periodic table are non-metals. Elements in group 18 are called inert or noble gases. They are generally found in a gaseous state at room temperature. Some glow different colors when an electric current is passed through them, making them useful in neon lights. They are also called inert gases because they don't react easily with other elements.

A noble gas is least likely to

• exist as a gas at room temperature

• glow

• be used in a neon light

• react with a metal

Chemical changes occur when atoms or molecules share, lose or gain electrons. These interactions are called chemical reactions. During a chemical reaction atoms interact to create a new substance with different physical and chemical properties than the two or more elements that are combining. Common examples of chemical reactions include rusting iron, souring milk, burning charcoal or even baking cookies. The starting substances in a chemical reaction are called reactants; the resulting substance is called a product. When chemical reactions occur, new products are formed. But the number of atoms always stays the same. That’s the law called the conservation of mass.

In the chemical reaction of baking cookies, the product is

• the ingredients that are mixed together before baking.

• the temperature at which the oven is set.

• the cookies that are baked at the end

• frosting that can be added after the cookies cool

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Chemical bonding is the joining of two or more atoms to form a new substance. A chemical bond is a force of attraction that holds the two atoms together, a force that involves the electrons that whir around the nucleus of every atom. In some atoms different electrons are closer or farther from the nucleus. The electrons in the outermost energy level of an atom are called its valence electrons. The number of valence electrons is one of the properties that elements in the same group on the periodic table share.

Which of these are an atom’s valence electrons?

• the electrons closest to the nucleus.

• the electrons farthest from the nucleus.

• the electrons at the inner-most energy level.

• electrons that are never involved in chemical bonding.

When an atom is missing an electron or has an extra electron, it is no longer neutrally charged. We call this positively or negatively charged atom an ion. When positive and negative ions interact, they form a new substance. For example, when sodium and chlorine interact in a solution, they combine to form sodium chloride, or salt.

Sometimes atoms can contain more or fewer neutrons in their nucleus than other atoms of that element. These variations of an element are called isotopes. One isotope, called deuterium or heavy water, is found among the hydrogen atoms in water. Heavy water is used in nuclear reactors to slow down neutrons and thus control the rate of a nuclear reaction.

A carbon atom usually has 6 protons and 6 neutrons. Carbon-14 is a type of carbon with atoms that contain 6 protons and 8 neutrons. Carbon-14 can be used to find the age of certain fossils. Carbon-14 is an example of a(n)

• positive ion.

• negative ion.

• salt.

• isotope.

The smallest unit of matter is a(n) ________.

• molecule

• atom

• covalent bond

• electron

Which are characteristics of all metals?

• They are malleable, ductile, and conduct electricity.

• They are malleable, ductile, and form ionic bonds.

• They are ductile, inert, and conduct electricity.

• They are noble, ductile, and form ionic bonds.

Noble gases are known as ________.

• ionic gases

• inert gases

• metalloid gases

• ductile gases