Chemistry Study Guide for McGraw-Hill's ASVAB (page 2)
Everything is made up of matter. Matter is anything that takes up space and has energy. Energy causes change. Matter can exist as a solid, liquid, or gas- the three states of matter. An atom is the smallest building block of matter. Atoms are made of neutrons (with no electric charge), protons (with a positive electric charge), and electrons (with a negative electric charge). The center of an atom is the nucleus. The nucleus contains protons and neutrons. The nucleus of an atom is tiny in comparison to the atom itself. Electrons fly around the nucleus in orbits. Most of an atom is empty space. Energy holds the atom together. Atoms have the same number of electrons and protons, making them stable.
If a substance is made up entirely of the same kind of atom, it is called an element. An element cannot be broken down into a simpler form. The element carbon is made up of only carbon atoms, and the element sulfur is made up of only sulfur atoms.
Scientists have given all elements a one- or two-letter symbol. For example, oxygen is O, sulfur is S, calcium is Ca, and magnesium is Mg. All elements are arranged in a periodic table according to their similarities, providing important information about each element. You will not need to know the periodic table for the ASVAB test, but you may need to know that each block of the table has certain information.
All elements in the periodic table are ordered by their atomic number. The atomic number is the number of protons in the atom. In an atom with a neutral charge, the number of electrons equals the number of protons. The atomic mass number is equal to the number of protons plus the number of neutrons.
Presence of Elements Most of the human body is made up of water, H2O. Your cells contain about 65 to 90 percent water by weight. As much as 99 percent of the mass of the human body is made up of just six elements: oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus.
By contrast, Earth's crust is made up of lesser amounts of oxygen, but a lot of silicon and aluminum.
In a compound, two or more elements combine chemically in a fixed proportion to create a new substance that has characteristics different from those of the combining elements. For example, water is a compound made up of two hydrogen atoms and one oxygen atom. It is commonly written as H2O. This ratio holds whether you have a gram of water or an ocean full of water.
There are two types of compounds, covalent compounds and ionic compounds. A covalent compound is made up of molecules (in most cases). A molecule is a group of atoms that are held together by chemical bonds (mainly covalent bonds). Chemical reactions occur when the bonds are broken and new bonds form, making different molecules. Ions are atoms that are positively or negatively charged. If an ion has more electrons than protons, it is negatively charged. If it has more protons than electrons, it is positively charged. Ions that are positively charged are attracted to ions that are negatively charged. Often ions will come together to form neutral ionic compounds. Table salt is a good example of this, as it has a positive ion of sodium (Na+) that is attracted to a negatively charged chlorine (Cl–), forming NaCl.
A mixture is composed of two or more substances that do not combine with each other, but instead retain their original characteristics. Also, the substances in a mixture do not have to be in any fixed proportion. For example, if you poured flour and salt together into a bowl, you would create a mixture. Mixing a cup of sugar into a gallon of water would create a mixture called a solution, where the sugar dissolves into the water. Mixtures can be solids, liquids, or gases or any combination of those.
Another type of mixture is a suspension. A suspension is a mixture of a liquid or gas with another substance spread evenly through it. Common suspensions include salt in water, fine soot or dust in air, droplets of oil in air, and whole blood. Particles in a suspension precipitate to the bottom if the suspension is left standing.
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