Reading ¹²₆C

The idea

In standard notation an atom is written with its mass number (A) over its atomic number (Z), followed by the symbol of the element.

A short code for an atom

Carbon has the symbol C. Its atomic number is 6 and its mass number is 12.

How can all three be written together in a compact form?

• Mass number above atomic number, then the symbol
• Symbol, then both numbers in a row
• Atomic number above mass number, then the symbol

Read the notation

Open each tab.

Standard notation

The mass number goes above the atomic number, and both are written before the symbol. For carbon, ¹²₆C.

In standard notation, an atom is written with its mass number (A) over its atomic number (Z), followed by the symbol of the element. For example, carbon: ¹²₆C.

Notes

Standard notation: mass number A over atomic number Z, then the symbol, as in ¹²₆C.

Check yourself

In ¹²₆C, what does the upper number 12 show?

In ²³₁₁Na, what does the lower number 11 show?

How many neutrons are there in an atom of ¹²₆C?

Answer: 6

Mass number minus atomic number: 12 − 6 = 6.

In the standard notation, what is the order?

• The mass number — correct. Yes!
• The atomic number. The atomic number is written below, as 6.
• The number of neutrons. The number of neutrons is 12 − 6 = 6, not 12.
• The mass number. The mass number is the upper number, 23.
• The atomic number, the number of protons — correct. Yes!
• The number of neutrons. The number of neutrons in this atom is 23 − 11 = 12.
• Atomic number over mass number, then the symbol. The mass number is the upper number.
• Symbol first, then the two numbers. The symbol is written after the numbers.
• Mass number over atomic number, then the symbol — correct. Yes!