Decimals and Spatial Reasoning

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My second grade students just started a unit on decimals.  Based on the pre-assessment, most students have no problem with identifying the value and place value position of digits in the ones – hundred-thousands place.  It’s a different story for numbers to the left of the decimal point.

Earlier in the week students explored the tenths and hundredths place.  Students connected money concepts to place value and fractions.  They compared 1/2 with 0.50 and $1.50 with 1.50.    They completed similar activities where they needed create benchmarks on number lines and place numbers.  Some were still having trouble and I believe this is partially due to exposure.  Also, I was finding that their were issues with spatial awareness.  Students were looking placing able to approximate benchmarks of half, but placing 0.1 close to the half.  Student practiced using number lines and using benchmarks.  The most tricky piece was looking at the differences between the hundredths and thousandths.  This challenge reminded me of how students develop an understanding of the magnitude of numbers.

Today I grouped students into teams and they used dice to create different decimals.  The decimals ranged between 0 and 3.  Students were given a horizontal and vertical number line on a 11 by 17 paper.  This gave students room to work.  The two number lines were different sizes.  An indicator line was placed at the beginning and end of each line.

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After students created their decimals they started to place benchmarks.  Some students had to get out the erasers as realization set in that the maximum would be three instead of two.  Students also reevaluated their benchmark placement.  Groups noticed that the two number lines were different sizes and had to adjust their benchmarks accordingly.  I found it interesting that some students used the vertical number line top down, while other went bottom to top.

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We’ll be reviewing the number lines on Monday.  I’m looking forward to the discussion and we might even break out the rulers to evaluate the reasonableness between benchmarks.