# atomic number

## atomic number

The number of protons in an atom is called its atomic number. This number is very important because it is unique for atoms of a given element. All atoms of an element have the same number of protons, and every element has a different number of protons in its atoms. For example, all helium atoms have two protons, and no other elements have atoms with two protons. In the case of helium, the atomic number is 2. The atomic number of an element is usually written in front of and slightly below the elements symbol, like in the Figure 1.2 for helium. Atoms are neutral in electrical charge because they have the same number of negative electrons as positive protons. Therefore, the atomic number of an atom also tells you how many electrons the atom has. This, in turn, determines many of the atoms properties.

## mass number

There is another number in the box above for helium. That number is the mass number, which is the mass of the atom in a unit called the atomic mass unit (amu). One atomic mass unit is the mass of a proton, or about 1.67 1027 kilograms, which is an extremely small mass. A neutron has just a tiny bit more mass than a proton, so its mass is often assumed to be one atomic mass unit as well. Because electrons have virtually no mass, just about all the mass of an atom is in its protons and neutrons. Therefore, the total number of protons and neutrons in an atom determines its mass in atomic mass units. Consider helium again. Most helium atoms have two neutrons in addition to two protons. Therefore the mass of most helium atoms is 4 atomic mass units (2 amu for the protons + 2 amu for the neutrons). However, some helium atoms have more or less than two neutrons. Atoms with the same number of protons but different numbers of neutrons are called isotopes. Because the number of neutrons can vary for a given element, the mass numbers of different atoms of an element may also vary. For example, some helium atoms have three neutrons instead of two. Therefore, they have a different mass number than the one given in the box above. Q: What is the mass number of a helium atom that has three neutrons? A: The mass number is the number of protons plus the number of neutrons. For helium atoms with three neutrons, the mass number is 2 (protons) + 3 (neutrons) = 5. Q: How would you represent this isotope of helium to show its atomic number and mass number? A: You would represent it by the elements symbol and both numbers, with the mass number on top and the atomic number on the bottom: 5 2 He

## telling atoms apart

Its often useful to have ways to signify different people or objects like athletes on teams. The same is true of atoms. Its important to be able to distinguish atoms of one element from atoms of other elements. Elements are pure substances that make up all other matter, so each one is given a unique name. The names of elements are also represented by unique one- or two-letter symbols, such as H for hydrogen, C for carbon, and He for helium. You can see other examples in the Figure 1.1. Q: The table shown above is called the periodic table of the elements. Each symbol stands for a different element. What do you think the symbol K stands for? A: The symbol K stands for the element potassium. The symbol comes from the Latin name for potassium, which is kalium. The symbols in the table above would be more useful if they revealed more information about the atoms they represent. For example, it would be useful to know the numbers of protons and neutrons in the atoms. Thats where atomic number and mass number come in.

## instructional diagrams

No diagram descriptions associated with this lesson

## questions

all atoms of a given element have the same number of protons.

``````-->  a. true

b. false
``````

atoms of different elements always have different numbers of protons.

``````-->  a. true

b. false
``````

which statement about the mass of subatomic particles is false?

``````a) a proton has a mass of 1 amu.

b) a neutron has slightly more mass than a proton.

-->  c) an electron has slightly less mass than a proton.

d) none of the above
``````

a carbon atom has six protons and six neutrons. what is its atomic number?

``````-->  a) 6

b) 12

c) 36

d) none of the above
``````

what is the mass number of the carbon atom in question 8?

``````a) 6

-->  b) 12

c) 18

d) 72
``````

## diagram questions

No diagram questions associated with this lesson