Introduction Chemical Bonds.
Of the three objects shown previously, only the copper wire consists of a single element. The other two objects are made of two or more elements. How are elements combined to make new substances having unique properties? What holds atoms of the elements together so that they do not easily come apart? These questions lead us into thinking about how atoms and ions are held together.
Chemical Bond Formation
In the example above, the copper wire contains copper atoms that are held together by chemical bonds. Water contains two hydrogen atoms bonded to one oxygen atom, and salt contains ions formed by sodium and chlorine that are also held together by chemical bonds. In each case, chemical bonds join together the atoms or ions that make up that substance. A chemical bond is the force of attraction that holds atoms (in molecules or metals) or ions (in salts) together.
The force of attraction between atoms or ions in a chemical bond is so strong that energy is required to break them apart. This happens because the atoms or ions achieve a greater stability when they become chemically bonded to other atoms or ions. For example, a hydrogen atom with its single electron can form a chemical bond with another hydrogen atom. The two atoms bonded together are more stable than the individual atoms.
Also, as mentioned above, atoms or ions of two different elements can also form a chemical bond. In the case of water, chemical bonds form by the sharing of valence electrons. In the case of salt, chemical bonds form when two oppositely charged ions come together as a result of the attractive force between them.
Whether an atom loses, gains, or shares electrons depends on its electron configuration and the electron configuration of the element it is bonding with. In all cases, atoms tend to acquire a stable noble gas configuration. When elements combine, they lose their properties and form a new compound with new properties. In other words, the properties of compounds are different from the properties of the individual elements that make them up. Table salt, which is very safe to eat, is made by combining sodium, a metal that reacts explosively, and chlorine, a green poisonous gas. In contrast, water is made by combining hydrogen and oxygen gases, which will certainly not satisfy your thirst!
How does the formation of bonds affect the stability of an atom?
It becomes more stable.
It becomes unstable.
It does not affect its stability.
It acquires a noble gas configuration.
Answer
it becomes more stable
it acquires a noble gas configuration
Elements and Compounds
Chemical bonds are found in any substance made up of atoms or ions that associate with one another to form stable units. For example, hydrogen gas is made up of molecules containing two hydrogen atoms that are linked by one chemical bond. Water is another example in which three atoms are linked through two chemical bonds to form one stable unit. In both cases, the stable unit formed is a molecule.
Although both hydrogen gas and water are composed of molecules, they differ significantly. Hydrogen is an element, whereas water is a compound. Like hydrogen, some elements exist as diatomic molecules (molecules consisting of two atoms). In these elements, the atoms in each molecule are the same.
A compound, on the other hand, consists of atoms of different elements. Elements combine in a specific ratio to form a compound. Water is a compound in which the atoms of hydrogen and oxygen are in a 2-to-1 ratio. Lithium chloride is a compound containing lithium and chloride ions in a 1-to-1 ratio.
Molecular compounds are composed of molecules in which electrons are shared between atoms. They are formed between nonmetals. The smallest unit of water is a molecule, and so water is a molecular compound.
Ionic compounds (or salts) are composed of ions of opposite charges.They are formed when a metal combines with a nonmetal. The smallest unit of lithium chloride is a unit containing one lithium ion and one chloride ion, and so lithium chloride is an ionic compound. Earlier, the term salt was used to describe such compounds, and both of the terms salt and ionic compound refer to the same type of compound. In most molecular compounds, molecules exist as separate units, but the units of ionic compounds are part of a continuous, three-dimensional structure called a crystal lattice.
Figure 2 Hydrogen gas is an example of an element because it contains only one type of atom.
Water and lithium chloride are both compounds because they each contain more than one type of atom.
Water is a molecular compound and lithium chloride is an ionic compound (or salt).
Water and lithium chloride are both compounds because they each contain more than one type of atom.
Water is a molecular compound and lithium chloride is an ionic compound (or salt).
Fill in the blank.
A(n)
is formed of the same type of atoms.
answer
element
Fill in the blank.
A(n)
is formed of different types of atoms.
answer
compound
2.1 Introducing Chemical Bonds
Pure Substance
Chemists classify substances in many different ways because the classifications help them work with the huge variety of materials they encounter. Chemists are often concerned about the purity of their materials and distinguish pure substances from mixtures.
A pure substance is composed of only one type of particle or unit. When pure substances are examined at the smallest level, the particles or units are found to be identical. The unique structures can be atoms, molecules, or ionic units, but they are all identical in a pure substance. Examples of two pure substances are shown in Figure 3.
In the glass of water on the left, only one type of molecule is present, water molecules. In the helium-filled balloon only one type of atom is present, helium atoms. In both cases, the smallest unit making up the substance is uniform throughout the substance, with no other particles present.
Because a pure substance is composed of only one type of particle or unit, samples from that substance have the same physical properties, which can be used to identify that substance. Physical properties such as melting point, boiling point, and density are often used to identify pure substances. For example, water has a characteristic freezing point of 0°C, a boiling point of 100°C, and a density of 1.0 g/mL. If you have a liquid that has these characteristics, you can identify the liquid as water.If you dissolve salt in the water, its freezing point decreases but its boiling point and density increase. The water is no longer a pure substance; the changes in its physical properties provide evidence of this change.
A chemist has a piece of metal that looks like aluminum. What physical properties can he rely on in order to make sure it's aluminum?
melting point
color
odor
density
texture
Answer
density and melting point
Answer
Pure
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