Elements and Atoms
An
atom is the smallest particle of an element that retains the chemical properties of that element, and an
elementis defined as a substance that can’t be broken down or separated intosimpler substances through a chemical reaction. Elements contain justone type of atom, and each different element contains a different typeof atom. Take the element sulfur (S). A pile of sulfur (a yellow,powdery or crystallized substance) sitting on a table represents asingle element—sulfur—and this pile of sulfur is made up of only onetype of atom—sulfur atoms.
Each atom, regardless of its identity, is made up of three types of subatomic particles.
Protons, which are positively charged and situated at the center of the atom (also known as the atomic
nucleus);
neutrons, which are electrically neutral (meaning that they have no charge) and are also in the nucleus of the atom; and
electrons,which are negatively charged and are situated outside the nucleus. Themajority of the mass of an atom is contained in its nucleus: whileelectrons are about the same size as protons and neutrons, an electronhas 1/837th the mass of protons or neutrons. You should also be awarethat the nucleus of an atom is much, much smaller and more dense thanthe space occupied by an atom’s electrons—if an atom were the size of afootball field, the nucleus would be the size of a flea on the 50-yardline!
The number of protons an atom possesses iswhat gives the atom its identity—all atoms of a particular element havethe same number of protons in their nuclei. For example, all of thesulfur atoms in the pile of sulfur we looked at above have 16 protonsin their nucleus. If they had one more proton in their nucleus, theywould have a different identity—they’d be chlorine (Cl) atoms, and withone less, they’d be phosphorus (P) atoms.
Atoms of a given element can, however,differ in the number of neutrons they contain, and atoms of the sameelement that have different numbers of neutrons are known as
isotopes.Most elements have at least two isotopes that occur naturally, althougha few have just one. Now take a look at how atoms are usuallysymbolized:
This represents a carbon atom that has 6 protons and 6 neutrons. In this notation, the
atomic number (A), which is the number of protons the atom contains, is indicated by the subscript, and the
mass number (Z ),which is the number of the atom’s protons plus the number of itsneutrons, is indicated by the superscript. Some relatively commonisotopes of carbon can contain 5, 7, or 8 neutrons, so although theiratomic numbers would all be 6, their mass numbers, respectively, wouldbe 11 (6 + 5), 13 (6 + 7), and 14 (6 + 8). Isotopes can also be writtenas carbon-14, carbon-15, carbon-16, etc., or C-14, C-15, C-16, wherethe number represents the mass number of the atom.
The last thing you should know about thebasic structure of an atom is that atoms have the same number ofprotons and electrons, and since protons are positively charged andelectrons are negatively charged,
neutral atoms have no net electrical charge.
Example
The atomic number of a certain element is11, and its atomic mass number is 23. How many protons and neutronsdoes this atom have, and what is its chemical symbol?
Explanation
If the atomic number is 11, this element issodium and its symbol is Na. If the atomic mass number is 23, thenumber of neutrons is equal to 23 - 11 = 12.
Atoms and the Periodic Table
The day of the
SAT II Chemistry exam, youwill be given a periodic table to use while answering the questions.However, this periodic table will most likely be much simpler than theones you use in class or have seen in your chemistry text. It will giveyou only two pieces of information for each element: the element’satomic number and the element’s atomic weight, which is written belowthe element’s symbol in each box. The
atomic weight of anelement represents its average atomic mass based on the relativeabundance of various isotopes of that element in nature. So, when wesay that the atomic weight of carbon is 12.0107, we mean that theaverage weights of all of the isotopes of carbon that exist in nature,whether the carbon is carbon-11, -12, -13, or -14, is 12.0107.
But what does it mean to say that theisotopes “weigh” 12.0107? 12.0107 what? Certainly not grams, or theisotopes would be a lot bigger than they are. Atomic weights have theunit amu, or
atomic mass unit, and one atomic mass unit is equal to 1.66054
10-24 g.
