Matter is anything taking up space and having mass. The amount of matter in an object is its mass. 3 main states of matter include: SOLID: definite mass, definite volume, definite shape LIQUID: definite mass, definite volume, indefinite shape GAS: indefinite mass, indefinite volume, indefinite shape The amountof disorder in atoms or molecules is known as entropy. Example: SOLID LIQUID = Entropy increasing Example: GAS SOLID = Entropy decreasing
All matter has 2 types of distinguishing characteristics: Physical properties Chemical properties Physical properties can be observed without changing the composition of the substance. Examples: density, color, hardness, freezing point, boiling point, and the ability to conduct electricity. Chemical properties describe how a substance reacts with other substances. Examples: oxidation (iron reacting with oxygen to form rust).
An element is a substance that cannot be broken down into simpler, stable substances by ordinary chemical means. About 90 elements occur naturally on Earth. 8 of those make up more than 98% of Earth’s crust. Elements consist of atoms, which are the smallest units of an element having the chemical properties of that element. A single atom is so small, its size is difficult to imagine. Look at the thickness of a sheet of notebook paper about 1,000,000 atoms lined up side by side would be equal to that thickness.
Atoms are made up of even smaller parts called subatomic particles. 3 major kinds include: Protons – subatomic particle having a positive charge located in the nucleus of an atom. Neutrons – subatomic particle having no charge located in the nucleus of an atom. Electrons – subatomic particle having a negative charge located in energy levels surrounding the nucleus of an atom.
The maximum number of electrons an energy level can occupy is expressed by the formula 2n2, where n= the energy level.
The number of protons in the nucleus of an atom is called the atomic number. All atoms of any given element have the same atomic number thus the same number of protons. Since an uncharged atom has an equal number of protons and electrons, the atomic umber is also equal to the number of electrons.
The Periodic Table of Elements is a system for classifying elements and was first devised by the Russian chemist, Dmitri Mendeleev, in 1869. Elements onthe Periodic Table are ordered according to their atomic numbers. Rows are called periods. Columns are called groups. Elements in the same column have similar arrangements of electrons around their atom, therefore having similar chemical Mendeleev properties.
The sum of the number of protons and neutrons in an atom is the mass number. This is the atomic mass rounded to the nearest whole number. The mass of a subatomic particle is too small to be expressed in grams. So, we use a special unit called the atomic mass unit (amu). Protons and neutrons each have an atomic mass close to 1 amu. In contrast, electrons have much less. The mass of 1 proton is equal to the combined mass of about 1840 electrons!
Although atoms of a given element contain the same number of protons, the number of neutrons may differ. An atom of the same element having the same number of protons (atomic number), but a different number of neutrons (different mass number) is called an isotope.
A Carbon-14 atom having 8 neutrons is more massive than a Carbon- 11 atom having only 5 neutrons. Different isotopes of the same element have slightly different properties. Unstable isotopes are subject to radioactive decay, where they will emit energy and change into another element. Since isotopes of an element have different masses, the Periodic Table uses an average atomic mass, which is the weighted average of the atomic masses of the naturally occurring isotopes. Isotopes of Hydrogen (H): 1H, 2H (Deuterium), and 3H (Tritium). On the Periodic Table, the average atomic mass of Hydrogen (H) is 1.00794 u.
An atom’s chemical properties are largely determined by the number of the outermost electrons, valence electrons, in an atom’s electron cloud. Within each group (column), the atoms of each element generally have the same number of valence electrons. For Groups 1 and 2, the number of valence electrons in each atom is the same as that atom’s group number. Atoms of elements in Groups 3-12 (Transition Metals) have 2 or more valence electrons. For Groups 13-18, the number of valence electrons in each atom is the same as that atom’s group number minus 10, except for Helium (He), which only has 2 valence electrons.
Elements whose atoms have only 1, 2, or 3 valence electrons tend to lose electrons rather easily. These elements have metallic properties and are generally classified as metals. Elements whose atoms have from 4-7 valence electrons are more likely to gain electrons. Many of these elements are classified as nonmetals. When an atom has 8 valence electrons, it is considered stable, or chemically un-reactive. It’s outer shell is full … Octet Rule
Elements rarely occur in pure form in Earth’s crust, but rather as a combination with other elements we discussed Quartz, SiO2 A substance made of two or more elements chemically combined is called a compound. A group of atoms held together by chemical forces is called a molecule. All compounds are molecules, but not all molecules are compounds! Some elements occur as diatomic molecules, which are molecules made up of only two atoms. Mnemonic: I Have No Bright Or Clever Friends Iodine (I2), Hydrogen (H2), Nitrogen (N2), Bromine (Br2), Oxygen (02), Chlorine (Cl2), and Fluorine (F2).
A chemical formula is a combination of letters and numbers showing which elements make up a compound. Also shows the number of atoms of each element that are required to make a molecule of a compound. The chemical formulafor water is H20, indicating each water molecule consists of 2 atoms of Hydrogen and 1 atom of Oxygen, shown by the subscript. x2 x2 Superscript Subscript
Another example of a chemical formula is sodium chloride, NaCl, halite, or table salt … they all mean the same thing! Sodium (Na) is a silvery metal and Chlorine (Cl) is a poisonous greenish-yellow gas. They both make up table salt, which you can eat. + =
A chemical reaction showing the combination of elements and compounds is called a chemical equation. The reactants are on the left-hand side of the arrow, combining to form the products, on the right-hand side of the arrow. The arrow means “yields” or “gives”.
Chemical equations mustbe balanced to show accurate chemical information. An equation is balanced when the number of atoms of each element on the right side equal the number of atoms of the same element on the left side. You cannot change the chemical formulas, but rather put numbers called coefficients in front of chemical formulas. A coefficient multiplies the subscript in a formula; 4 Hydrogen (H) atoms are in the formula 2H20.
The forces holding the atoms in molecules together are called chemical bonds. They form due to the attraction between positive and negative charges. Atoms form chemical bonds by either transferring or sharing valence electrons from one atom to another. When this happens, the atoms become charged. A particle, such as an atom or molecule, carrying a charge is called an ion.
An attraction between oppositely charged ions resulting in the transfer of electrons from one atom to another is called an ionic bond.
Let’s look again at NaCl, or sodium chloride:
The Sodium (Na) atom transfers 1 electron, becoming positively charged (Na+1). The Chlorine (Cl) atom accepts the 1 electron, becoming negatively charged (Cl-1). This chemical reaction results in table salt, NaCl.
An attraction between atoms sharing electrons is called a covalent bond. The positive nucleus of each atom is attracted to the shared negative electrons.
On Earth, elements and compounds are generally mixed together. A mixture is a combination of 2 or more substances not chemically combined. Mixtures in which 2 or more substances are not uniformly distributed are called heterogeneous mixtures. Examples include: igneous rock, granite (contains the minerals quartz, feldspar, hornblende, and biotite mica). Mixtures in which 2 or more substances are uniformly dispersed throughout are called homogeneous mixtures. Examples include: Sea water (NaCl is dissolved in H2O).
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